Atom-diatom van der Waals complexes

Hutson J M and Howard B J 1980 Spectroscopic properties and potential surfaces for atom-diatom Van der Waals complexes Mol. Phys. 41 1123-41... 

The present chapter mainly discusses the simplest class of atom-diatom Van der Waals molecules, the molecular hydrogen-inert gas complexes. While experimental information on the vibrational predissociation of these species is as yet relatively limited, our knowledge of the potential energy surfaces which govern their dynamics (9,10) is unequalled for any other systems. Moreover, the small reduced mass and large monomer level spacings make accurate calculations of their properties and propensities relatively inexpensive to perform. For these reasons, these species have come to be treated as prototype systems in theoretical studies of vibrational predissociation (17-25). 

Each of the potentials shown in Figure 12.5 supports at least one bound or quasi-bound state which can be labeled by quantum numbers (j, Cl, J). These zeroth-order states correspond to almost free rotation of HF within the van der Waals complex with quantum numbers j = 0,1,2,... and Cl = 0,1,2,..., min(j, J). In analogy with the nomenclature for electronic states, they are termed E and n for Cl = 0 and 1, respectively. For j = 1 and Cl = 0 the diatom rotates in the plane defined by the three atoms. In contrast, for j = 1 and Cl = 1 it rotates in a plane perpendicular to the intramolecular vector R. As J increases, the centrifugal potential h2[J(J + 1) + j(j + 1) — 2Cl2]/2mR2 increases as well and eventually Veff(R j,Cl,J) becomes purely repulsive and the sequence of bound or quasi-bound states breaks off. 

We shall discuss some examples of reactions of excited van der Waals complexes. Up to now, only a few examples of a atom-diatom reactions have been studied—the atom being Hg, Ca, or Xe, and the diatom being H2 and halogen-containing molecules. These examples show clearly the new features in the reaction dynamics, such as orbital specificity, selectivity in the products, products state distribution, and observation of the intermediate states. 

Zhao and Rice started their analysis by defining a Hamiltonian for a model system designed to mimic a van der Waals complex of the diatom BC and the atom X. The full classical Hamiltonian for that system can be represented, as a function of the variables P,p,l,j,R,r, qi, qj, in the form... 

Interesting in this context are also theoretical studies of vibrational predissociation in van der Waals complexes by Beswick and Jortner. They conclude that the rates of vibrational predissociation of the rare gas atom-diatomic molecule complexes should be enhanced with decreasing mass of the rare-gas atom. If one could view relaxation of the guest molecule as a predissociation in a polyatomic van der W lals complex involving the guest and the nearest-neighbor rare-gas atoms, the observed trends would again be correctly predicted. 

Are atoms racist, as implied by the fact that atoms of like color prefer to bind each other The answer turns out to be a resounding no To see this, we recognize that reaction (2) represents only a part of the entire hypersurface. On the reactant side, the two diatomics can combine to form a van der Waals complex, while, on the product side, two... 

DYNAMICS OF VAN DER WAALS COMPLEXES BEYOND ATOM-DIATOM SYSTEMS... 

The energy levels of the free monomer axe of course more complicated than in the atom-diatom case. It is interesting to consider the specific cases of Ar-NHa and Ar-H20. NH3 is a symmetric top, and it is convenient to choose the monomer-fixed z axis to lie along the C3 axis. The interaction potential with therefore have threefold symmetry, and the only non-zero terms in equation (16) with be those for which // is a multiple of 3. The potential can in principle couple states with AA = 3, but any mixing due to such coupling will be very small because of the large a rotational constant of NH3 k will remain a nearly good quantum number in the Van der Waals complex. 

It is possible to formulate the bound state problem for a wide range of Van der Waals complexes in a form reminiscent of that for atom-diatom systems. The similarities between the different sets of coupled equations will be helpful in understanding the dynamical approximations that may be applied to the larger complexes. In particular, the helicity decoupling approximation, which has proved to be accurate for most atom-diatom systems, is equally applicable to the larger systems. 

The dipole moments of the atom-diatomic complexes X2-Y, in contrast to the atom-atomic ones, depend also on the angle 0 between the axis of diatomic molecule and the axis passing through the atom Y and the molecule X2 (Fig. 3.2a). Note that if the nonrigidity of the diatomic molecule X2 is taken into account, then the additional dependence of dipole moment of the complex on r appears. So, in general we have a surface of the dipole moment for such complex. Nevertheless, these van der Waals complexes are relatively simple yet and they are studied intensively up to now because of their importance (see, for instance, [24—30]). 

The references given in the preamble of this chapter give exhaustive enough representation of hyperpIarizabiUties for small van der Waals complexes including the atom-atomic and atom-diatomic ones. Here, we dwell in more detail on the CH4—N2 van der Waals complex which help us to illustrate the calculation technique to be applied to the first hyperpolarizability of a molecule or any molecular complex. The CH4-N2 complex is very important and interesting complex that exists in methane-nitrogen atmosphere of Titan. 

In this section we will explain the essential mechanism of vibrational predissociation by virtue of a linear atom-diatom complex such as Ar H2. Figure 12.1 illustrates the corresponding Jacobi coordinates, t In particular, we consider the excitation from the vibrational ground state of H2 to the first excited state as illustrated in Figure 12.2. The close-coupling approach in the diabatic representation, summarized in Section 3.1, provides a convenient basis for the description of this elementary process. For simplicity of presentation we assume that the coupling between the van der Waals coordinate R and the vibrational coordinate r is so weak that it suffices to include only the two lowest vibrational states, n = 0 and n = 1, in expansion (3.4) for the total wavefunction,... 

A different class of noble gas compounds bound by van der Waals interactions are small molecules NgX, where X is either an atom or a diatomic or triatomic species such as HF, HCl, HBr, CIF, NO, HCN, CI2, Brj. The experimental and theoretical techniques for studying such weakly bound complexes and the concepts and theoretical models to understand them have recently been reviewd in several articles and monographs [151-154] and will not be discussed here. It should be mentioned, however, that detailed structural and dynamical... 

ABSTRACT. Predictions are presented of spectra for excitation of the van der Waals rovibrational modes in ArHCl, ArHCN, H2DF, ArOH and NeC2H4. For ArHCN, H2DF and ArOH the potential energy surfaces used in the spectral computations have been obtained from CEPA calculations with large basis sets. Comparisons with experiment illustrate the power and usefulness of ab initio methods in predicting spectra for van der Waals molecules. The results also demonstrate that predictions of spectra can now be made for van der Waals molecules more complicated than the complexes of atoms with closed-shell diatomics. 

Van der Waals molecules containing a chemically bonded diatomic molecule weakly held to an atom have also been studied. The coordinates and quantum numbers needed to specify such a complex are shown in Fig. 2. We mean A-H B to represent all complexes of this type. The chemically bonded molecule is given by A-H and may be H2, HCl, or a molecule not containing hydrogen such as NO, N2, O2, and so on. 

Some detailed mappings of potential surfaces for diatomic -atom complexes in form of equation (3) have appeared and are reviewed elsewhere. For van der Waals molecules containing two chemically... 

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