Atmospheric Components and Their Dimers

Implications for the interpretation of recent laboratory [11,12] and atmospheric [13] spectroscopic observations, as well as current measurements of high pressure behavior of oxygen [14], are amenable to be discussed in this framework. This work provides also the ground for the interpretation of complicated band features in rotational spectra, as exemplified by the case of oxygen. Still appears to be valid the statement [15,16] that spectral analysis alone is not sufficient to extract information on structure and bonding, and the combined use of scattering and gaseous properties information is therefore confirmed to be crucial . 

The measurement of quantum mechanical interference effects in the velocity dependence of scattering cross-sections provides data which, together with accurate second virial coefficients, yield information on the intermolecular poten- 

Along these lines, a simultaneous analysis of our experimental cross-sections and of available second virial coefficients has been carried out to obtain a reliable interaction for O2-O2 [5]. The same approach had been applied to the N2-N2 and 

O2-N2 systems, although in considerable less detail. Previous results on the N2-N2 system [44] have been reanalyzed, and a characterization of the N2-O2 complex has been presented a subsequent paper [10]. These results indicate that most of the bonding in the dimers comes from van der Waals (repulsion - - dispersion) and electrostatic (permanent quadrupole-permanent quadrupole) forces. On the other hand, chemical (spin-spin) contributions are not negligible for O2-O2, which is an open-shell-open-shell system [4,5]. Therefore the geometrical properties of the three dimers have been found to show interesting differences, to be seen in the next paragraph. 

It has been found useful to represent the interaction potential for a dimer of homonuclear diatomic molecules [4,5,46,58] as a spherical harmonic expansion, separating radial and angular dependencies. The radial coefficients include different types of contributions to the interaction potential (electrostatic, dispersion, repulsion due to overlap, induction, spin-spin coupling). For the three dimers of atmospheric relevance, we provided compact expansions, where the angular dependence is represented by spherical harmonics and truncating the series to a small number of physically motivated terms. The number of terms in the series are six for the N2-O2 systems, corresponding to the number of configurations of the dimer (for N2-N2 and O2-O2 this number of terms is reduced to five and four, respectively). 

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