Excited states multi-electron processes

Excitation to the repulsive electronic state may also involve a multi-electron process. For example, creation of a core hole on a metal atom in an oxide may lead to an interatomic Auger transition which ultimately results in a positive oxygen ion which desorbs because it is now in a strongly repulsive Madelung well. Knotek and Feibelman have reported results which they interpret in this manner. Core ionization in the adsorbed molecule can also lead to an Auger process which leads to desorption. 

These potentials theoretically allow water photolysis. However, multi-electron processes have to occur at the catalyst in order to photolyze water with this complex. The lifetime of the excited state is 650 ns, and the excited state is quenched efficiently through electron transfer with redox reagents. The conversion model with this complex is described in Chapter 4. 

Higher-order processes contribute in both the excitation of the core electron and relaxation of the core hole. These multi-electron processes are significant because of the strong perturbation caused by the creation or the annihilation of a core hole. In these processes, additional electrons are excited (shake-up) or ionized (shake-off). Two electron processes, double autoionization and double Auger decay, were mentioned above. The final states reached in core hole decay may be excited states and also may autoionize. It is clear that excitation of a core electron and the relaxation of the core hole provide many paths leading to multiple-electron excited states. These states have a unique chemistry relative to the single-electron excited states produced by arc lamp, laser, or vacuum ultraviolet (VUV) excitation. 

XANES spectra contain information about the electronic states of a specific target (X-ray absorbing) atom and the local structure surrounding it. The near-edge structure is associated with the excitation process of a core electron to bound and quasi-bound states. Because of the multi-electron process of the excitation, the theoretical assignment of XANES spectra has several difficulties. 

It is often supposed that this final simple decomposition is the end result of a multi-stage process involving photo-ejection of an electron to give radical cations, which then recombine with the electrons to give excited molecules which undergo homolysis rather than dropping to the ground state. 

In Fig. 13.13, the CR rates in the Marcus inverted region are much slower than the CS rates from both the singlet and triplet excited states in the Marcus normal region. This allows a subsequent electron transfer from an additional electron donor such as ferrocene (Fc) to ZnP+ in the triad molecule (Fc-ZnP+ -C60 ) to produce the final CS state, Fc+-ZnP-C60 , in competition with the back electron transfer in the initial CS states [41]. Such multi-step electron-transfer processes are expanded to the tetrad molecule (Fc-ZnP-H2P-C60) as shown in Fig. 13.16a [50], In the final CS state, Fc+-ZnP-H2P-C60 , charges are separated at... 

Until recently organic photochemistry has only partially focused on stereoselective synthesis, one of the major challenges and research areas in modern organic synthesis. This situation has dramatically changed in the last decade and highly chemo-, regio-, diastereo- as well as enantioselective reactions have been developed. Chemists all over the world became aware of the fascinating synthetic opportunities of electronically excited molecules and definitely this will lead to a new period of prosperity. Photochemical reactions can be performed at low temperatures, in the solid or liquid state or under gas-phase conditions, with spin-selective direct excitation or sensitization, and even multi-photon processes start to enter the synthetic scenery. 

When desorption takes place from a metal surface, many hot charge carriers are generated in the substrate by laser irradiation and are extended over the substrate. Then, the desorption occurs through substrate-mediated excitation. In the case of semiconductor surfaces, the excitation occurs in the substrate because of the narrow band gap. However, the desorption is caused by a local excitation, since the chemisorption bond is made of a localized electron of a substrate surface atom. When the substrate is an oxide, on the other hand, little or no substrate electronic-excitation occurs due to the wide band gap and the excitation relevant to the desorption is local. Thus, the desorption mechanism for adsorbed molecules is quite different at metal and oxide surfaces. Furthermore, the multi-dimensional potential energy surface (PES) of the electronic excited state in the adsorbed system has been obtained theoretically on oxide surfaces [19, 20] due to a localized system, but has scarcely been calculated on metal surfaces [21, 22] because of the delocalized and extended nature of the system. We describe desorption processes undergoing a single excitation for NO and CO desorption from both metal and oxide surfaces. 

The ab initio calculation of NLO properties has been a topic of research for about three decades. In particular, response theory has been used in combination with a number of electronic structure methods to derive so-called response functions [41 8], The latter describe the response of a molecular system for the specific perturbation operators and associated frequencies that characterize a particular experiment. For example, molecular hyperpolarizabilities can be calculated from the quadratic and cubic response functions using electric dipole operators. From the frequency-dependent response functions one can also determine expressions for various transition properties (e.g. for multi-photon absorption processes) and properties of excited states [42]. 

Of first priority is the proof or disproof of the conjecture about the spin density. It is hoped in the not-to-distant future, several significant systems of multi-electron atoms in the ground and excited states, through polyatomic molecules will be calculated and used as evidence of the efficacy of this method. Eventually, a fully relativistically covariant model will be developed. Beside philosophical interest, and fundamental scientific interest, there is potential commercial interest (Eberhart 1994)24. The process... 

It should be realized that a photochemically induced reaction may have a multi-step mechanism in which, perhaps, only one step may involve light absorption. For example, an excited molecule may transfer an electron to some acceptor molecule in its ground state to produce two odd-electron species. Both these free radicals may then take part in subsequent dark reactions. Although it is often stated that photochemistry is relatively insensitive to temperature, this is strictly only correct for the initial, light absorbing step and the rapid internal rearrangements of the excited state. Subsequent processes may be very susceptible to temperature effects. 

The very nature of the photochemical processes the multi-state multiconfigurational character and the occurrence of non-adiabatic behavior -leads to the fact that the field of computational photochemistry is still far from saturated with respect to computational tools and method developments. The need to treat several excited states, of different electronic character (covalent, ionic, charge-transfer, Rydberg, etc.), without bias requires the applied theory to be developed at a rather high level of sophistication. While TD-DFT to a large extent is attractable for Born-Oppenheimer molecular dynamics and trajectory surface hopping (TSH), due to its speed, it has its limitations in what type of transitions are correctly... 

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