2009. "Excitation, Ionization, and Desorption: How Sub-band gap Photons Modify the Structure of Oxide Nanoparticles." Journal of Physical Chemistry C 113(4):1274-1279. Abstract Nanoparticles of wide-band-gap materials MgO and CaO, subjected to low-intensity ultraviolet irradiation with 266 nm (4.66 eV) photons, emit hyperthermal oxygen atoms with kinetic energies up to ~ 0.4 eV. We use ab initio embedded cluster methods to study theoretically a variety of elementary photoinduced processes at both ideal and defect-containing surfaces of these nanoparticles and develop a mechanism for the desorption process. The proposed mechanism includes multiple local photoexcitations resulting in sequential formation of localized excitons, their ionization, and further excitations. It is suggested that judicious choice of sub-band-gap photon energies can be used to selectively modify surfaces of nanomaterials.
2009. "Photoinduced Formation of Zinc Nanoparticles by UV Laser Irradiation of ZnO." Langmuir 25(4):1930-1933. Abstract Simple exposure of single-crystal ZnO to 193 11m excimer laser radiation at room temperature results in unexpected coloration. The gray to nearly black colored material. seen principally in the irradiated laser spot, is superficial. We present unambiguous evidence that this coloration is due to high densities of metallic Zn nanoparticles growing on the exposed surface of the crystal. Higher fluence laser exposure generates accumulated surface metal just outside of the irradiated spot. We suggest that the near surface bulk is photodecomposing; thermally driven diffusion leads to surface Zn metal aggregation.
2009. "Excitons in Potassium Bromide: A Study using Embedded Time-dependent Density Functional Theory and Equation-of-Motion Coupled Cluster Methods." Chemical Physics Letters 470(4-6):353-357. doi:10.1016/j.cplett.2009.01.073 Abstract We present a study of the electronic excitations in insulating materials using an embedded- cluster method. The excited states of the embedded cluster are studied systematically using time-dependent density functional theory (TDDFT) and high-level equation-of-motion coupled cluster (EOMCC) methods. In particular, we have used EOMCC models with singles and doubles (EOMCCSD) and two approaches which account for the e®ect of triply excited con¯gurations in non-iterative and iterative fashions. We present calculations of the lowest surface excitations of the well-studied potassium bromide (KBr) system and compare our results with experiment. The bulk-surface exciton shift is also calculated at the TDDFT level and compared with experiment.
2008. "Electronic Energy Transfer on CaO Surfaces." Journal of Chemical Physics 129(12):124704. doi:10.1063/1.2980049 Abstract We excite low-coordinated surface sites of nanostructured CaO samples using tunable UV laser pulses and observe hyperthermal O-atom emission indicative of an electronic excited-state desorption mechanism. The O-atom yield increases dramatically with photon energy, between 3.75 and 5.4 eV, below the bulk absorption threshold. The peak of the kinetic energy distribution does not increase with photon energy in the range 3.9 to 5.15 eV. These results are analyzed in the context of a laser desorption model developed previously for nanostructured MgO samples. The data are consistent with desorption induced by exciton localization at corner-hole trapped surface sites following either direct corner excitation or diffusion and localization of excitons from higher coordinated surface sites.
2008. "Energy and Site Selectivity in O-Atom Photodesorption from Nanostructured MgO." Surface Science 602(11):1968-1973. doi:10.1016/j.susc.2008.03.046 Abstract Electronic excitation of wide gap ionic solids can induce desorption of neutral atoms with distinct hyperthermal and thermal kinetic energy distributions. Hyperthermal atomic desorption results from electronic surface excitation while thermal desorption is initiated primarily by bulk excitation. Calculations indicate that surface-localized transitions can be excited independently from bulk transitions using selected photon energies. The photon energy required to excite specific surface sites depends upon the site coordination with successively lower energies required to excite terrace, step, and corner sites. Here, we excite low-coordinated surface sites of nanostructured MgO samples using 4.7 eV UV laser pulses and observe dominant hyperthermal O-atom emission. We then selectively excite bulk sites of nanostructured MgO, using a 7.9 eV laser, and observe dominant thermal O-atom desorption. These results are analyzed in terms of laser desorption models developed previously for alkali halide crystals. We propose a multi-step mechanism for hyperthermal O-atom desorption, under surface selective excitation, based on hole trapping at 3C (corner) O-atom sites followed by exciton decomposition. The proposed “hole plus exciton” model has similarities to the surface exciton desorption model, established for alkali halides, but is more complex and requires more steps. Nonetheless, the principle of site-specific photoreaction, established for alkali halide crystals, is clearly extendable to a prototypical metal oxide.
2007. "" Abstract Laser ablation encompasses a wide range of delicate to extreme light interactions with matter that present considerably challenging problems for scientists to study and understand. At the same time, laser ablation also represents a basic process of significant commercial importance in laser material processing—defining a multi-billion dollar industry today. These topics were widely addressed at the 8th International Conference on Laser Ablation (COLA), held in Banff, Canada on 11–16 September 2005. The meeting took place amongst the majestic and natural beauty of the Canadian Rocky Mountains at The Banff Centre, where delegates enjoyed many inspiring presentations and discussions in a unique campus learning environment. The conference brought together world leading scientists, students and industry representatives to examine the basic science of laser ablation and improve our understanding of the many physical, chemical and/or biological processes driven by the laser. The multi-disciplinary research presented at the meeting underlies some of our most important trends at the forefront of science and technology today that are represented in the papers collected in this volume. Here you will find new processes that are producing novel types of nanostructures and nano-materials with unusual and promising properties. Laser processes are described for delicately manipulating living cells or modifying their internal structure with unprecedented degrees of control and precision. Learn about short-pulse lasers that are driving extreme physical processes on record-fast time scales and opening new directions from material processing applications. The conference papers further highlight forefront application areas in pulsed laser deposition, nanoscience, analytical methods, materials, and microprocessing applications.
