Collision-induced dipole profile

Method of moments. In rare gas mixtures, the induced dipole consists of just one B component, with Ai AL = 0001, Eq. 4.14. Alternatively, one particular B(c) component may cause the overwhelming part of a measured spectrum, like the quadrupole-induced component in mixtures of small amounts of H2 in highly polarizable rare gases ((c) = Ai AL = 2023, Eq. 4.59) in a given spectral range, other components (like 0001, 2021,...) are often relatively insignificant. In such cases, one can write down more or less discriminating relationships between certain spectral moments of low order n that are obtainable from measurements of the collision-induced spectral profile, g Al(o>),... 

Theory. The theory of collision-induced absorption profiles of systems with anisotropic interaction [43, 269] is based on Arthurs and Dalgamo s close coupled rigid rotor approximation [10]. Dipole and potential functions are approximated as rigid rotor functions, thus neglecting vibrational and centrifugal stretching effects. Only the H2-He and H2-H2 systems have been considered to date, because these have relatively few channels (i.e., rotational levels of H2 to be accounted for in the calculations). The... 

Beyond the binary systems. Spectroscopic signatures arising from more than just two interacting atoms or molecules were also discovered in the pioneering days of the collision-induced absorption studies. These involve a variation with pressure of the normalized profiles, a(a>)/n2, which are pressure invariant only in the low-pressure limit. For example, a splitting of induced Q branches was observed that increases with pressure the intercollisional dip. It was explained by van Kranendonk as a correlation of the dipoles induced in subsequent collisions [404]. An interference effect at very low (microwave) frequencies was similarly explained [318]. At densities near the onset of these interference effects, one may try to model these as a three-body, spectral signature , but we will refer to these processes as many-body intercollisional interference effects which they certainly are at low frequencies and also at condensed matter densities. 

It is, therefore, interesting to point out that in a recent molecular dynamics study, shapes of intercollisional dips of collision-induced absorption were obtained. These line shapes are considered a particularly sensitive probe of intermolecular interactions [301]. Using recent pair potentials and empirical pair dipole functions, for certain rare-gas mixtures spectral profiles were obtained that differ significantly from what is observed... 

In the framework of the impact approximation of pressure broadening, the shape of an ordinary, allowed line is a Lorentzian. At low gas densities the profile would be sharp. With increasing pressure, the peak decreases linearly with density and the Lorentzian broadens in such a way that the area under the curve remains constant. This is more or less what we see in Fig. 3.36 at low enough density. Above a certain density, the l i(0) line shows an anomalous dispersion shape and finally turns upside down. The asymmetry of the profile increases with increasing density [258, 264, 345]. Besides the Ri(j) lines, we see of course also a purely collision-induced background, which arises from the other induced dipole components which do not interfere with the allowed lines its intensity varies as density squared in the low-density limit. In the Qi(j) lines, the intercollisional dip of absorption is clearly seen at low densities, it may be thought to arise from three-body collisional processes. The spectral moments and the integrated absorption coefficient thus show terms of a linear, quadratic and cubic density dependence,... 

The dipole forbidden transitions, S-S and S-D transitions of alkali atoms, are induced by the collisions with the rare gas atoms [82]. This phenomenon is called collision-induced absorption. The position and the profile of the collision-induced absorption sensitively reflect the interatomic potentials of the ground and excited states of the alkali-noble gas system and the (induced) transition moments between them. The SAC-CI method elucidated the detailed mechanism of the collision-induced absorption spectra of CsRg (Rg = Ne, Ar, Kr, Xe) system [83,84]. 

The leading term on the right-hand side of Eq. 5.88 is, of course, the intracollisional profile, g(co), that is the spectrum that would be observed if no correlations existed between the dipoles induced in successive collisions. The remaining term describes the intercollisional process, which is the central theme of this subsection. 

Because of the low collision rate in the high vacuum environment of a Fourier transform mass spectrometer (FTMS), vibrationally excited molecular ions cool predominantly by IR fluorescence. For typical IR transition dipole moments and frequencies in the mid-IR, spontaneous emission is expected to occur at a rate in the range of 1-100 s To energize an ion efficiently using IR multiple-photon excitation (MPE), the rate of photon absorption - the product of absorption cross section and photon flux - should exceed the emission rate. From such a back-of-an-envelope estimate, one finds that radiation sources producing several Watts/cm are required to induce efficient dissociation [141], Note that the demands on laser power may further increase because of the limited residence time of the ions in the laser field, collisional deactivation in traps at higher pressures, limited spectral overlap between molecular absorption and laser emission profiles, etc. 

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