Translational-energy-resolved studies of photogenerated carrier-induced reactions on UHV semiconductor surfaces
This year we have observed and understood the central phenomena of adsorbate photochemistry of semiconductor-adsorbate interfaces. In particular, it was observed that a major channel of photochemistry on a semiconductor surface is through the transfer of photoexcited substrate electrons to the adsorbed molecules. In contrast, photochemical processes by direct intra-adsorbate excitation are largely quenched for molecules in direct contact with the surface. The electron-transfer mediated process leads to both desorption and intra-adsorbate bond cleavage. The transfer may be by hot electrons or thermalized electrons in the conduction band. Dynamics of the electron-transfer reaction vary greatly for molecules in the first and second layer and for molecules of different orientations in the first layer. While molecules in the second layer can be dissociated only through the transfer of hot electrons or direct photon bond cleavage, those in the first layer react by thermalized electrons as well as hot electrons, depending on the orientation of the molecules. The two processes are differentiated experimentally by the different characteristic energy and angular distribution of the photofragments. In addition, we have discovered that reaction self-quenching can be an important process in thermalized electron-induced reactions on semiconductor surfaces. This behavior stems from effects on lifetime of the thermalized carriers by reaction products left on the surface. Finally, importance of electron-transfer-induced reactions at surfaces arises in part from shift in lowering of unoccupied electronic orbitals of adsorbates in image region of semiconductor surfaces. This latter effect is also largely responsible for a red shift observed for photochemical processes on surfaces.
- Research Organization:
- Columbia Univ., New York, NY (United States)
- Sponsoring Organization:
- USDOE, Washington, DC (United States)
- DOE Contract Number:
- FG02-90ER14104
- OSTI ID:
- 10107822
- Report Number(s):
- DOE/ER/14104-4; ON: DE94004169; BR: KC0202020/KC0301010
- Resource Relation:
- Other Information: PBD: [1993]
- Country of Publication:
- United States
- Language:
- English
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