Collision-induced properties

FROM INTERMOLECULAR POTENTIALS AND COLLISION-INDUCED PROPERTIES TO THE MEASURED PROPERTIES OF ISOLATED COMPLEXES AND CONDENSED PHASES... 

Collision-induced Properties and Modelling of Raman Spectra of Atomic Gases... 

P.W. Fowlta-, A.J. Sadlej, Long-range and ovtalap effects on collision-induced properties. Mol. Phys. 77(4), 709-725 (1992)... 

Spectroscopic techniques have been applied most successfully to the study of individual atoms and molecules in the traditional spectroscopies. The same techniques can also be applied to investigate intermolecular interactions. Obviously, if the individual molecules of the gas are infrared inactive, induced spectra may be studied most readily, without interference from allowed spectra. While conventional spectroscopy generally emphasizes the measurement of frequency and energy levels, collision-induced spectroscopy aims mainly for the measurement of intensity and line shape to provide information on intermolecular interactions (multipole moments, range of exchange forces), intermolecular dynamics (time correlation functions), and optical bulk properties. 

Of a special astronomical interest is the absorption due to pairs of H2 molecules which is an important opacity source in the atmospheres of various types of cool stars, such as late stars, low-mass stars, brown dwarfs, certain white dwarfs, population III stars, etc., and in the atmospheres of the outer planets. In short absorption of infrared or visible radiation by molecular complexes is important in dense, essentially neutral atmospheres composed of non-polar gases such as hydrogen. For a treatment of such atmospheres, the absorption of pairs like H-He, H2-He, H2-H2, etc., must be known. Furthermore, it has been pointed out that for technical applications, for example in gas-core nuclear rockets, a knowledge of induced spectra is required for estimates of heat transfer [307, 308]. The transport properties of gases at high temperatures depend on collisional induction. Collision-induced absorption may be an important loss mechanism in gas lasers. Non-linear interactions of a supermolecular nature become important at high laser powers, especially at high gas densities. 

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. 

G. Birnbaum. Determination of molecular constants from collision-induced far-infrared spectra and related methods. In J. van Kranendonk, ed., Intermolecular Spectroscopy and Dynamical Properties of Dense Systems - Proceedings of the International School of Physics Enrico Fermi , Course LXXV, p. Ill, 1980. 

The dielectric properties of gases are closely related to collision-induced absorption. It is well known that collisions modify molecular properties. Specifically, we are here interested in the dipole moments induced by collisions (Chapter 4) which manifest themselves not only in collision-induced absorption, but also in the dielectric virial properties of gases. 

Ternary and Other Induced Spectra. Three-particle induced dipoles and the associated ternary collision-induced absorption spectra and dipole autocorrelation functions have been studied for fluids composed of mixtures of rare gases, and for neat fluids of nonpolar molecules — that is for systems that are widely thought to interact with radiation only by virtue of interaction-induced properties. A convenient framework is thus obtained for understanding the variety of experimental observations. The computer simulation studies permit an insight into the involved basic processes, but were not intended for direct comparison with measurements [57]. Methods have been developed for computer... 

J. D. Poll. Spectral moments, presentation at the Conference on Collision-induced Phenomena, Absorption, Light Scattering, and Static Properties, held at Florence, Italy, September 2-5, 1980. 

Other means of manipulating ions trapped in the FTMS cell include photodissociation (70-74), surface induced dissociation (75) and electron impact excitation ("EIEIO")(76) reactions. These processes can also be used to obtain structural information, such as isomeric differentiation. In some cases, the information obtained from these processes gives insight into structure beyond that obtained from collision induced dissociation reactions (74). These and other processes can be used in conjunction with FTMS to study gas phase properties of ions, such as gas phase acidities and basicities, electron affinities, bond energies, reactivities, and spectroscopic parameters. Recent reviews (4, 77) have covered many examples of the application of FTMS and ICR, in general, to these types of processes. These processes can also be used to obtain structural information, such as isomeric differentiation. 

Silylenium ions are common in gas-phase organosilicon chemistry, where they may be generated by various techniques including electron impact (29-33), photoionization (34-36), chemical ionization (37-45), collision-induced dissociation (25,46), and chemical-nuclear methods (15). Although this article is concerned with reactions in solution, a short account of gas-phase studies cannot be omitted, since they provide important information about chemical and physical properties of silylenium ions, which are so elusive in condensed phases. 

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. 

Barocchi F, Zoppi M (1981) Experimental determination of two-body collision-induced light scattering spectrum of helium. In Van Kranendonk J (ed) Intermolecular spectroscopy and dynamical properties of dense systems, Proceedings of the International School Enrico Fermi , Course LXXV, Amsterdam, North-Holland, pp263-274... 

In a very detailed study, using a combined theoretical/experimental approach. Squires et examined the gas phase properties of isomeric C3H5 anions. They were able to generate stable alkyl, 2-propenyl, 1-propenyl and cyclopropyl anions, as well as parent vinyl anions, in the gas phase by collision induced dissociation of the corresponding carboxylate anions, using the reaction sequence in Scheme 36 which was monitored by Fourier transform mass spectrometry. 

The Raman spectra of solids have a more or less prominent collision-induced component. Rare-gas solids held together by van der Waals interactions have well-studied CILS spectra [656, 657]. The face-centered, cubic lattice can be grown as single crystals. Werthamer and associates [661-663] have computed the light scattering properties of rare-gas crystals on the basis of the DID model. Helium as a quantum solid has received special attention [654-658] but other rare-gas solids have also been investigated [640]. Molecular dynamics computations have been reported for rare-gas solids [625, 630, 634]. 

G. Birnbaum. Study of Atomic and Molecular Interactions from Collision-Induced Spectra. In J. V. Sengers (ed.), Thermophysical Properties of Fluids, American Society of Mechanical Engineers, New York, 1981, pp. 8-17. 

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