Scientific Publications 2007
2007. "Assignment of Rovibrational Transitions of Propyne in the Regionof 2934–2952 cm⁻1 Measured by Two-color IR–vacuum Ultravioletlaser photoion-photoelectron methods." Journal of Chemical Physics 127:044313 1-5. doi:10.1063/1.2748403 Abstract The research described in this product was performed in part in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. The infrared (IR) spectrum of propyne in the region of 2934–2952 cm−1 has been recorded by the IR–vacuum ultraviolet (VUV)–photoion method. The spectrum is shown to consist of two near-resonant, but noncoupled vibrational bands: the Ʋ2 symmetric methyl C–H stretching vibrational band and a combination vibrational band Ʋcs. The previously unobserved Q line of the Ʋcs band is observed. The rotational transition lines of the Ʋ₂=1 band produces IR-VUV–pulsed field ionization–photoelectron (IR-VUV-PFI-PE) signal at the C₃H₄ + (Ʋ₂⁺=1) photoionization threshold. The rotational transition lines associated with the Ʋcs band do not produce IR-VUV-PFI-PE signal. Rotational transition lines of both vibrational bands are assigned and simulated; and ab initio calculations further confirm the assignment.
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. "Kinetic Monte Carlo simulations of Pd deposition and islandgrowth on MgO(100)." Surface Science 601(14):3133-3142. doi:10.1016/j.susc.2007.05.027 Abstract The research described in this product was performed in part in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. The deposition and ripening of Pd atoms on the MgO(10 0) surface are modeled using kinetic Monte Carlo simulations. The density of Pd islands is obtained by simulating the deposition of 0.1 ML in 3 min. Two sets of kinetic parameters are tested and compared with experiment over a 200–800 K temperature range. One model is based upon parameters obtained by fitting rate equations to experimental data and assuming the Pd monomer is the only diffusing species. The other is based upon transition rates obtained from density functional theory calculations which show that small Pd clusters are also mobile. In both models, oxygen vacancy defects on the MgO surface provide strong traps for Pd monomers and serve as nucleation sites for islands. Kinetic Monte Carlo simulations show that both models reproduce the experimentally observed island density versus temperature, despite large differences in the energetics and different diffusion mechanisms. The low temperature Pd island formation at defects is attributed to fast monomer diffusion to defects in the rate-equation-based model, whereas in the DFT-based model, small clusters form already on terraces and diffuse to defects. In the DFT-based model, the strong dimer and trimer binding energies at charged oxygen vacancy defects prevent island ripening below the experimentally observed onset temperature of 600 K.