Energy loss processes

By scattering within molecular solids and at their surfaces, LEE can excite with considerable cross sections not only phonon modes of the lattice [35,36,83,84,87,90,98,99], but also individual vibrational levels of the molecular constituents [36,90,98-119] of the solid. These modes can be excited either by nonresonant or by resonant scattering prevailing at specific energies, but as will be seen, resonances can enhance this energy-loss process by orders of magnitude. We provide in the next two subsections specific examples of vibrational excitation induced by LEE in molecular solid films. The HREEL spectra of solid N2 illustrate well the enhancement of vibrational excitation due to a shape resonance. The other example with solid O2 and 02-doped Ar further shows the effect of the density of states on vibrational excitation. 

Excitation Eunctions of O2 and 02-Doped Ar Eilms. Resonances can be best identified by the structures they produce in excitation functions of a particular energy-loss process (i.e., the incident-electron energy dependence of the loss). Fig. 7 is reproduced from a recent study [118] of the electron-induced vibrational and electronic excitation of multilayer films of O2 condensed on the Pt(lll) surface and shows the incident electron energy dependence of major losses at the indicated film thickness and scattering angles. Also shown in this figure is the scattered electron intensity of the inelastic background... 

Also shown in Fig. 10(c)-(g) are the anion yield functions for submonolayer quantities of O2 deposited onto various multilayer atomic and molecular solids. The data represent part of a study [41] on the environmental factors involved in the DEA process. As can be seen, the yield of desorbed ions can vary greatly with substrate composition. Such variations can be attributed to the so-called extrinsic factors that modify the ESD process at times before attachment and after dissociation, for example, electron energy-loss processes in the substrate and postdissociation interactions (PDI) of ions with the surrounding medium [41]. These processes can be contrasted with intrinsic factors, which... 

The dotted lines in Fig. 1 show the stopping powers for the different ions at a constant velocity in units of MeV/amu. This unit of energy is very often used in heavy ion radiolysis and it is based on the classical formula for kinetic energy, E = V2 MV, where M is the heavy ion mass. As seen in Eq. (1), the ion velocity is a dominant parameter in energy loss processes and the MeV/amu energy unit is more convenient to use than converting to absolute velocity units. Remember that MeV/amu is actually proportional to the square of the velocity. 

In thicker uniform samples the peaks at each mass broaden out with a relatively flat-topped distribution. By calculations based upon the energy loss process described above, it is possible to relate the energy of the scattered ion to the depth below the surface for the scattering interaction. In this way the observed distribution may be related to the composition vs. depth of the particular element within the sample layer. 

According to Eq. (1), the thermodynamic driving force of a PET process increases with the solvent polarity and therefore, photoreactions can be simply switched from energy transfer to electron transfer by changing the solvent [8]. However, back electron transfer (BET) often diminishes the yields of radical ions formed and therefore various efforts have been undertaken to circumvent this energy loss process [14]. Among these approaches two processes have been widely used and will thus be described in more detail. 

Phosphorescence most commonly follows population of Ti via ISC from Si, itself excited by absorption of light. The Ti state is usually of lower energy than Si, and the long-lived (phosphorescent) emission is almost always of longer wavelength than the short-lived (fluorescent) emission. The relative importance of fluorescence and phosphorescence depends on the rates of radiation and ISC from Si the absolute efficiency depends also on intermolecu-lar and intramolecular energy-loss processes, and phosphorescent emission competes not only with collisional quenching of Ti but also with ISC to So-... 

The kinetics of the emission process has been developed in terms of excitation, emission, and collisional deactivation steps. If intramolecular energy-loss processes (IC or ISC) occur, then additional first-order terms must be added to the denominator of Eq. 24. A similar, but more complex and extended, steady-state treatment can be developed to predict the intensity of phosphorescent emission. 

Fig. 9.10. XPE spectra of LaFeOj in the La(3d) region showing the charge-transfer satellites at an energy separation of approximately 4 eV from 3d5/2 and 3d3/2 signals. The weaker satellites at an energy separation of 13-16 eV are attributed to energy loss processes [50].

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