Collision-induced spectra

By CH4-chemical ionization two predominant ions from DDS were formed in the ion source m/z 163 (M + 1) and m/z 165 (M + 3), which were accelerated and separated in the first quadrupole. Decomposition by collision activation with argon then occurred in the second quadrupole and the resulting daughter ions were separated in the third quadrupole to give rise to collision-induced spectra (equations 41-43). When carrying out the analysis with a solid inlet MS-MS system (TSQ) it was observed that, when operating TSQ in the multiple ion detection mode and selecting the ions m/z 63, 83, 85,99 and 101, only in the third quadrupole was there no interference from other m/z 163 and 165 precursor ions . ... 

For the purpose of this work, we will call a complex of two or more interacting atoms/molecules a supermolecule. Supermolecules may exist for a short time only, e.g., the duration of a fly-by encounter ( 10-13 s). Alternatively, supermolecules may be bound by the weak van der Waals forces and thus exist for times of the order of the mean free time between collisions ( 10-10 s), or longer. In any case, it is clear that, in general, supermolecules possess a spectrum of their own, in excess of the sum of the spectra of the individual (non-interacting) molecules that make up the supermolecule. These spectra are the collision-induced spectra, the subject matter of this monograph. 

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. 

Binary and ternary spectra. We will be concerned mainly with absorption of electromagnetic radiation by binary complexes of inert atoms and/or simple molecules. For such systems, high-quality measurements of collision-induced spectra exist, which will be reviewed in Chapter 3. Furthermore, a rigorous, theoretical description of binary systems and spectra is possible which lends itself readily to numerical calculations, Chapters 5 and 6. Measurements of binary spectra may be directly compared with the fundamental theory. Interesting experimental and theoretical studies of various aspects of ternary spectra are also possible. These are aimed, for example, at a distinction of the fairly well understood pairwise-additive dipole components and the less well understood irreducible three-body induced components. Induced spectra of bigger complexes, and of reactive systems, are also of interest and will be considered to some limited extent below. 

The spectral line shapes of collision-induced spectra (like in Fig. 1.3) resemble a Lorentzian, but the profile shown in Fig. 1.1 looks quite... 

We emphasize the line shape problem perhaps a little more than usual in the spectroscopic literature. Collision-induced spectra have little structure. Yet, the diffuse line and band spectra extend over wide frequency bands and must often be subtracted, say from the complex spectra of planetary or stellar atmospheres, for a more detailed analysis of other, less well known components. The subtraction requires accurate knowledge of the profile and its variation with temperature, composition, etc., often over frequency bands of hundreds of cm-1. 

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. 

Spectral moments. For the analysis of collision-induced spectra and the comparison with theory, certain integrals of the spectra, the spectral moments, are of interest. Specifically, we define the nth moment of the spectral function, g(v), by... 

Measurements of collision-induced spectra reflect certain details about intermolecular interactions. If analyzed with care, such information will enhance knowledge of molecular interactions. Furthermore, for specific applications, laboratory measurements of collision-induced spectra taken at a few fixed temperatures must be interpolated, often even extrapolated to the temperatures of interest. We will, therefore, discuss the tools available for analysis and further use of laboratory measurements, for example, for astrophysical applications. 

The shapes of collision-induced spectra are, however, not fully characterized by one or two averages of the distribution. The far wings of a collision-induced line show logarithmic slopes that are gently decreasing in magnitude with increasing frequency this behavior is, however, not... 

Not all of these induced dipole types may exist in any given system. The components that exist generally couple in different ways to the translational, rotational, vibrational, etc., states of the complex and usually are associated with different selection rules, thus generating different parts of the collision-induced spectra. 

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. 

We start with the basic relationships ( Ansatz ) of collision-induced spectra (Section 5.1). Next we consider spectral moments and their virial expansions (Section 5.2) two- and three-body moments of low order will be discussed in some detail. An analogous virial expansion of the line shape follows (Section 5.3). Quantum and classical computations of binary line shapes are presented in Sections 5.4 and 5.5, which are followed by a discussion of the symmetry of the spectral profiles (Section 5.6). Many-body effects on line shape are discussed in Sections 5.7 and 5.8, particularly the intercollisional dip. We conclude this Chapter with a brief discussion of model line shapes (Section 5.10). 

IT model. A line shape based on information theory (IT) has been proposed for the collision-induced spectra [159, 203], The profile was really never intended to be used to represent collision-induced spectra with the best accuracy possible. Rather, the emphasis is on simplicity it is the result of a qualitative theory of the line shape in situations where only... 

