Van Kranendonk

LeRoy R J and van Kranendonk J 1974 Anisotropic intermolecular potentials from an analysis of spectra of H2- and D2-inert gas complexes J. Chem. Phys. 61 4750... 

Van Kranendonk J. Intercollisional interference effects in pressure induced infrared spectra. Can. J. Phys. 46, 1173-9 (1968). 

The most extensive potential obtained so far with experimental confirmation is that of Le Roy and Van Kranendonk for the Hj — rare gas complexes 134). These systems have been found to be very amenable to an adiabatic model in which there is an effective X—Hj potential for each vibrational-rotational state of (c.f. the Born Oppenheimer approximation of a vibrational potential for each electronic state). The situation for Ar—Hj is shown in Fig. 14, and it appears that although the levels with = 1) are in the dissociation continuum they nevertheless are quasi bound and give spectroscopically sharp lines. 

The important point used by Le Roy and Van Kranendonk is that if the overall angular momentum state J = 0 is considered then the position of the levels is determined only by the terms in (50) and if J - 2 is considered then Pq and Pj are rele-... 

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. 

Interesting line narrowing has been observed of quadrupole-induced lines of hydrogen-rare gas mixtures. These have been explained by van Kranendonk and associates [428] in terms of the mutual diffusion coefficient of H2 in a rare-gas environment, as an effective lengthening of the interaction times of H2-atom complexes. 

Later studies showed the same phenomena in deuterium and deuterium-rare gas mixtures [335, 338, 305], and also in nitrogen and nitrogen-helium mixtures [336] in nitrogen-argon mixtures the feature is, however, not well developed. The intercollisional dip (as the feature is now commonly called) in the rototranslational spectra was identified many years later see Fig. 3.5 and related discussions. The phenomenon was explained by van Kranendonk [404] as a many-body process, in terms of the correlations of induced dipoles in consecutive collisions. In other words, at low densities, the dipole autocorrelation function has a significant negative tail of a characteristic decay time equal to the mean time between collisions see the theoretical developments in Chapter 5 for details. 

U. Buontempo, S. Cunsolo, P. Dore and P. Maselli, Molecular motions in liquids. In J. van Kranendonk, ed., Intermodular Spectroscopy and Dynamical Properties of Dense Systems - Proceedings of the Int. School of Physics Enrico Fermi , Course LXXV, p. 211, 1980. 

J. van Kranendonk, Solid Hydrogen, Plenum, New York and London, 1983. 

The dipole moment induced in a molecule, or in a group of molecules, is a finite range function of the intermolecular separations, R, which falls off faster than R-3 for R —> oo. Van Kranendonk has argued that, therefore, it is possible to expand the above equation in a series of cluster functions [400, 402]. If ft( 1 n) designates the dipole moment induced in the cluster of molecules 1 n when these are present alone in the given volume V, cluster functions 1/(1 n) can be defined according to... 

The theory of collision-induced absorption developed by van Kranendonk and coworkers [405] and other authors [288, 289, 81, 126, 125] has emphasized spectral moments (sum formulae) of low order. These are given in closed form by relatively simple expressions which are readily evaluated. Moments can also be obtained from spectroscopic measurements by integrations over the profile so that theory and measurement may be compared. A high degree of understanding of the observations could thus be achieved at a fundamental level. Moments characterize spectral profiles in important ways. The zeroth and first moments, for example, represent in essence total intensity and mean width, the most striking parameters of a spectral profile. 

Collision-induced absorption takes place by /c-body complexes of atoms, with k = 2,3,... Each of the resulting spectral components may perhaps be expected to show a characteristic variation ( Qk) with gas density q. It is, therefore, of interest to consider virial expansions of spectral moments of binary mixtures of monatomic gases, i.e., an expansion of the observed absorption in terms of powers of gas density [314], Van Kranendonk and associates [401, 403, 314] have argued that the virial expansion of the spectral moments is possible, because the induced dipole moments are short-ranged functions of the intermolecular separations, R, which decrease faster than R 3. We label the two components of a monatomic mixture a and b, and the atoms of species a and b are labeled 1, 2, N and 1, 2, N, respectively. A set of fc-body, irreducible dipole functions U 2, Us,..., Uk, is introduced (as in Eqs. 4.46), according to... 

The binary moments, yjjab and the various ternary ones, namely y(naab>, y(nabb>, yf,aaaK yj,bbbK may thus be computed. Poll and van Kranendonk give the first moments, yi, according to... 

Results. The theory of ternary processes in collision-induced absorption was pioneered by van Kranendonk [402, 400]. He has pointed out the strong cancellations of the contributions arising from the density-dependent part of the pair distribution function (the intermolecular force effect ) and the destructive interference effect of three-body complexes ( cancellation effect ) that leads to a certain feebleness of the theoretical estimates of ternary effects. 

It has been argued that, in the low-density limit, intercollisional interference results from correlations of the dipole moments induced in subsequent collisions (van Kranendonk 1980 Lewis 1980). Consequently, intercollisional interference takes place in times of the order of the mean time between collisions, x. According to what was just stated, intercollisional interference cannot be described in terms of a virial expansion. Nevertheless, in the low-density limit, one may argue that intercollisional interference may be modeled as a sequence of two two-body collisions in this approximation, any irreducible three-body contribution vanishes. 

An explanation was offered by van Kranendonk many years after the experimental discovery. Van Kranendonk argued that anticorrelations exist between the dipoles induced in subsequent collisions [404], Fig. 3.4. If one assumed that the induced dipole function is proportional to the intermolecular force - an assumption that is certainly correct for the directions of the isotropic dipole component and the force, and it was then thought, perhaps even for the dipole strength - an interference is to be expected. The force pulses on individual molecules are correlated in... 

J. C. Lewis and J. van Kranendonk. Intercollisional interference effects in collision-induced light scattering. Phys. Rev. Lett., 24 802, 1970. 

J. van Kranendonk. Theory of induced infrared absorption. Physica, 23 825, 1957. 

J. van Kranendonk. Induced infrared absorption in gases. Calculation of the binary absorption coefficients of symmetrical diatomic molecules. Physica, 24 347, 1958. 

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