2007. "Photoemission Electron Microscopy of TiO2 Anatase Films Embedded with Rutile Nanocrystals." Advanced Functional Materials 17(13):2133-2138. doi:10.1002/adfm.200700146 Abstract Photoemission electron microscopy (PEEM) excited by x-ray and UV sources is used to investigate epitaxial anatase thin films embedded with rutile nanocrystals, a model system for the study of heterocatalysis on mixed-phase TiO2. Both excitation sources show distinct contrast between the two TiO2 phases, however, the contrast is reversed. Rutile nanocrystals appear darker than the anatase film in X-ray PEEM images but brighter in UV-PEEM images. Topography-induced contrast is dominant X-ray PEEM imaging, whereas work function contrast, dominates for UV-PEEM. Work function contrast results from the differences in work function and surface defect state densities between the two phases near the Fermi level. This assertion is confirmed by UPS data that shows the rutile work function to be 0.2 eV lower and a greater occupied valence band density-of-states in rutile (100) than in anatase (001). Since the boundaries between rutile nanocrystals and the anatase film are clearly resolved, these results indicate that PEEM studies of excited state dynamics and heterocatalysis are possible at chemically intriguing mixed-phase TiO2 interfaces and grain boundaries.
2007. "Study of Copper Diffusion Through Ruthenium Thin Film by Photoemission Electron Microscopy." Applied Physics Letters 90:111906. doi:10.1063/1.2712832 Abstract Photoemission electron microscopy (PEEM) is employed to study Cu diffusion in real time through a Ru barrier in a Cu/Ru bilayer system. The PEEM images display large contrast between Cu and Ru due of the differences in work function between the two metals, making PEEM an ideal methodology to study diffusion in real time. At low temperature (175-290 °C), Cu mainly diffuses through the defective sites in the Ru film. Uniform diffusion of Cu through a Ru thin film occurs at approximately 300 °C. The results are confirmed by X-ray photoemission spectroscopy (XPS) depth profiling and scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS) analysis.
2007. "Synthesis and Photoexcited Charge Carrier Dynamics of beta-FeOOH Nanorods." Applied Physics Letters 90(10):Art. No. 103504. doi:10.1063/1.2711395 Abstract Akaganeite(B-FeOOH) nanorods of dimensions 15 nm diameter and 200 nm length were prepared by aqueous synthesis. Charge carrier dynamics following femtosecond excitation displays three timescales. The first is a sub-picosecond decay of initially excited carriers to the band edge followed by trapping or nonradiative decay within 2 ps. The trapped electrons and holes persist for significantly longer times (at least tens-of-ps), similar to previous results from a-Fe2O3 materials. The short carrier lifetimes in these materials are attributed to fast trapping to Fe d-d and midgap states.
2007. "Study of Martensitic Phase transformation in a NiTiCu Thin Film Shape Memory Alloy Using Photoelectron Emission Microscopy." Advanced Functional Materials 17(1):161-167. doi:10.1002/adfm.200600611 Abstract The thermally-induced martensitic phase transformation in a polycrystalline NiTiCu thin film shape memory alloy was probed by photoelectron emission microscopy (PEEM). In situ PEEM images reveal distinct changes in microstructure and photoemission intensity at the phase transition temperatures. In particular, images of the low temperature, martensite phase are brighter than that of the high temperature, austenite phase, due to the relatively lower work function of the martensite. Ultra-violet photoelectron spectroscopy shows that the effective work function changes by about 0.16 eV during thermal cycling. In situ PEEM images also show that the network of trenches observed on the room temperature film disappear suddenly during heating and reappear suddenly during subsequent cooling. These trenches are also characterized by atomic force microscopy at selected temperatures. We describe implications of these observations with respect to the spatial distribution of phases during thermal cycling in this thin film shape memory alloy.
2007. "An In Situ Study of the Martensitic Transformation in Shape Memory Alloys Using Photoemission Electron Microscopy." Journal of Nuclear Materials 361(2-3):306-312. doi:10.1016/j.jnucmat.2006.12.008 Abstract Thermally-induced martensitic phase transformations in polycrystalline CuZnAl and thin-film NiTiCu shape memory alloys were probed using photoemission electron microscopy (PEEM). Ultra-violet photoelectron spectroscopy shows a reversible change in the apparent work function during transformation, presumably due to the contrasting surface electronic structures of the martensite and austenite phases. In situ PEEM images provide information on the spatial distribution of these phases and the evolution of the surface microstructure during transformation. PEEM offers considerable potential for improving our understanding of martensitic transformations in shape memory alloys in real time.
2006. "Laser-Induced Oxygen Vacancy Formation and Diffusion on TiO2(110) Surfaces Probed by Photoemission Electron Microscopy." Physica Status Solidi. C 3(10):3598-3602. Abstract Photoemission electron microscopy is used to probe photon-induced oxygen vacancies generated on TiO2 (110)-(1×2) surfaces. An increased oxygen vacancy concentration within the irradiated region leads to an increase of local photoelectron emission. The local oxygen deficient region can be compensated by exposing the surface to molecular oxygen at 1×10-5 Torr, or via surface diffusion at 450 K in vacuum. The surface diffusion coefficient was estimated to be on the order of 10-12 m2/s. Photoemission electron microscopy allows in situ studies of surface electronic defect formation and removal.
2006. "Introduction to Photoelectron Emission Microscopy: Principles and Applications." Journal of Chinese Electron Microscopy Society 25(1):15-25. Abstract In the past decade, photoelectron emission microscopy (PEEM) has undergone major instrument development and become commercially available. PEEM probes photoelectrons in a high-contrast imaging technique that is sensitive to the surface electronic structure. In this paper we illustrate the principles of PEEM and analyze important PEEM contrast mechanisms. We briefly summarize the applications of PEEM to areas such as surface structure analysis, surface chemistry, magnetism, and semiconductor device characterization. Two important new directions in PEEM development are multiphoton and time-resolved PEEM. Multiphoton PEEM is capable of imaging materials with work functions greater than the incident photon energy, while time-resloved PEEM enables study of fast relaxation dynamics of surface intermediate states. We discuss our recent progress on implementing femtosecond time-solved PEEM and multiphoton PEEM to investigate the silver nanostructured film coated on silicon. The multiphoton images consist of some “hot spots” with far greater photonelectron intensity than is observed in single-photon derived images. We surmise that this is due to the highly-selective excitation of surface localized plasmon of silver. To illustrate the utility of PEEM, we also describe an in-situ thermal-induced structural phase transformation of CuZnA1 shape memory alloy.
