Induced dipole collision

These can be determined experimentally to very high accuracy from the Stark effect and molecular beam studies. The experimental accuracy is far beyond the capabilities of ab initio studies. At the other extreme, the original route to these quantities was through studies of the dielectric polarization of species in solution, and there is currently interest in collision-induced dipole moments. In either case, the quantities deduced depend critically on the model used to interpret the experiment. 

It is a remarkable fact that the translational transitions of virtually all supermolecules are infrared active - even if the individual molecules are not. The only exceptions are supermolecules that possess a symmetry which is inconsistent with the existence of a dipole moment. Pairs of like atoms, e.g., He-He, have inversion symmetry, implying a zero dipole moment and, hence, infrared inactivity. But dissimilar atomic pairs, e.g., He-Ar, or randomly oriented molecular pairs, e.g., H2-H2, generally lack such symmetry. As a consequence, more or less significant collision-induced dipoles exist for the duration of the interaction which generate the well known collision-induced spectra. 

Spectral moments of allowed molecular absorption bands vary in general nearly linearly with the gas density, y /q constant. At sufficiently high pressures, a small, linear increase with density of the ratio y /g is, however, discernible, e.g., [8]. This quadratic absorption component is largely due to apparent induced absorption, resulting from the long range interaction of dipoles induced by the incident radiation field [400], Moreover, a true induced absorption component is believed to exist which arises from collision-induced dipole components (Chapter 4) [146, 210]. It was argued, however, that in most measurements true induced absorption was too weak to be identified positively in this way. Recent experimental and... 

Besides the collision-induced dipoles, we will occasionally refer to field-induced dipoles, or to rotation-induced dipoles, that is dipoles induced by an external electric field, or by centrifugal forces distorting certain symmetries of rotating molecules. Moreover, we will be interested in the dipoles induced in binary, ternary, etc., systems as we proceed. 

Collision-induced dipoles manifest themselves mainly in collision-induced spectra, in the spectra and the properties of van der Waals molecules, and in certain virial dielectric properties. Dipole moments of a number of van der Waals complexes have been measured directly by molecular beam deflection and other techniques. Empirical models of induced dipole moments have been obtained from such measurements that are consistent with spectral moments, spectral line shapes, virial coefficients, etc. We will briefly review the methods and results obtained. 

W. Meyer. Ab initio calculations of collision-induced dipole moments. In G. Birnbaum, ed., Phenomena Induced by Intermolec. Interactions, p. 29, Plenum Press, New York, 1985. 

X. Li and K. L. C. Hunt. Transient collision-induced dipoles in pairs... 

G. Birnbaum, M. Krauss, and L. Frommhold. Collision-induced dipoles of rare gas mixtures. J. Chem. Phys., 80 2669, 1984. 

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,... 

Potential energy surfaces of weakly bound dimers and trimers are the key quantities needed to compute transition frequencies in the high resolution spectra, (differential and integral) scattering cross sections or rate coefficients describing collisional processes between the molecules, or some thermodynamic properties needed to derive equations of state for condensed phases. However, some other quantities governed by weak intermolecular forces are needed to describe intensities in the spectra or, more generally, infrared and Raman spectra of unbound (collisional complexes) of two molecules, and dielectric and refractive properties of condensed phases. These are the interaction-induced (or collision-induced) dipole moments and polarizabilities. 

It is worth noting that the contribution from the fcth-order contribution to the collision-induced dipole moment, A/a , can be written in terms of the complete orthogonal set of angular functions labeled by A = LA, KA, LB, KB, L, X and m,... 

Collision-induced Dipoles. Even in materials whose molecules possess no permanent dipole, molecular collisions, either of unlike molecules or of like molecules in skew attitudes, produce fluctuating dipoles whose time variation reflects the translational movement of the colliding molecules rather than any characteristic vibration of the molecules. This effect appears in its purest form in m lxtures of rare gases. 

The analysis of Fig. (5.1) shows that the analytical description of A y may be effectively used for / > 11 a.u. Naturally, for shorter R the analytical values of APg, y are noticeably differ from ab initio values due to exchange interactions appeared in this range. It is also noticeable that the exchange interaction is the most important for the Afi component. Such behavior of Aj is consistent with results of the asymptotic model of exchange interactions for van der Waals complexes [54, 60]. The strong anisotropy of the exchange effects was also observed for the case of collision-induced dipole moments and dipole polarizabilities in ion-atom pairs [61]. 

X. Li, M.H. Champagne, K.L.C. Hunt, Long-range, collision-induced dipoles of -Dooh molecule pairs theory and numerical results for CH4 or CF4 interacting with H2, N2, CO2, or CS2. J. Chem. Phys. 109(19), 8416 (1998)... 

A reference calculation of the collision-induced dipole polarizability of Hc2 was reported by Cencek et al. ° These authors reported calculations of the interaction-induced properties via variation-perturbation calculations... 

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