Photoabsorption processes

The surface sensitivity is ensured by detecting the decay products of the photoabsorption process instead of the direct optical response of the medium (transmission, reflection). In particular one can measure the photoelectrons, Au r electrons, secondary electrons, fluorescence photons, photodesorbed ions and neutrals which are ejected as a consequence of the relaxation of the system after the photoionization event. No matter which detection mode is chosen, the observable of the experiment is the interference processes of the primary photoelectron with the backscattered amplitude. 

The nature of the molecular system implements a change in the physical mechanism of the photoabsorption process. Once again we may, however, employ the golden rule expression. We use it in a general sense to express the absorption rate in the form of ... 

Recently in our laboratory we have initiated a program to study the photoabsorption processes of metal vapors throughout the UV and EUV region. Our research interests are (1) to obtain the absolute cross section measurement of atomic and molecular metal vapors, (2) to study the photoionization processes of molecular metal species, and (3) to study the photodissociation processes of molecular metal ions. Several experimental methods such as the heat-pipe absorption spectroscopy, photoionization mass spectroscopy, and electron-ion coincidence technique, will be used in the study. This report summarizes our first experiment using heat-pipe absorption spectroscopy. 

In their study of the photodissociation of formic acid, Tian and Fang compared the gas-phase reaction with that of an aqueous solution. For the solvent they used a combined model with a supermolecule treatment of the solute plus the nearest water molecules together with a polarizable continuum model for the long-range solvent effects. They could demonstrate that the explicit treatment of water molecules closest to the solute was important for the theoretical study of the photoabsorption process. 

For many years, X-UV emission and absorption spectroscopy has been known as an appropriate method for the determination of electronic distributions in solids. An analysis of X-ray emission spectra that are due to transitions between an inner nl atomic level of known characteristics and the various levels either in the energy bands or in the molecular orbitals provides a direct method of determining the distribution of occupied conduction or valence states in the material to be studied. Absorption spectra on the other hand describe the photoabsorption process and provide a description of unoccupied states over a wide energy range. These two types of analysis are complementary. 

Absorption. During the X-UV photoabsorption process, an electron of a more or less deep atomic level is excited to unoccupied states of suitable syrmnetry. 

To understand the physical mechanism undei lying these processes, it is helpful to first consider the broad significance of the energy-time Uncertainty Principle for photoabsorption processes. It is well-known that for a molecular excited state with an average lifetime St and an average energy displacement SE from the ground state, there exists the relation (Finkel 1987)... 

It is useful to express photoabsorption processes in terms of phase shifts, and to partition the total phase shift A into one part which is due to the resonances (Ao, say) and one part (say 6) which is the phase shift of the background continuum ... 

To clarify the physical implication of the cross-section of the photoabsorption process discussed in the previous sections, it is better to use the Green s function approach with a generalized optical theorem (Natoli et al. 1986), and to write the following expression for the cross section... 

The possibility to use laser radiations to achieve the so-called "coherent control" of molecular dissociation or of atomic photoionization has been predicted since the advent of laser sources in the early sixties. It was expected that, thanks to the coherence and monochromaticity properties of the laser light, one could selectively choose a dissociation channel and the spatial orientation of ejection of the fragments (either ions or electrons or even neutrals) in an elementary chemical process. However, earlier attempts, based on simple photoabsorption processes, have been unsuccessful and it is only recentiy that experiments have been shown to enable one to achieve such a goal in some selected systems. Amongst the various scenarios which have been explored, one of the most promising is based on the realization of quantum interferences in so-called "two-colour" photodissociation or... 

Allen and Martin (1982) and Lavagna et al. (1982, 1983) proposed that the y-a transition in Ce was related to the Kondo effect. Since Kondo systems have densities of states with two peaks, one very close to the Fermi level and one below, this offered a possible explanation of the two peaks in the photoelectron spectra, assuming that the photoabsorption process would transfer the structure in the density of states to the emitted spectrum. Detailed calculations of a photoelectron spectrum were not, however, carried out. The Kondo (or Abrikosov-Suhl) resonance in the density of states can be obtained from the Anderson Hamiltonian in the limit U CO (Lacroix 1981). For a Fermi level in the center of a valence band of width 2D and a constant density of states coupled by a constant matrix element V to an Nf-fold degenerate localized level at energy Sf below two peaks can arise in the density of states of the coupled system at low temperatures if is not too small. As... 

It is essential to underline that the time-dependence considered here must not be mixed up with the time-dependence of the actual photoabsorption process in which the electric field Efieid( ) steadily pumps the molecule from the ground electronic state to the excited state. The formalism described above corresponds to a picture in which an infinitely short light pulse instantaneously promotes the molecule to the excited-state PES. Because the initial wave packet is not an eigenfunction of the upper-state Hamiltonian, it immediately starts to move according to the equations of motion on the PES V ... 

Fig. 8. In EXAFS, the absorption of X-ray radiation is determined as a function of photon energy. The photoelectrons generated in the photoabsorption process at an adsorbate-atom are backscattered from substrate atoms and lead to modulation of the absorption cross section through interference phenomena. In the center figure the lines of equal phase of the backscattered radiation go through the excited atom (see top) resulting in constructive interference, whereas in the bottom figure the photoelectron kinetic energy is slightly different, resulting in destructive interference at the position of the excited atom.

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