2006. "In Situ Photoelectron Emission Microscopy of a Thermally Induced Martensitic Transformation in a CuZnAI Shape Memory Alloy." Applied Physics Letters 88(9):Art. No. 091910. Abstract Photoemission electron microscopy, in conjunction with photoemission spectroscopy, reflectivity, and surface roughness measurements, is used to study the thermally-induced martensitic transformation in a CuZnAI shape memory alloy. Real-time phase transformation is observed as a nearly instantaneous change of photoelectron intensity, accompanied by microstructural deformation and displacement due to the shape memory effect. The difference in the photoelectron intensity before and after the phase transformation is attributed to the concomitant change of work function as measured by photoelectron spectroscopy. Photoemission electron microscopy is shown to be a valuable new technique facilitating the study of phase transformations in shape memory alloys, and provides real-time information on microstructural changes and phase-dependent electronic properties.
2006. "Excited Carrier Dynamics of α-Cr2O3/α-Fe2O3 Core-Shell Nanostructures." Journal of Physical Chemistry B 110(34):16937-16940. doi:10.1021/jp062507n S1520-6106(06)02507-7 Abstract In this work α-Cr2O3/α-Fe2O3 core-shell polycrystalline nanostructures were synthesized using α-Cr2O3 nanoparticles as seed crystals during aqueous nucleation. The formation of α-Fe2O3 polycrystallites on α-Cr2O3 surfaces were confirmed by x-ray diffraction, transmission electron microscopy, and energy dispersive x-ray analysis. The excited state relaxation dynamics of as-grown core-shell structures and \pure" α-Fe2O3 particles of the same size were measured using femtosecond transient absorption spectroscopy. The results show the carrier lifetimes decay within a few picoseconds regardless of sample. This is likely due to fast recombination/trapping of carriers to defects and iron d-states.
2006. "Two-Hole Localization Mechanism for Electron Bond Rupture of Surface Atoms by Laser-Induced Valence Excitation of Semiconductors." Physical Review. B, Condensed Matter and Materials Physics 74(3):Art. No. 035337. doi:10.1103/PhysRevB.74.035337 Abstract We examine the mechanism of electronic bond rupture on semiconductor surfaces induced by laser-generated three-dimensional non-equilibrium valence excitation associated with strong carrier diffusion. For such excited systems, the density of sub-surface valence holes that contribute to two-hole localization on the surface is characterized by quasi Fermi level and effective temperature. The rate of two-hole localization, formulated for equilibrated two-dimensional electronic systems by Sumi [Surf. Sci, 248, 382 (1991)], is re-formulated, and a simple analytical expression is yielded for moderate excitation densities. The resulting theoretical model has been successfully applied in the analysis of recent laser-induced atomic desorption experiments on InP and Si surfaces
2006. "Probing Electron Transfer Dynamics at MgO Surfaces by Mg-Atom Desorption." Journal of Physical Chemistry B 110(37):18093-18096. doi:10.1021/jp064092b Abstract Desorption of a weakly bound adsorbate from a porous solid was studied for the case of N2 on amorphous solid water (ASW). Porous ASW films of different thickness were grown on Pt(111) by ballistic deposition. N2 adsorption and desorption kinetics were monitored mass-spectrometrically. Temperature programmed desorption spectra show that with the increasing film thickness, the N2 desorption peak systematically shifts to higher temperatures. The results are explained and quantitatively reproduced by a simple model, which assumes that the N2 transport within the film is faster than the depletion rate to vacuum. The local coverage at the pore mouth determines the desorption rate. For thick ASW films (>1 μm), the assumption of the fast equilibration within the film is shown to be no longer valid due to diffusion limitations. The mechanisms of the adsorbate transport are discussed.
2006. "Carrier Dynamics in a-Fe2O3 (0001) Thin Films and Single Crystals Probed by Femtosecond Transient Absorption and Reflectivity." Journal of Applied Physics 99(5):Article: 053521 (6 pages). Abstract Femtosecond transient reflectivity and absorption is used to measure the carrier lifetimes in α-Fe2O3 thin films and hematite single crystals. The results from the thin films show that initially excited hot electrons relax to the bandedge within 300 femtoseconds and then recombine with holes or trap within 5 pioseconds. The trapped electrons have a lifetime of hundreds of picoseconds. The trapped electrons have a lifetime of hundreds of picoseconds. Transient reflectivity measurements from hematite (α-Fe2O3) single crystals show similar but slightly faster dynamics. In hematite, the transient reflectivity displays oscillations due to the formation of longitudinal acoustic phonons generated following absorption of the ultrashort excitation pulse.
2006. "Site-Specific Laser Modification of MgO nanoclusters: Towards Atomic-Scale Surface Structuring." Physical Review. B, Condensed Matter 74:045404 (5 pages). Abstract Atomic emission from MgO nanostructures is induced using laser light tuned to excite specific surface sites at energies well below the excitation threshold of the bulk material. We find that near UV excitation of MgO nancrystalline films and nanocube samples desorbs neutral Mg-atoms with hyper-thermal kinetic energies in the range of 0.1-0.4 eV. Our ab initio calculations suggest that metal atom emission is induced predominantly by electron trapping at surface 3-coordinated Mg sites followed by electronic excitation at these sites. The proposed general mechanism can be used to control atomic scale modification of insulating surfaces.
2005. "A Mechanism of Photo-Induced Desorption of Oxygen Atoms From MgO Nano-Crystals." Surface Science 593(1-3):210-220. Abstract In a series of recent experimental and theoretical papers we reported the results of our studies of photo-induced hyper-thermal halogen atom desorption from alkali halide surfaces. There we demonstrated that the yield, electronic state and velocity distributions of desorbed atoms can be controlled by carefully choosing parameters of photo-irradiation such as laser photon energy and pulse power [ ]. To achieve laser control over desorption process one must have clear understanding of possible desorption mechanisms and parameters responsible for their selective excitation. For alkali halides, as it has been shown through a combination of theory and experiment, such selectively is observed if the laser energy is tuned to preferentially excite surface excitons. If similar mechanisms could be demonstrated for a wider variety of materials, this approach could become a new method for controlling surface processes and hence modifying surface structures on an atomic scale. In this paper we report the first experimental observation of the hyper-thermal oxygen atom emission from an of MgO nano-clusters and thin films using frequency selected laser pulses oxide surface and investigate theoretically the mechanisms of this process. On this way we demonstrate a new concept that can be applied to studying surface reactions and desorption of binary oxides.
