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Pulsed molecular beam cavity microwave Fourier transform spectroscopy of some fundamental van der Waals dimers and trimers Xu, Yunjie


The pure rotational spectra of several van der Waals dimers and trimers, namely Ar-Cl₂, Ne-Kr, Ne-Xe, Ar-Xe, Kr-Xe, Ne₂-Kr and Ne₂-Xe, as well as Ar₂-OCS and Ar₂-CO₂, have been investigated using a pulsed molecular beam cavity microwave Fourier transform spectrometer. Ar-Cl₂ has been found to have a T-shaped geometry dominated by pairwise additive forces, in contrast to the previously studied linear complex Ar-ClF. An effective van der Waals bond distance has been obtained, as well as an average vibrational angle for Cl₂. This study has not only confirmed the structure obtained from electronic spectroscopy, but has also provided a reliable estimate of the ³⁵Cl nuclear quadrupole coupling constant in the free chlorine molecule. In the microwave investigation of four mixed rare gas dimers, the newest available mixed rare gas pair potentials have been adjusted to obtain the equilibrium distances (re) between the rare gas atoms. The results have been compared with those obtained from isotopic data. It has been found that the present literature re values for these dimers had been underestimated in the order of 1~2%. The quadrupole coupling constants of ⁸³Kr and ¹³¹Xe have been determined for several different species containing these nuclei. The magnitudes of these constants are in the order of several hundreds of kHz, which are surprisingly large. The induced dipole moments are estimated to be in the order of 0.01 D. Transitions of four isotopomers of ²⁰Ne₂-Kr and of six isotopomers of ²⁰Ne₂-Xe, all with C₂v symmetry, as well as two mixed isotopomers of ²⁰Ne²²Ne-Kr, have been measured. A recent theoretical model derived for such floppy systems has been used to obtain structural parameters. The structures, harmonic force constants, ⁸³Kr and ¹³¹Xe nuclear quadrupole coupling constants and dipole moments have been compared with the corresponding parameters of the dimers. Possible nonpairwise additive contributions to these properties have been deduced from deviations of the experimental values from those obtained from corresponding dimers assuming pairwise additivity. The complexes Ar₂-OCS and Ar₂-CO₂ have been found to have distorted tetrahedral geometries (C₂ and C₂v symmetry, respectively), with the linear molecule lying in the symmetry plane. Harmonic force field analyses have been performed for Ar₂-OCS and Ar₂-CO₂. The structures and force constants obtained indicate a dominance of pairwise additivity. The basic geometric trend in Ar₂-molecule trimers is discussed.

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