Spectroscopic investigation of the ternary Ne–Ne–OCS van der Waals cluster: additive and non-additive interactions

Abstract

The ternary Ne–Ne–OCS cluster was generated in a pulsed molecular beam and was probed using a Fourier transform microwave spectrometer. In all, nine isotopomers, namely ²⁰Ne–²⁰Ne–OCS, ²⁰Ne–²²Ne–OCS, ²²Ne–²²Ne–OCS, ²⁰Ne–²⁰Ne–OC³⁴S, ²⁰Ne–²²Ne–OC³⁴S, ²²Ne–²²Ne–OC³⁴S, ²⁰Ne–²⁰Ne–O¹³CS, ²⁰Ne–²²Ne–O¹³CS, and ²²Ne–²²Ne–O¹³CS were studied in their natural abundances, and their rotational transitions were observed and assigned. The measured transition frequencies could be fitted adequately using a semirigid rotor model. The resulting rotational constants were used to derive structural parameters. A harmonic force field analysis based on the quartic centrifugal distortion constants was performed and was used to predict the harmonic vibrational frequencies of the van der Waals vibrational modes. For the isotopomers with C_s symmetry, i.e. ²⁰Ne–²⁰Ne and ²²Ne–²²Ne containing species, both a- and c-type transitions were observed, while for the asymmetric isotopomers, i.e. species with ²⁰Ne–²²Ne, additional, weak b-type transitions were detected. These observations are qualitatively in accord with a geometry based on a pairwise additive model. More detailed structural analyses indicate that the ternary cluster is indeed dominated by additive intermolecular interactions. However, a significant lengthening of the Ne–Ne bond as compared with the Ne–Ne dimer was detected. This is indicative of the presence of three-body non-additive contributions to the interaction energy of the ternary cluster.