2005. "Interaction of Wide-Band-Gap Single Crystals With 248-nm Excimer Laser Radiation: XI. The Effect of Water Vapor and Temperature on Laser Desorption of Neutral Atoms From Sodium Chloride." Journal of Applied Physics 97:043502-1-10. Abstract We investigate the effect of water vapor and temperature on the desorption of neutral Na and C1 from cleaved, single crystal NaC1 during pulsed laser irradiation at 248-nm (KrF excimer). Neutral emissions in the presence of ~10-5 Pa of water vapor are much more intense than in ultra high vacuum (total pressure <10-7 Pa). Emission intensities are also increased by raising the substrate temperature or the laser fluence. The neutral time-of-flight signals are well described by Maxwell Boltzmann velocity distributions for effusing particles, which we use to estimate the peak surface temperatures during the laser pulse. The neutral emission intensities display Arrhenius behavior when plotted against both the background substrate temperature and the peak surface temperature. The resulting activation energies correspond to different, rate limiting processes, one of which is enhanced in the presence of water vapor. We propose a mechanism for the effect of water on these neutral emissions and discuss the implications.
2005. "Interaction of Wide-Band-Gap Single Crystals with 248-nm Excimer Laser Irradiation: X. Laser-Induced Near-Surface Absorption in Single-Crystal NaC1." Journal of Applied Physics 97(4):Article no. 043501. Abstract Ultraviolet laser-induced desorption of neutral atoms and molecules from nominally transparent, ionic materials can yield particle velocities consistent with surface temperatures of a few thousand Kelvin, even in the absence of visible surface damage. The origin of the laser required for this surface heating has been often overlooked. In this work, we report simultaneous neutral emission and laser transmission measurements on single crystal NaCI exposed to 248-nm excimer laser radiation. As much as 20% of the incident radiation at 248 nm must be absorbed in the near surface region to account for the observed particle velocities. We show that the laser absorption grows from low values over several pulses and saturates at values sufficient to account for the surface temperatures required to explain the observed particle velocity distributions. The growth of absorption in these early pulses is accompanied by a corresponding increase in the emission intensities. Diffuse reflectance spectra acquired after exposure suggest that near surface V-type centers are responsible for most of the absorption at 248 nm in single crystal NaCI.
2005. "Laser Control of Desorption Through Selective Surface Excitation." Journal of Physical Chemistry B 109(42):19563-19578. doi:10.1021/jp0523672 Abstract We review recent developments in controlling photo-induced desorption processes of alkali halides and magnesium oxide. We focus primarily on hyperthermal desorption of halogen (and oxygen) atoms and show that the yield, electronic state, and velocity distributions of desorbed atoms can be selected using tunable laser excitation. We demonstrate that the observed control is due to preferential excitation of surface excitons. This approach takes advantage of energetic differences between surface and bulk exciton states and probes the surface exciton directly. We demonstrate that desorption of these materials leads to controlled modification of their surface geometric and electronic structures. Laser desorption can serve as a solid-state source of halogen and oxygen atoms in well defined electronic states and with controlled velocity distributions. The latter can be used for studying chemical processes in the gas phase and at surfaces. We demonstrate that the exciton mechanism of desorption developed for alkali halides can be extended to cubic oxide surfaces.
2005. "Surface Electronic Properties and Site-Specific Laser Desorption Processes of Highly Structured Nanoporous MgO Thin Films." Surface Science 593(1-3):242-247. Abstract The surface electronic properties of metal oxides critically depends on low-coordinated sites, such as kinks, corners and steps [1]. In order to characterize experimentally those surface states as well as their role for laser desorption processes, we prepare defect enriched surfaces by growing thin MgO films using reactive ballistic deposition [2] on crystalline dielectric substrates. With samples held at room temperature, the resulting MgO films are highly textured and consist of porous columns with column lengths ranging from tens of nanometers up to six micrometers. Measurements by x-ray photoelectron spectroscopy (XPS) are carried out in-situ for MgO films, vacuum-cleaved MgO crystals, and water vapor exposed samples. In the case of thin films, we observe O 1s spectra with a significant shoulder feature at 2.3 eV higher binding energy (HBE) than the corresponding peak at 530.0 eV representing regular lattice oxygen. We evaluate this feature in terms of non-stoichiometric oxygen and formation of an oxygen-rich layer at the topmost surface of the MgO columns. In contrast, no HBE-features are detectable from clean single crystal MgO surfaces, while the hydroxyl O 1s band peaks at 531.6 eV. Under excitation with 266-nm-laser-pulses, known to be resonant with low-coordinated surface anions [3], we observe preferential depletion of defective oxygen-states (HBE signal) and temporary restoration of ideal surface stoichiometry. Furthermore, auxiliary signals are observed on several micrometer thick films, acting like satellites to major photoelectron-peaks (O 1s, Mg 2s, and Mg 2p) but shifted by approximately 4 eV towards lower kinetic energy. These features are depleted by UV-light exposure, pointing to the occurrence of surface-charge imbalance, accompanied by photon stimulated charge-transfer reactions. These results are in line with desorption experiments of neutrals, stimulated by laser excitation at 266 nm. According to the low-coordination nature of nanoporous MgO films, we find that the laser fluence required for oxygen desorption is much lower in comparison to the single-crystal MgO (100) surface. The detected neutral oxygen desorption occurs with a single photon power dependence from nanoporous MgO thin films. This contrasts to the two-photon power dependence observed from MgO single crystals. In fact, the single photon energy of the laser lies within the charge-transfer transition in surface ion pairs with three-coordinated anions (~ 4.7 eV), while a two-photon absorption regime easily lies within the four-coordinated anion charge transfer transition near 5.4 eV [3]. In summary, our XPS studies and laser desorption experiments indicate new surface-site-specific excitation regimes, which might eventually allow for site-specific manipulation of surface morphology.
