Collision-induced dipole electronic

J. Schafer and W. Meyer. Collision induced dipole radiation of normal hydrogen gas in frequency range of the cosmic background. In J. Eichler, I. V. Hertel, and N. Stolterfoht, eds., Electronic and Atomic Collisions,... 

Content. After a brief overview of molecular collisions and interactions, dipole radiation, and instrumentation (Chapter 2), we consider examples of measured collision-induced spectra, from the simplest systems (rare gas mixtures at low density) to the more complex molecular systems. Chapter 3 reviews the measurements. It is divided into three parts translational, rototranslational and rotovibrational induced spectra. Each of these considers the binary and ternary spectra, and van der Waals molecules we also take a brief look at the spectra of dense systems (liquids and solids). Once the experimental evidence is collected and understood in terms of simple models, a more theoretical approach is chosen for the discussion of induced dipole moments (Chapter 4) and the spectra (Chapters 5 and 6). Chapters 3 through 6 are the backbone of the book. Related topics, such as redistribution of radiation, electronic collision-induced absorption and emission, etc., and applications are considered in Chapter 7. 

If, however, one replaces the H2 molecules by deuterium hydride (HD), new effects appear. While the electronic structure of HD still does not differ much from those of H2 or D2, a small, permanent dipole moment exists in the case of HD which gives rise to the allowed transitions with J — J + 1, the so-called Rq(J) lines. In Fig. 3.19, three such lines with J = 0... 2 are clearly discernible. These are superimposed with a collision-induced background, the So(0) line of HD, that peaks around 280 cm-1. Although this is not obvious from the figure, a detailed analysis shows an interference between allowed and induced lines that will concern us below. 

Both photon-assisted collisions and collision-induced absorption deal with transitions which occur because a dipole moment is induced in a collisional pair. The induction proceeds, for example, via the polarization of B in the electric multipole field of A. A variety of photon-assisted collisions exist for example, the above mentioned LICET or pair absorption process, or the induction of a transition which is forbidden in the isolated atom [427], All of these photon-assisted collision processes are characterized by long-range transition dipoles which vary with separation, R, as R n with n — 3 or 4, depending on the symmetry of the states involved. Collision-induced spectra, on the other hand, frequently arise from quadrupole (n = 4), octopole (n = 5) and hexadecapole (n = 6) induction, as we have seen. At near range, a modification of the inverse power law due to electron exchange is often quite noticeable. The importance of such overlap terms has been demonstrated for the forbidden oxygen —> lD emission induced by collision with rare gases [206] and... 

Perturbations affect the rate of absorption and emission of radiation in a fully understood and exactly calculable manner. They also affect the rates of chemical and collisional population/depopulation processes, but in a less easily estimated way. Perturbation effects on steady-state populations can be very large and level-specific. Although collision-induced transitions and chemical reactions are not governed by rigorous selection rules as are electric dipole transitions and perturbation interactions, some useful propensity rules have been suggested theoretically and confirmed experimentally. Gelbart and Freed (1973) suggested that the cross sections for collision-induced transitions between two different electronic states, E and E, are... 

The propensity rules for collision-induced transitions between electronic states and among the fine-structure components of non-1E+ states depend on the identity of the leading term in the multipole expansion of the molecule/collision-partner interaction potential. Alexander (1982a) has considered the dipole-dipole term, which included both permanent and transition dipole contributions. In the limit that first-order perturbation theory applies (not the usual circumstance for thermal molecular collisions), the following collisional propensity rules for the permanent dipole term can be enumerated from the selection rules for both perturbations and pure rotational transitions... 

A more recent suggestion " takes into account the possibility of interaction at each electron-deficient site in a solute molecule and orientation of the benzene solvent molecules by local dipole-induced dipole interactions. This precludes the idea of a single 1 1 collision complex, but provided there is no phase relationship between the various 1 1 associations in the molecule, the A values would still be proportional to the mole fraction of benzene added, as is suggested by the dilution studies. In the case of /i-ATN-dimethylaminobenzaldehyde (9B), solvation at the carbonyl carbon may be drastically reduced or even non-existent due to the influence of the electron-donating di-methylamino-group, so that the local dipole-induced dipole concept reduces to that shown in (9B). However, for -nitrobenzaldehyde the observation that is greater than (see 9A) will be a... 

For dielectric relaxation to be observed there has to be a change in the polarizability of the media under the influence of an applied field. This selection rule implies that dispersion will only be detected in polar materials. However, in certain cases contributions to the polarizability have been detected in non polar materials and are ascribed to collisional polarization effects. If two non polar molecules collide there is the possibility that distortion of the electron density nd atomic positions may result in the formation of a tiansient dipole or multipole. The induced polarization can be destroyed by further collision with other non activated molecules. The lifetime of these collisionally activated molecules may be many times the collision frequency and it then becomes possible to observe the reorientational motion of the induced dipoles, which act as though they were permanent dipoles. 

Dephasing is another important broadening process for spectral lines of adsorbates. Elastic collisions of phonons and conduction electrons with adsorbed atoms or molecules disrupt the phases of their induced dipole moments and thus provide surface-specific pathways for phase relaxation. If an adsorbed particle can be considered as a two-level system, both the lifetime of its excited state, T, and the dephasing time, T, contribute to the spectral linewidth 7 as ... 

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