The numerator A represents the density of states function, which in practice is set equal to a constant [326]. In this way, profiles may be obtained for the collision-induced spectra. However, one should not expect the IT shape to approximate real line shapes well [83, 274]. The basic assumption of IT mentioned above ignores the wealth of information... 

For some time, the line profiles used to construct collision-induced spectra were believed to satisfy detailed balance , Eq. 5.73,... 

Collision-induced electronic spectra have many features in common with rovibrotranslational induced absorption. In this Section, we take a look at the electronic spectra. We start with a historical note on the famous forbidden oxygen absorption bands in the infrared, visible and ultraviolet. We proceed with a brief study of the common features, as well as of the differences, of electronic and rovibrotranslational induced absorption. Recent work is here considered much of which was stimulated by the advent of the laser - hence the name laser-assisted collisions. The enormous available laser powers stimulated new research on laser-controlled, reactive collisions and interactions of supermolecules with intense radiation fields. In conclusion, we attempt a simple classification of various types of electronic collision-induced spectra. 

Collision-induced spectra are usually studied at high densities, indeed often at densities that are so high that many-body interactions (as opposed to just binary interactions, Eq. 7.2) dominate the spectra [45, 47], For example, liquids have strong many-body components [364, 376]. On the... 

Both photon-assisted collisions and collision-induced absorption deal with transitions which occur because a dipole moment is induced in a collisional pair. The induction proceeds, for example, via the polarization of B in the electric multipole field of A. A variety of photon-assisted collisions exist for example, the above mentioned LICET or pair absorption process, or the induction of a transition which is forbidden in the isolated atom [427], All of these photon-assisted collision processes are characterized by long-range transition dipoles which vary with separation, R, as R n with n — 3 or 4, depending on the symmetry of the states involved. Collision-induced spectra, on the other hand, frequently arise from quadrupole (n = 4), octopole (n = 5) and hexadecapole (n = 6) induction, as we have seen. At near range, a modification of the inverse power law due to electron exchange is often quite noticeable. The importance of such overlap terms has been demonstrated for the forbidden oxygen —> lD emission induced by collision with rare gases [206] and... 

Collision-induced Spectra of Binary Systems. A brief overview of new measurements of collision-induced spectra is given in Table 1. Theoretical calculations of such spectra are also important, e.g., for the comparison of measurement with the fundamental theory, for easy frequency and/or temperature interpolation (and, sometimes, extrapolation) for many applications, or to predict or estimate such spectra that have not been measured or cannot be measured readily. 

S. Weiss. Simulation of gas phase collision-induced spectra quadrupole-... 

E. Bar-Ziv and S. Weiss. Collision-induced spectra of rare gas mixtures Experimental and empirical relations. J. Chem. Phys., 64 2412, 1976. 

G. Birnbaum. A study of atomic and molecular interactions from collision-induced spectra. Proc. 8 Symp. Therm. Phys., 1 8, 1982. 

A. Borysow, L. Frommhold, and W. Meyer. Collision induced spectra of H2-He dependence of the radial dipole transition elements on the rotational states. Phys. Rev. A 41 264, 1990. 

U. Buontempo, S. Cunsolo, and G. Jacucci. Memory effects in the collision-induced spectra of the noble gases in liquid argon. Can. J. Phys., 49 2870, 1971. 

P. S. Julienne and L. Frommhold. Notes and comments on roundtable discussion on laser-assisted collisions and collision induced spectra. In J. Szudy, ed., Spectral Line Shapes 5, University of Torun Press, 1989. 

A. Rare-gas Compounds.—The recent preparation of a number of rare-gas compounds has led to a significant number of calculations on these molecules, particularly on diatomic species. The repulsive interaction and dispersion interaction between like rare-gas atoms has been dealt with above. Several years ago Matcha and Nesbet284and Gilbert and Wahl276 presented calculations on the species NeHe, ArHe, andNeAr. The interaction of two dissimilar rare-gas diatoms gives rise to a dipole moment, which leads to far-i.r. collision-induced spectra of rare-gas mixtures. For this reason, the dipole moments of such species are of interest, but dispersion contributions are not described in the HF approximation and have to be computed by an empirical expression ft — B exp (—Rjp). Much more work is needed on this problem. Wahl and co-workers have reported the numerical results of OVC calculations on HeNe,240 but no detailed discussion was given. In cases like these, the HF model should yield reliable interaction potentials, since A and B are both closed systems. 

Proffitt M, Keto JW, Frommhold L (1980) Collision-induced spectra of the helium isotopes. Phys Rev Lett 45 1843-1846... 

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. 

J. Jonas. Density effects on collision induced spectra in fluids. J. Chem. Soc. Faraday Trans. 2, 53 1777-1789 (1987). 

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