2004. "Temperature-Dependent Yield of Frenkel-Pairs Generated by Valence Excitation in NaCl." Physical Review. B, Condensed Matter 69(15):155102-1-155102-7. Abstract The dynamics of formation of self-trapped excitons (STEs) and Frenkel-pair upon band-gap excitation have been studied in NaCl by femtosecond time-resolved spectroscopy at temperatures ranging from 6 to 297 K. The relative yields of Frenkel-pairs and triplet STEs are governed by the process of exciton relaxation that terminates within 10 ps of excitation at any temperature studied. Although the Frenkel-pair yield increases with temperature, there is no correlation between the formation time of the F center and the decay time of the STE. Therefore, the thermal coversion of the STEs into Frenkel-pairs is not the origin of the temperature-dependent Frenkel-pair yield in NaCl. Temperature-dependent branching during relaxation is shown to be the origin of the temperature effects.
2004. "Temperature-Dependent Yield of Frenkel Pairs Generated by Valence Excitation in NaCl." Physical Review. B, Condensed Matter 69:155102. Abstract The dynamics of the formation of self-trapped excitons (STEs) and Frenkel pairs upon band-gap excitation have been studied in NaC1 by femtosecond time-resolved spectroscopy at temperatures ranging from 6 to 330K. We found that the primary yield of Frenkel pairs was governed by the process of exciton relaxation that terminated within 10 ps of excitation for all temperatures studied. The yield increases with temperature, and is anticorrelated with the yield of triplet STEs, showing a strong effect of temperature on branching during exciton relaxation. We discuss the mechanism of the temperature-dependent branching of excitons based on the present results and knowledge accumulated from studies of other crystals.
2004. "Determination of Surface Exciton Energies by Velocity Resolved Atomic Desorption." Surface Science 564(1-3):62-70. Abstract We have developed a new method for determining surface exciton band energies in alkali halides based on velocity-resolved atomic desorption (VRAD). Using this new method, we predict the surface exciton energies for K1, KBr, KC1, and NaC1 within +0.15 eV. Our data, combined with the available EELS data for alkali fluorides, demonstrate a universal linear correlation with the inverse inter-atomic distance in these materials. The results suggest that surface excitons exist in all alkali halides and their excitation energies can be predicted from the known bulk exciton energies and the obtained correlation plot.
2004. "Laser-Induced Damage of Calcium Fluoride." Journal of Undergraduate Research Volume IV, 2004:60-65. Abstract Radiation damage of materials has long been of fundamental interest, especially since the growth of laser technology. One such source of damage comes from UV laser light. Laser systems continue to move into shorter wavelength ranges, but unfortunately are limited by the damage threshold of their optical components. For example, semiconductor lithography is making its way into the 157nm range and requires a material that can not only transmit this light (air cannot), but also withstand the highly energetic photons present at this shorter wavelength. CaF2, an alkaline earth halide, is the chosen material for vacuum UV 157 nm excimer radiation. It can transmit light down to 120 nm and is relatively inexpensive. Although it is readily available through natural and synthetic sources, it is often times difficult to find in high purity. Impurities in the crystal can result in occupied states in the band gap that induce photon absorption [2] and ultimately lead to the degradation of the material. In order to predict how well CaF2 will perform under irradiation of short wavelength laser light, one must understand the mechanisms for laser-induced damage. Laser damage is often a two-step process: initial photons create new defects in the lattice and subsequent photons excite these defects. When laser light is incident on a solid surface there is an initial production of electron-hole (e-h) pairs, a heating of free electrons and a generation of local heating around optically absorbing centers [3]. Once this initial excitation converts to the driving energy for nuclear motion, the result is an ejection of atoms, ions and molecules from the surface, known as desorption or ablation [3]. Secondary processes further driving desorption are photoabsorption, successive excitations of self-trapped excitons (STE’s) and defects, and ionization of neutrals by incident laser light [3]. The combination of laser-induced desorption and the alterations to the electronic and geometrical structure of the lattice result in defect formation. In the material CaF2 some of these defects take the form of F-centers, an electron trapped at a halogen vacancy [4], and H-centers, a F2- molecular ion at a single lattice site [5]. While the F-centers are stable, the H-centers are transient but can form into aggregates that are stable. There are many different configurations the defects can take based on the relative position of F and H centers in the lattice and this is extensively discussed in literature [1,4,5]. Once these defects have formed they cause further absorption of light, which ultimately induces particle emission and the production of even more defects. Various forms of laser-induced damage of CaF2 have been studied. For example, the mechanism for photon-stimulated desorption (PSD) of F+ from CaF2 (111) is discussed in ref. 6 and the energy threshold, distribution and kinetics governing electron-stimulated desorption (ESD) is investigated in ref. 7. The desorption of neutral Ca and F atoms has also been explored [1]. In this paper I focus on the emission of ions and neutrals from CaF2 under the irradiation of pulsed laser light at 266 nm, in addition to a brief study of its purity and transmittance.
2004. "Laser Control of Product Electronic State: Desorption from Alkali Halides." Journal of Chemical Physics 120(5):2456-2463. Abstract We demonstrate laser control of the electronic product state distribution of photodesorbed halogen atoms from alkali halide crystals. Our general model of surface exciton desorption dynamics is developed into a simple method for laser control of the relative halogen atom spin orbit laser desorption yield. By tuning the excitation laser photon energy in a narrow region of the absorption threshold, the relative C1(2P1/2) yield can be made to vary from near 0 to 80% from KCI and from near 0 to 60% for NaC1. We described the physical properties necessary to obtain a high degree of product state control and the limitation induced when these requirements are not met. These results demonstrate that laser control can be applied to solid state surface reactions and provide strong support for surface exciton-based desorption models.
2003. "Broad Distribution of Crystal Field Environments for Nd3+ in Calcite." Physics and Chemistry of Minerals 30(7):440 - 448. Abstract Calcite micro-crystals were grown from solution with single crystal dimensions up to 3 mm and doped up to ~0.1% with Nd3+ ions. Phase purity was verified by powder x-ray diffraction. The concentration of Nd3+ was measured by energy-dispersive spectrometry and Rutherford backscattering spectrometry. Micro x-ray fluorescence mapping of the calcite grains indicates uniform Nd distribution in as-grown crystal grains. X-ray absorption fine structure indicates that Nd3+ is substituted for Ca2+ with local lattice distortion. Temperature dependent near-infrared spectroscopy of Nd3+ impurities in calcite reveals large inhomogeneous line widths. These results suggest a broad distribution of crystal-field environments for Nd3+ as a result of charge compensation and inhomogeneous lattice strain.
