Millimeterwave Absorption Spectroscopy with Supersonic-Jet Expansion Technique, Direct Observation of van der Waals Modes of the ArHCN Cluster

Abstract

Millimeterwave spectrometer cooperated with a pulsed supersonic jet nozzle and dc discharge was designed for the detection of transient species such as molecular cluster, radical and ion. Several atm of Ar seeded molecule to 1-3% was injected into a cell through a circular nozzle with a repetition rate of 10-30 Hz. Millimeterwave radiation whose frequency was modulated by a 83 kHz bipolar square wave was focused just below the nozzle and phase sensitively detected together with time-resolved technique. The rotational spectrum of methylacetylene $(CH_{3}CCH) J = 8\leftarrow 7$ was observed as a test in the 137 GHz region. The rotational temperature evaluated from the line intensities of the K = 1 and 2 components was less than 12 K at the point 15 mm below the nozzle. We employed the spectrometer for the direct observation of the absorption spectrum of the van der Waals (vdW) bend band of the ArHCN cluster. So far we have observed the five P-branch (J = 1-5) lines of the $\Sigma_{1} - \Sigma_{0}$ band in the 154 - 162 GHz region. The line intensities are comparable to those reported by `indirect' method, molecular beam electric resonance optothermal spectroscopy (1). The measurements are now carrying out for the more intense Q and R-branch lines of the $\Pi_{1} - \Sigma_{0}$ band in the 210 - 260 GHz region. The present technique will be applicable to the detection of the vdW mode bands of ArCS $(\omega_{b} - 8 cm^{-1})$ and ArHCCH $(\omega_{b} - 8.9 cm^{-1})$ whose bending vibration is estimated to be in the millimeterwave region.