Interaction-induced absorption

Supermolecular spectra could perhaps be studied with state-selection using adequate molecular beam techniques. That would not be easy, however, because of the smallness of the dipole moments induced by in-termolecular interactions. For the purpose of this book, we will mostly deal with bulk spectra, or interaction-induced absorption of pure and mixed gases. A great variety of excellent measurements of such spectra exists for a broad range of temperatures, while state-selected supermolecular absorption beam data are virtually non-existent at this time. Furthermore, important applications in astrophysics, etc., are concerned precisely with the optical bulk properties of real gases and mixtures. 

Interaction-induced absorption (as the new features were called early on [353]) has stimulated considerable interest. For a long time, explanations were attempted in terms of weakly bound (02)2 polarization molecules (that is, van der Waals molecules), but some of the early investigators argued that unbound collisional pairs might be responsible for the observed absorption. More recently, a study of the temperature dependence of the induced intensities has provided evidence for the significance of collisional complexes. The idea of absorption by collisionally interacting, unbound molecular pairs was, however, not widely accepted for decades. 

Hence, the process was named collision-induced absorption. Whereas the term interaction-induced absorption used by some early on seems to cautiously leave the question open whether free or bound complexes generate the absorption, Welsh and associates bravely state their conclusion as collisional interactions. Since then, other names have also been used, such as pressure-induced and supermolecular absorption. 

According to theory, most collision-induced absorption spectra should not only consist of contributions of the free-state to free-state transitions typical of collisional pairs, but also of contributions arising from bound-to-free and bound-to-bound transitions involving van der Waals molecules. In the rotovibrational spectra such dimer bands have been known for some time, but in CIA studies of the rototranslational band, where path lengths have generally been limited to a few meters, dimer features have been seen only recently [268], The dimer spectra are an integral part of the interaction-induced absorption. 

The induced bands mentioned are known from spectral studies with liquid, solid and compressed gaseous oxygen. Condensed oxygen is blue, because of the red 0-0 absorption bands near 760 and 630 nm, and their 1-0 counterparts, that is, because of interaction-induced absorption. 

Interesting ripples were first seen in the interaction-induced absorption spectra of compressed nitrogen [107] and later also of oxygen and in various gas mixtures in the infrared [73, 78, 84]. It has been suggested that these weak, roughly sinusoidal structures that are superimposed with the quasi-continuous, induced background arise possibly from line mixing, due to the anisotropy of... 

Interaction-induced absorption by the vibrational or rotational motion of an atom, ion, or molecule trapped within a Ceo cage, so-called endohedral buckmin-sterfullerene, has excited considerable interest, especially in astrophysics. The induced bands of such species are unusual in the sense that they are discrete, not continuous they may also be quite intense [127]. Other carbon structures, such as endohedral carbon nanotubes, giant fullerenes, etc., should have similar induced band spectra [128], but current theoretical and computational research is very much in flux while little seems to be presently known from actual spectroscopic measurements of such induced bands. 

R. Vallauri. Molecular Dynamics Studies of Interaction Induced Absorption and Light Scattering in Diatomic Systems. In G. Birnbaum (ed.). Phenomena Induced by Inter-molecular Interactions, Plenum Press, New York, 1985, pp. 451-413. 

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At low water content, w = 3 and w = 5, a decrease in nanocrystallite size (blue shift of absorption onset) is observed in presence of low CTAC concentrations (<2 X 10-3 M). This is directly related to the decrease in interdroplct attractive interactions induced by CTAC addition. 

Spectroscopy is concerned with the interaction of light with matter. This monograph deals with collision-induced absorption of radiation in gases, especially in the infrared region of the spectrum. Contrary to the more familiar molecular spectroscopy which has been treated in a number of well-known volumes, this monograph focuses on the supermolecular spectra observable in dense gases it is the first monograph on the subject. 

Figure 1.2 illustrates the difference between the transitions involved in van der Waals dimer bands which Welsh and associates hoped to find, and the collision-induced absorption spectra that were discovered instead. Intermolecular interaction is known to be repulsive at near range and attractive at more distant range. As a consequence, a potential well exists which for most molecular pairs is substantial enough to support bound states. Such a bound state is indicated in Fig. 1.2 (solid curve b). When infrared radiation of a suitable frequency is present, the dimer may undergo various transitions from the initial state (solid curve) to a final state which may have a rather similar interaction potential (dashed curve b ) and dimer level spacings. Such transitions (marked bound-bound) often involve a change of the rotovibrational state(s) EVj of one or both molecule(s),... 

Induced spectra actually consist of contributions arising from free-to-free, free-to-bound, bound-to-bound, and bound-to-free transitions. At temperatures much greater than the well depth of the intermolecular potential, kT e, the observed induced absorption is nearly fully due to free-to-free transitions as Welsh and associates suggest, but individual dimer lines or bands may still be quite prominent unless pressure broadening and perhaps other processes (like ternary interactions) have obliterated such structures. However, at lower temperatures, kT collision-induced is a poor choice. 

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. 

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. 

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. 

Time scales. For an understanding of spectral line shapes of induced absorption, at not too high gas densities, it is useful to distinguish three different times associated with collisions, namely the average time between collisions, the duration of a molecular fly-by and the duration of the spectroscopic interaction. 

Sample cells. Variable temperature. Temperature control has been essential in much of the collision-induced absorption studies. Temperature variation accesses different parts of the intermolecular interaction potential and redistributes the relative importance of overlap and multipolar induction. Furthermore, at low temperatures, collision-induced line shapes are relatively sharp induced lines may be resolved at low temperatures whose structures may be masked at higher temperatures. 

Our main interest is the absorption which arises from complexes of interacting atoms and/or molecules, i.e., the absorption which exceeds the simple sum of the absorption spectra of the individual (non-interacting) atoms and molecules (where such monomer spectra exist). This excess absorption is of a supermolecular nature. It will be called interaction-induced, or briefly induced absorption. 

We have argued above that like rare-gas pairs (Ar-Ar) do not absorb in the infrared because the inversion symmetry which is inconsistent with a dipole moment. Like molecular pairs, on the other hand, absorb in the broad vicinity of the molecular transition frequencies (and at sums or differences of these). At the lower densities where binary interactions dominate, absorption in pure hydrogen is proportional to the density squared (see Fig. 3.15 below) which suggests that H2-H2 pairs, just like pairs of virtually all other molecules, do in fact possess an induced dipole moment (except for certain molecular orientations of high symmetry). 

Above we have stated that over a substantial range of gas densities, essential parts of the profiles of collision-induced absorption spectra are invariant if normalized by density squared, a/q2, in pure gases, or by the product of densities, cl/q Q2, in mixed gases. Induced spectra that show this density-squared dependence may be considered to be of a binary origin. Above, we have seen examples that at very low frequencies many-body effects may cause deviations from the density-squared behavior at any pressure, over a limited frequency band near zero frequency (intercol-lisional effect). Furthermore, with increasing densities, a diffuse N-body effect with N > 2 more or less affects most parts of the observable spectra. It is interesting to study in some detail how the three-body (and perhaps higher-order) interactions modify the binary profiles. 

It has been known since the early days of collision-induced absorption that spectral moments may be represented in the form of a virial expansion, with the coefficients of the Nth power of density, qn, representing the N-body contributions [402, 400], The coefficients of qn for N = 2 and 3 have been expressed in terms of the induced dipole and interaction potential surfaces. The measurement of the variation of spectral moments with density is, therefore, of interest for the two-body, three-body, etc., induced dipole components. 

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