2003. "The Role of O(1D) in the Oxidation of Si(100)." Journal of Vacuum Science and Technology B--Microelectronics and Nanometer Structures 21(2):895-899. Abstract Oxidation of silicon with neutral atomic oxygen species generated in a rare gas plasma has recently been shown to produce high-quality thin oxides. It has been speculated that atomic oxygen in the first excited state, O(1D), is a dominant reactive species in the oxidation mechanism. In this study, we investigate the role of O(1D) in silicon oxidation in the absence of other oxidizing species. The O(1D) is generated by laser-induced photodissociation of N2O at 193 nm. We find that, at 400?C, O(1D) is effective in the initial stages of oxidation, but the oxide growth rate falls dramatically past 1.5 nm. Oxide films thicker than 2 nm were unachievable regardless of oxidation time or N2O partial pressure (0.5-90 mTorr), indicating O(1D) cannot be a dominant reactive species in thicker oxidation mechanisms. We suggest that quenching of O(1D) to O(3P) (ground state) during diffusion through thicker oxides results in drastically slower oxidation kinetics. In contrast, oxidation with a vacuum ultraviolet (VUV) excimer lamp operating at 172 nm resulted in oxide thicknesses up to 4 nm. Thus, other species produced in plasmas and excimer lamps, such as molecular and atomic ions, photons, and free and conduction band electrons, play a dominant role in the rapid oxidation mechanism of thicker oxides (> 2 nm).
2003. "Surface Electronic Spectra Detected by Atomic Desorption." Surface Science 544(1):L683-L688. Abstract Using continuously tunable laser excitation of KI we measure the velocity profiles and the yield of desorbing hyperthermal iodine atoms as a function of photon energy. Based on the theoretical model of desorption we demonstrate that these spectra display a signature of a surface exciton and constitute a new sensitive method of surface specific desorption spectroscopy. Our results demonstrate that creation of surface excitions can be a much more general phenomenon than was previously thought based on extant spectroscopic measurements.
2003. "Photon stimulated desorption from KI: Laser control of I-atom velocity distributions." Surface Science 528 (1-3):219-223. Abstract Abstract: Irradiation of cleaved KI single crystals with photons near the bulk absorption threshold between 5.1 and 6.0 eV produces hyper-thermal I(2P3/2) emission with a minor thermal component, and with little spin orbit excited I(2P1/2) emission. The I-atom kinetic energy distribution may be tuned by choice of photon energy indicating that control of photon energy and pulse power can produce I-atoms in selectable quantities and velocity distributions.
2003. "Synergistic Effects of Exposure of Surfaces of Ionic Crystals to Radiation and Water." Applied Surface Science 208:2-14. Abstract We present studies of the consequences of simultaneous exposure of inorganic single crystals to radiation and water. The first case consists of a biomineral, CaHPO4-2H2O (brushite), which is a wide band gap, hydrated inorganic single crystal. We examine the laser-induced ion and neutral emissions accompanying 248 nm excimer laser radiation.
2002. "What do Matrix-Assisted Laser Desorption/Ionization Mass Spectra Reveal about Ionization Mechanisms?" Journal of Mass Spectrometry 37:639-647. Abstract We describe the results of experiments designed to test several of the most widely discussed mechanistic models for matrix-assisted laser desorption and ionization. By comparing ion mass spectra from the same matrix-analyte systems across various wavelengths from ultraviolet to mid-infrared and pulse durations from nanosecond to femtosecond, we have evaluated the plausibility of such ion-formation mechanisms as multiphoton ionization, excited state ionization, and photothermal ionization. We conclude that some of these models are not plausible for the matrix-analyte systems we studied. However, the fundamental principles of the laser-materials interaction also suggest that inspection of the mass spectra alone can only serve to exclude certain mechanisms, not to establish which of several competing mechanisms is actually occurring. This is particularly true with respect to variations in pulse duration and wavelength.
2002. "Nitric acid-water complexes: Theoretical calculations and comparison to experiment ." Journal of Physical Chemistry 106(33):7628-7635. Abstract The formation of HNO3?(H2O)n complexes in the gas phase and in thin films is studied both theoretically and experimentally. First principles electronic structure calculations were used to produce minimum structures and harmonic vibrational frequencies of HNO3?(H2O)n and DNO3?(H2O)n complexes (n=0-4). They also provide insight into the ionization of HNO3 in water predicting that ionization in isolated clusters occurs for n=4 or larger. Vibrational absorption spectra of matrix isolated nitric acid/water complexes were obtained using an FTIR spectrometer - based instrument. By incrementally increasing the amount of H2O in the matrix, we have been able to study nitric acid/water complexes and the ionization of HNO3. New spectral assignments, the first ones for the n=2 and 3 complexes are suggested based on the results of the electronic structure calculations.
2002. "Femtosecond Time-Resolved Photo-Stimulated Desorption from Ionic Crystals." Applied Surface Science 186(1-4):339-344. Abstract We have used the pump-probe technique to measure the positive ion yield, from ionic crystals, as a function of time-delay between two femtosecond laser pulses. The two-pulse technique allows direct observation of solid state and surface dynamics on a femtosecond timescale. We find the ion yield, from 265 nm irradiated MgO and KBr, depends critically on the time delay between pulses. For example, the Mg+ desorption yield displays three distinct features; a coherence peak followed by a fast rise and decay features. In contrast, the yield of K+ from KBr displays only the coherence peak and picosecond decay features. The observed thresholds suggest that, although the nanosecond laser ion desorption mechanism may be dominated by defect photoabsorption, significant electron-hole pair production may contribute to the femtosecond laser desorption mechanism. By determining the ultrafast time-dependence we hope to reveal the mechanism of the laser ion desorption for both regimes.
