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.
2008. "Laser and Electrical Current Induced Phase Transformation of In2Se3 Semiconductor thin film on Si(111) ." Applied Physics A, Materials Science and Processing 93(1):93-98. doi:10.1007/s00339-008-4776-8 Abstract Phase transformation of thin film (~30 nm) In2Se3/Si(111) (amorphous crystalline) was performed by resistive annealing and the reverse transformation (crystalline amorphous) was performed by nanosecond laser annealing. As an intrinsic-vacancy, binary chalcogenide semiconductor, In2Se3 is of interest for non-volatile phase-change memory. Amorphous InxSey was deposited at room temperature on Si(111) after pre-deposition of a crystalline In2Se3 buffer layer (6.4 Å). Upon resistive annealing to 380°C, the film was transformed into a y-In2Se3 single crystal with its {0001} planes parallel to the Si (111) substrate and parallel to Si , as evidenced by scanning tunneling microscopy, low energy electron diffraction, and x-ray diffraction. Laser annealing with 20 nanosecond pulses (0.1 milliJoules/pulse) re-amorphized the region exposed to the laser beam, as observed with photoemission electron microscopy (PEEM). The amorphous phase in PEEM appears dark, likely due to abundant defect levels inhibiting electron emission from the amorphous InxSey film.
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. "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. "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. "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. "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. "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. "SubNanosecondTime Resolved XAFS of Laser Excited Thin Ge Films." Physica Scripta T115:1044. Abstract A facility at PNCCAT in the Advanced Photon Source measures with subnanosecond time resolution both XAFS and diffraction on femtosecond laserexcited samples. XAFS measures with relatively high efficiency the time for the laser excitation to couple to the lattice, the sample temperature after reaching thermal equilibrium, any ablation of the sample with time, and, in many cases can distinguish between the amorphous and crystalline states. Preliminary measurements on 200nm thick polycrystal Ge films indicate that the time for transferring the laser excitation to thermal heating of the lattice is less than 2nanoseconds when the initial temperature is 560K.
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. "Surface-Induced Dissociation of Ions Produced by Matrix-Assisted Laser Desorption Ionization in a Fourier Transform Ion Cyclotron Resonance Mass Spectrometer." Analytical Chemistry 76(2):351-356. Abstract Intermediate pressure matrix assisted laser ionizaton (MALDI) source was constructed and interfaced with a 6T Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS) specially configured for surface-induced dissociation (SID) studies.
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 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 Pacific Northwest Consortium (PNC) CAT in Sector 20: High-Repetition-Rate Laser System for Performing Time-Resolved XAFS." Synchrotron Radiation News 16(4):30-31. Abstract XAFS (X-ray absorption fine structure) is a valuable probe of material dynamics because with it one can determine the behavior of a sample directly without dependence on long-range order, in contrast to X-ray diffraction. Dynamics of heating and phase changes induced by laser excitation may therefore be directly monitored by XAFS regardless of material state. In addition, the near-edge portion of the XAFS (the XANES—X-ray absorption near-edge spectroscopy) contains electronic information about the material and is, for example, sensitive to the shielding between the core hole and photoelectron, which can be modified by the laser pulse in a semiconductor, since the excited carrier density can be initially similar to that of a metal. The XANES technique therefore has the potential to study dynamics of this type of phenomenon, regardless of material state.
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.
2002. "Preparation of Pt/TiO2 Nancomposite Films by 2-Beam Pulsed Laser Deposition." Applied Surface Science 197-198:619-623. Abstract Pt/TiO2 nanocomposite films were prepared using 2-beam pulsed laser deposition (2B-PLD) technique, where sequential irradiation of ArF excimer laser for ionization after Nd:YAG laser for ablation of the target was utilized. The effect of the time delay of the ArF laser after the ablation laser irradiation on the structures of Pt/TiO2 nanocomposite films was studied. It was inferred from X-Ray photoelectron spectroscopy (XPS) of the as-deposited films that the coordination structures around Ti atoms in the films were affected by the time delays. Single phase TiO2 in anatase form was obtained in the annealed Pt/TiO2 nancomposite films prepared at 200-250 ns of the time delays, which suggests that the effective photoionization of the ablated species could take place by the ArF laser irradiation in this time range and affect the structure of the deposits.
