Direct valence excitation

Irradiation of adsorbate-covered surfaces with higher energy photons (typically up to 6.4 eV) with lower intensities opens the possibility of direct valence excitation. Since the lifetimes of electronic excitations at metal surfaces are much shorter than those for nuclear motion, photochemical reactions appear rather improbable. Surprisingly, however, the cross sections determined for photodesorption were found to be comparable to those found for reactions with free molecules, mainly because the short lifetime of the excited state is compensated by a much larger cross section for absorption of the light [32,62-64]. This process takes place in the near-surface region of the metal (within about 10 nm), where relaxation of the photoexcited electrons leads to rapid establishment of a transient energy distribution. As depicted in Fig. 4.11, these hot electrons may scatter at the surface or are resonantly attached to an empty level of the adsorbate. 

Due to the simplicity and the ability to explain the spectroscopic and excited state properties, the MO theory in addition to easy adaptability for modern computers has gained tremendous popularity among chemists. The concept of directed valence, based on the principle of maximum overlap and valence shell electron pair repulsion theory (VSEPR), has successfully explained the molecular geometries and bonding in polyatomic molecules. 

The low threshold energies for the production of D( S), 0( P), and 0( D2) show the importance of valence excited states in the BSD of neutral fragments [47]. The pathway for D( S) desorption probably involves D O D -I- OD. Ffowever, the thresholds for producing 0( P2) and 0( D2), which are the same within experimental error, are lower than the 9.5-and 11.5-eV thermodynamic energies required to produce 0( P2) + 2D( S) and 0( D2) + 2D( S), respectively. The low threshold values therefore indicate that the formation of 0( P2) and 0( D2) must occur by a pathway which involves simultaneous formation of D2. Kimmel et al. have in fact reported [46] a threshold for the production of D2 from D2O ice at — 6 to 7 eV, which supports this conclusion. Above the ionization threshold of amorphous ice, these excited states can be formed directly or via electron-ion recombination. 

D. M. Neumark I would like to make a comment to Prof. Schlag. One expects an anion ZEKE spectrum to have the same overall intensity profile as the corresponding photoelectron spectrum only if direct detachment is the only process that occurs. However, FeO has several dipole-bound and valence-excited states near the detachment threshold. 

Results of photoemission studies of polyethylene have shown definite evidence for wide energy bands among deep valence orbitals ( ), but the nature of the fundamental absorption edge has not been resolved. Band structure calculations predict direct interband excitations to occur above 12.6 eV (.8) whereas the absorption threshold is at 7.2 eV and a strong peak in e occurs at 9.0 eV. The momentum dependence of the absorption threshold indicates that the threshold is of excitonic origin, i.e. the excitation is localized by the strong electron-hole or configuration inter-... 

In the radiolysis by f3- or y-radiation with maximum energy of about 1 Mev. the most typical process (see Table II) is the electronic transitions of valence electrons of molecules. Other processes like direct vibrational excitations of the ground state or electron capture by molecules are characteristic only for the subexcitation electrons with kinetic energy below the threshold E0 of electronic excitations. By the interactions of subexcitation electrons a fraction of about 10 to 15% of the total absorbed energy is dissipated. This fraction may partially be utilized for chemical changes however, the corresponding yield should always be considered separately since the mechanisms are much more intricate here and depend strongly on the nature of the medium. 

Figure 8a illustrates two processes that lead to a decrease of the negative surface charge (1) direct optical excitation of trapped electrons into the conduction band and (2) capture of photoexcited holes from the valence band (Goldstein and Szostak, 1980). At high light intensities this produces a neutral surface and AFjj = V,. However, depending on the relative capture... 

In this chapter we aim to diseuss the requirements, challenges, and pitfalls associated with attempting to theoretically model molecular properties for such systems that can be directly compared to experimental data, such as valence excitation spectra, core-excitation spectra, thermodynamics of chemical reactions, and redox properties. 

A DIET process involves tliree steps (1) an initial electronic excitation, (2) an electronic rearrangement to fonn a repulsive state and (3) emission of a particle from the surface. The first step can be a direct excitation to an antibondmg state, but more frequently it is simply the removal of a bound electron. In the second step, the surface electronic structure rearranges itself to fonn a repulsive state. This rearrangement could be, for example, the decay of a valence band electron to fill a hole created in step (1). The repulsive state must have a sufficiently long lifetime that the products can desorb from the surface before the state decays. Finally, during the emission step, the particle can interact with the surface in ways that perturb its trajectory. 

If the work function is smaller than the ionization potential of metastable state (see. Fig. 5.18b), then the process of resonance ionization becomes impossible and the major way of de-excitation is a direct Auger-deactivation process similar to the Penning Effect ionization a valence electron of metal moves to an unoccupied orbital of the atom ground state, and the excited electron from a higher orbital of the atom is ejected into the gaseous phase. The energy spectrum of secondary electrons is characterized by a marked maximum corresponding to the... 

Electric current is conducted either by these excited electrons in the conduction band or by holes remaining in place of excited electrons in the original valence energy band. These holes have a positive effective charge. If an electron from a neighbouring atom jumps over into a free site (hole), then this process is equivalent to movement of the hole in the opposite direction. In the valence band, the electric current is thus conducted by these positive charge carriers. Semiconductors are divided into intrinsic semiconductors, where electrons are thermally excited to the conduction band, and semiconductors with intentionally introduced impurities, called doped semiconductors, where the traces of impurities account for most of the conductivity. 

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