2002. "Solid-State Halogen Atom Source for Chemical Dynamics and Etching." Applied Physics Letters 81(6):1140-1142. Abstract We describe a solid state Br atom source for surface etching, kinetics, or reaction dynamics studies. Pulsed laser irradiation of crystalline KBr, near the bulk absorption threshold at 6eV, produces hyperthermal Br atmos in dense plumes. The Br atom density and velocities may be controlled by choice of laser pulse power and photon energy. Single and multiple pulse excitation of KBr produces Br and Br* in controllable quantities and velocities, thus providing an attractive UHV compatible solid-state radical atom source. The solid-state atom source is in principle extensible to other halogens using other alkali halides and perhaps other materials.
2002. "Control of Laser Desorption Using Tunable Single Pulses and Pulse Pairs." Journal of Chemical Physics 116(18):8144-8151. Abstract Abstract: We desorb ground state Br and spin-orbit excited Br* atoms from KBr single crystals using single pulses and sequential pulse pairs of tunable nanosecond laser radiation. Irradiation of cleaved KBr crystals, near the bulk absorption threshold produces hyper-thermal Br emission without a significant thermal component, and with little spin orbit excited Br* emission. The Br kinetic energy distribution may be controlled either by choice of photon energy or by excitation of transient defect centers created within the crystal. In this latter scheme, a first laser pulse generates transient centers within the bulk crystal and in the vicinity of the surface, and a second delayed laser pulse then excites the transient centers leading to atomic desorption. The Br* to Br yield ratio is significantly enhanced using two-pulse excitation as compared to resonant single-pulse desorption. Single and multiple pulse excitation of KBr produces Br and Br* in controllable quantities, velocities, and spin state distributions.
2002. ""EXAFS Study of Rare-Earth Element Coordination in Calcite"." Geochimica et Cosmochimica Acta 66(16):2875-2885. Abstract X-ray absorption fine-structure (XAFS) spectroscopy is used to characterize the local coordination of selected rare-earth elements (Nd3+, Sm3+, Dy3+, Yb3+) coprecipitated with calcite in minor concentrations from room-temperature aqueous solutions. Fitting results confirm substitution in the Ca site, but first-shell Nd-O and Sm-O distances are longer than the Ca-O distance in calcite, and longer than consistent with ionic radii sums for 6-fold coordination in the octahedral Ca site. In contrast, first-shell Dy-O and Yb-O distances are shorter than the Ca-O distance and consistent with ionic radii sums for 6-fold coordination. Comparison of Nd-O and Sm-O bond lengths with those in lanthanide sesquioxides and with ionic radii trends across the lanthanide series suggested that Nd3+ and Sm3+ impurities have 7-fold coordination in a modified Ca site in calcite. This would require some disruption of the local structure, with an expected decrease in stability and possibly a different charge compensation mechanism. A possible explanation for the increased coordination for the larger rare earth elements involves bidenate ligation from a CO3 group. Because trivalent actinides such as Am3+ and Cm3+ have ionic radii similar to Nd3+, their incorporation in calcite may result in similar defect structure.
2001. "Coprecipitation of Uranium (VI) with Calcite: XAFS, Micro-XAS,and Luminescence Characterization ." Geochimica et Cosmochimica Acta 65(20):3491-3503. Abstract X-ray absorption and luminescence spectroscopies have been used to characterize the local structure and coordination of uranium (VI) species coprecipitated with calcite (CaCO3) in room-temperature aqueous solutions. Different solution chemistries and pHs are found to result in defferences in the equatorial coordination of the uranyl species (UO2/2+) in the calcite, with multiple coordination environments of uranyl evident in one sample.
2001. "Ultraviolet Photochemistry of Hydrogen-Bonded HBr-Acetone Complexes in Argon Matrices." Journal of Chemical Physics 114(1):169-178. Abstract We have studied the effects of UV radiation on HBr-acetone Ar Matrices. Results indicate photoproducts formed from the irradiation of these matrices depend on the wavelength of light used. Irradiation at 266 and 309 nm results only in the loss of the H-bonded HBr-acetone complex and the photo-production of an HBr-1-propen-2-ol complex. Ab initio calculations of HBr-1-propen-2-ol complexes support this finding. Irradiation at 193 nm shows a high degree of fragmentation of all molecules as indicated by CH4, CO, and CO2 product peaks in the infrared spectrum and the loss of all peaks associated with HBr, acetone and their complex. In contrast, irradiation at 355 nm shows no change in the infrared spectra even after prolonged irradiation. For 266 and 309 nm light, a minor photoreaction channel to a non-H-bonded HBr-acetone complex is also noted.
2001. "Selective laser desorption of ionic surfaces: resonant surface excitation of KBr." Journal of Chemical Physics 115(20):9463 - 9472. Abstract We demonstrate evidence of selective laser-induced desorption of ground state Br(2P3/2) and spin-orbit excited state Br*(P1/2) atoms from single crystals at 6.4 eV. Laser excitation tuned selectively to a surface resonance below the first bulk absorption band excites surfaces states preferentially leading to surface specific reaction while inducing relatively insignificant bulk reaction. The experimental results are supported by embedded cluster ab initio calculations that indicate a reduced surface exciton energy compared to that of the bulk exciton with a slight further reduction for steps and kink sites. Low fluence irradiation of cleaved KBr crystals, near the calculated surface exciton energy of 6.2 eV, produces hyper-thermal Br(2P3/2) emissions without a significant thermal or Br*(2P1/2) component. The hyper-thermal emission is shown theoretically to be characteristic of surface induced reaction of exciton decomposition while thermal Br emission is attributed to bulk photoreaction.
2001. "Analysis of Hanford-Related Organics Using Matrix-Assisted Laser Desorption Ionization Time-Of-Flight Mass Spectrometry ." Journal of Radioanalytical and Nuclear Chemistry 250(2):247-253. Abstract Matrix-assisted laser desorption ionizaton coupled with time-of-flight mass spectrometry (MALDI/TOFMS) was used for the analysis of low-molecular phosphate compounds found in Hanford tank wastes. The mass spectra of these compounds indicate protonated ions as well as sodium adducts. Analytical methods presently utilized for the analysis of the phosphate-related organics are both time consuming and labor intensive. The MALDI process produces both positive and negative ions directly and very little sample is required. In addition, there is limited sample preparation and minimal hazardous waste production.