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. "Preparation of Pt/TiO2 Nanocomposite Thin Films by Pulsed Laser Deposition and their Photoelectrochemical Behaviors." Journal of Photochemistry and Photobiology. A, Chemistry 145(1-2 ):11-16. Abstract Pt/TiO2 nanocomposite films were prepared from the sintered mixture targets of Pt and TiO2 by pulsed laser deposition. Pt/Ti atomic ratios in the deposited films are more strongly dependent on the initial content of Pt in the target than on the laser fluence. It is inferred from transmission electron microscope, X-ray photoelectron spectroscopy and X-ray diffraction analyses that as-deposited Pt/TiO2 nanocomposite films are composed of metallic Pt nanoparticles with a diameter of about 30 nm and an amorphous TiO2 matrix, which are crystallized into the dominant crystal structure of the anatase pahse after the heating at 600?C. the optical bandgap of Pt/TiO2 nanocomposite films was less than that of the pure TiO2 film and photoluminescence emission was observed between 680 and 800 nm at 24 K. some energy levels can be formed by the interface between Pt nanoparticles and TiO2, which also affect the photoelectrochemical properties of Pt/TiO2 nancomposite electrodes. The anodic photocurrents at 1.0 V of Pt/TiO2 nanocomposite electrodes were observed in the visible light range.
2001. ""Preparation of Pt/TiO2 Nanocomposite Thin Films by Pulsed Laser Deposition and Their Photoelectrochemical Behaviors"." Journal of Photochemistry and Photobiology. A, Chemistry 145(1-2):11-16. Abstract Pt/TiO2 nanocomposite films were prepared from the sintered mixture targets of Pt and TiO2 by pulsed laser deposition. Pt/Ti atomic ratios in the deposited films are more strongly dependent on the initial content of Pt in the target than on the laser fluence. It is inferred from transmission electron microscope, X-ray photoelectron spectroscopy and X-ray diffraction analyses that as-deposited Pt/TiO2 nanocomposite films are composed of metallic Pt nanoparticles with a diameter of about 30nm and an amorphous TiO2 matrix, which are crystallized into the dominant crystal structure of the anatase phase after the heating at 600?C. The optical bandgap of Pt/TiO2 nanocomposite films was less than that of the pure TiO2 film and photoluminescence emission was observed between 680 and 800nm at 24K. Some energy levels can be formed by the interface between Pt nanoparticles and TiO2, which also affect the photoelectrochemical properties of Pt/TiO2 nanocomposite electrodes. The anodic photocurrents at 1.0V of Pt/TiO2 nanocomposite electrodes were observed in the visible light range.
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. "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. "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. "Comparison of Pt/TiO2 nanocomposite films prepared by sputtering and pulsed laser deposition." Chemical Physics Letters 301(1999):336-342. Abstract Pt/O2 nanocomposite films were prepared by sputtering and pulsed laser deposition (PLD). The Pt/Ti atomic ratios in the films deposited by the PLD were smaller than those in the ablation targets and those in the films prepared by sputtering. At low Pt concentration, Pt is deposited as PtO2 in the films sputtering, whereas metallic Pt is deposited in the films prepared by PLD. The Pt nanoparticle size in the as-deposited Pt/TiO2 nanocomposite films prepared by PLD was about 30 nm, which is quite large compared to the 1-2 nm size in the sputter-deposited films. Pt nanoparticles were produced in the growth processes via surface diffusion, which can be reflected by the kinetic energy of the chemical species spread from the target. It can be inferred from the opitical measurements of heated Pt/O2 nanocomposite films tha new energy levels are produced in the band gap of TiO2 by the homogeneous dispersion of Pt nanoparticles in the TiO2 matrix.
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. "Characterization of Nanocomposite Materials Prepared via Laser Ablation of Pt/TiO2 Bi-combinant Targets." Chemical Physics Letters 301(3-4):336-342. Abstract Pt/TiO2 thin film nanocomposites have been prepared using a novel process of bicombinant target PLD. Pt nanoparticles may have been produced in a post-deposition process of rapid surface diffusion.
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.