DIRECT ABSORPTION OBSERVATION OF THE VAN DER WAALS BENDING BAND OF ArHCN BY MILLIMETERWAVE SPECTROSCOPY COMBINED WITH PULSED-JET EXPANSION TECHNIQUE, EXTENSION TO THE $j=2.1$ HOT BAND

Abstract

Millimeterwave spectrometer combined with a pulsed supersonic jet nozzle was applied to observe the absorption spectra of the van der Waals bending band of ArHCN in the 150 - 270 GHz region. The transitions near band origin have been studied by EROS and the band origins were reported [1]. Seventeen rovibrational transitions split into hyperfine structure due to the nitrogen nucleus were newly observed for both subband $\Sigma_{1} - \Sigma_{0}$ and $\Pi_{1}$ of the $j = 1 - 0$ fundamental band. An improved set of molecular constants, including the band origins, rotational constants, quadrupole coupling constants, and the Coriolis coupling constant between the $\Sigma_{1}$ and $\Pi_{1}$ bending substates , was determined. More than one hundread of rather weak lines, of which intensities are about one third or less of the $j = 1 - 0$ fundamental liens, were observed. These lines are due to the hot band (j = 2 - 1 and 3 - 2) transitions of the vdW bending transitions. Some of the lines are tentatively assigned to the $R$-branch lines of the $\Sigma_{2} - \Pi_{1}$ band. Further analysis will give us the molecular structure in the excited state of vdw bend mode.