2001. "Analysis of Hanford-related Organics using Matrix-assisted Laser Desorption Ionization Time-of-flight Mass Spectrometry." Journal of Radioanalytical and Nuclear Chemistry 250(2):247-253. Abstract Matrix-assisted laser desorption/ionization coupled with time-of-flight mass spectrometry (MALDI/TOF-MS) was used for the analysis of low-molecular phosphate compounds found in Hanford tank wastes. The mass spectra of these compounds indicate protonated peaks as well as sodium adducts. Analytical methods presently utilized for the analysis of the phosphate-related organics are both time consuming and labor intensive. A promising alternative is MALDI/TOFMS. The MALDI process produces both positive and negative ions directly and very little sample is required. In addition,there is limited sample preparation and minimal hazardous waste production.
2001. "Evidence for a Surface Exciton in KBr via Laser Desorption ." Physical Review. B, Condensed Matter 63(12):125423(6). Abstract We demonstrate that direct photoexcitation of the KBr surface exciton leads to desorption of hyperthermal neutral bromine atoms. We have for the first time produced separately the hyperthermal and the near-thermal components of neutral halogen emission from an alkali halide. The source of hyperthermal bromine emission is attributed to decay of a surface exciton excited at photon energies below that of the bulk exciton. We further demonstrate that the frequently observed near-thermal component is derived from excitation within the bulk crystal. Our experimental data provides strong support to a theoretical emission model previously described in the literature.
2000. "Time-Resolved Femtosecond Laser-Induced Desorption from Magnesium Oxide and Lithium Fluoride Single Crystals." Surface Science 451(1-3):166-173. Abstract We have used the pump-probe technique to measure the photostimulated positive ion yield, as a function of time-delay between two sub-threshold femtosecond laser pulses. We find the ion yield, from 265 nm femtosecond irradiated MgO and LiF, depends critically on the laser pulse delay two-pulse experiments. For example, single pulse excitation of MgO produces a variety of ions including Mg+, MgO+, and H+. If the femtosecond laser pulse is split into two sub-threshold beams and then re-combined with a variable time delay, the Mg+ desorption yield displays three distinct lifetimes and persists for laser delays of over 100 picoseconds. A pulse delay of only 500 femtoseconds nearly eliminates ion desorption except for Mg+. In contrast, for LiF the majority of Li+ yields decays rapidly, largely within the femtosecond pulse duration. However, a weak but measurable decay component of approximately 2 picoseconds is indicated. We hypothesize the nonresonant two-photon excitation contributes to the ultrfast desorption mechanism through the creation of electron/hole pairs and allows direct observation of the surface electron/hole pair trapping dynamics and measurment of the surface free carrier lifetime.
2000. "Time-resolved Femtosecond Laser Induced Desorption from Magnesium Oxide and Lithium Fluoride Single Crystals ." Surface Science 451:166-173.
1999. "Photochemistry of Matrix-Isolated and Thin Film Acid Chlorides: Quantum Yields and Product Structures." Journal of Physical Chemistry A 103(7):965-970. Abstract ABSTRACT: Ultraviolet photoexcitation of matrix isolated acetyl, propionyl and valeryl chlorides at produces HCl-ketene, HCl-methyl-detene, and HCl-propyl-ketene complexes. We report precursor and matrix dependent reaction quantum yeilds. Quantum yield values decrease with increasing alkyl chain length due to a reduced number of (scientific) H-atoms available for the elimination reaction and steric hindrance. We measure quantum yields in neat matrices to be roughly half that in argon or xenon matrices and assign structures for HCl and ketene complexes in argon and xenon matrices by comparing IR spectra to ab-initio results...
1999. "Ultrafast and Nanosecond Laser Induced Desorption of Positive Ions from Lithium Fluoride Single Crystals." Applied Physics A, Materials Science and Processing 69:S153-S157. Abstract We compare desorption of positive ions from litium floride (LiF) single crystals following pulsed laser excitation using eith femtosecond (~300fs, 265 nm) or nanosecond (3ns, 266 nm) sources.
1999. "Femtosecond Time-Resolved Laser Induced Desorption of Positive Ions from MgO." Applied Physics A, Materials Science and Processing 69:S389-S393. Abstract We have used the pump-probe technique to measure the photostimulated positive ion yield, as a function of time-delay between two sub-threshold femtosecond laser pulses. We find the ion yield, from UV femtosecond irratdiated MgO, depends critically on the laser pulse delay, delta-t, in two-pulse experiments. n single pulse experiments, excitation of MgO produces a variety of ions including Mg+, MgO+, and a significant yield of H+. In contrast, if the femtosecond laser pulse is split into two sub-threshold beams and then re-combined with a variable time delay, the ion yield may be drastically may be drastically altered depending on the delay between pulses. The Mg+ desorption yield displays three distinct liftimes and persists for laser delays of over 100 picosecond.
1999. "Laser-induced positive ion and neutral atom/molecule emissions from single-crystal CaHPO4 center dot 2H20: The role of electron-beam-induced defects." Applied Physics A, Materials Science and Processing 69:S547-S552. Abstract We examine laser-induced ion and neutral emissions from single-crystal CaHPO4 center dot 2H2O (brushite), a wide-band-gap, hydrated inorganic single crystal, with 248-nm excimer laser radiation. Both laser-induced ion and neutral emissions are several orders magnitude higher following exposure to 2keV electrons at current densities of 200 uA/cm2 and doses of 1 C/cm2. In addition to intense Ca+ signals, electron-irradiated surfaces yield substantial CaO+, PO+, and P+ signals. As-grown and as-cleaved brushite show only weak neutral O2 and Ca emissions, whereas electron-irradiated surfaces yield enhanced O2, Ca, PO, PO2, and P emissions. Electron irradiation (i) significantly heats the sample, leading to thermal dehydration (CaHPO4 formation) and pyrolysis (Ca2P2O7 formation)and (ii) chemically reduces the surface via electron stimulated desorption. The thermal effects are accompanied by morphological changes, including recrystallization. Although complex, these changes lead to high defect densities, which are responsible for the dramatic enhancements in the observed laser desorption.
1998. "Quantum-State Resolved Products Via Vacuum Ultraviolet Photostimulated Desorption From Geologic Calcite." Applied Surface Science 127-129:21-25. Abstract We report the results of a photostimulated desorption (PSD) study of neutral CO products from room temperature geologic calcite utilizing fluences <300 (scientific equation...won't format) excimer laser radiation.