Optothermal-infrared and pulsed-nozzle Fourier-transform microwave spectroscopy of rare gas–CO2 complexes

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Abstract
Sub‐Doppler infrared spectra of Ne–CO2, Ar–CO2, and Kr–CO2 have been recorded near 3613 and 3715 cm− 1, in the region of the 2ν0 2+ν3/ν1+ν3 Fermi diad of CO2, using an optothermal molecular‐beam color‐center laser spectrometer. In addition, pulsed‐nozzle Fourier‐transform microwave spectra are reported for the ground vibrational states of the complexes. The infrared and microwave spectra are consistent with T‐shaped complexes as shown originally by Steed, Dixon, and Klemperer for Ar–CO2.1 The infrared band origins for the Ar and Kr complexes are red shifted, from that of free CO2, by 1.09 and 0.95 cm− 1 for Ar–CO2 and by 1.97 and 1.76 cm− 1 for 8 4Kr–CO2. For Ne–CO2, blue shifts of 0.15 and 0.19 cm− 1 are observed. The lower Fermi components are free of perturbations, whereas the upper components of Ar–CO2 and Kr–CO2 are perturbed. For Ar–CO2 the perturbation is strong, shifting the positions of the observed Q‐branch lines of the K a =1←0 subband by as much as 500 MHz.