Krishna G. Nath

Chemical-state analysis for low-dimensional Si and Ge films on graphite

Synchrotron radiation photoemission spectroscopy has been used to study the chemical interaction and the electronic structures of low-dimensional semiconducting Si and Ge on a graphite surface. The core level spectra, such as Si 1s, Ge 2p, and C 1s, show that no chemical interaction occurs between adatoms and the substrate atoms, and that the electronic structures of graphite remain intact upon deposition of the adatoms at room temperature. The Si 1s and Ge 2p photoemission spectra reveal that Si and Ge films on graphite remain in their elemental form. However, the electronic structures of both Si and Ge films are found to be thickness dependent. In the thicker films, such as Si 5.5 A or Ge 4.2 A, a single bulk-like component is detected. In the case of thinner films, such as Si 2.7 A or Ge 0.3 A, some additional components are observed at binding energies higher than the bulk-like component. We attribute these peaks to semiconductor nanostructures, for example, nanowires, and individual or polymerized ch...

Fragmentation in biological molecules by the irradiation of monochromatized soft X-rays

Abstract High-flux photons from FEL can be used not only for the structural analysis but also for the materials modification. In advance of future application of short wavelength light from FEL, we present the results for photo-fragmentation in biological molecules by monochromatized soft X-ray irradiation using synchrotron radiation. For sulfur-containing amino acids, the core-to-valence resonant excitation at the sulfur AT-edge induced the desorption of only S + ions, which is quite different from the fragmentation patterns by the valence excitation. On the basis of the Auger decay spectra, it is revealed that the resonance excitation from the S ls to σ * states is followed by the “spectator-type” Auger decay where the excited electrons remain in the σ * orbital in the course of Auger decay. We conclude that the specific fragmentation is caused by the fast bond scission due to the highly antibonding character of the σ * orbital. PACS codes: 32.80. Hd, 68.43. Tj, 82.S0. Kx

Fragmentation pathways caused by soft X-ray irradiation: The detection of desorption products using a rotatable time-of-flight mass-spectrometer combined with pulsed synchrotron radiation

Abstract High flux and pulsed photons from FEL are very fascinating for the study of photochemistry. In advance of future application of X-ray-FEL, we have investigated the mechanism of the bond-selective fragmentation occurring at the surface of fluorinated graphite following F Is core-excitation using time-of-flight mass-spectroscopy (TOF-MS) combined with pulsed synchrotron radiation. Excitation-energy dependence of F+ fragment yields revealed that C-F bonds are selectively broken by F Is resonant excitation. Furthermore, translation-energy-resolved excitation spectra of F+-yields were measured using TOF technique. Slow fragments were found to be involved in the specific dissociation. The result was explained assuming that Auger decay followed by F Is → O∗C-F resonance leaves a long-lived excited electron at O∗C-F orbital in precursor states, which should be highly antibonding and promote breaking C-F bonds. PACScodes: 32.80.Hd, 68.43.Tj, 82.50.Kx