Infrared diode-laser spectroscopy and Fourier-transform microwave spectroscopy of the (CO2,CO) dimer in a pulsed jet

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Abstract
Two techniques have been used to observe the spectrum of a weakly bound dimer (CO2, CO) formed in a pulsed jet of CO2 and CO seeded in argon. The v=1←0 vibrational transition in the modified asymmetric stretching mode of the CO2 subunit was first observed at Doppler‐limited resolution by investigating the pulsed jet with an infrared diode‐laser spectrometer. Then the ground‐state pure rotational spectrum was detected by applying Fourier‐transform microwave spectroscopy to such a jet. A combined fit of the two spectra of the parent isotopomer gave the following set of ground‐state (v=0) and excited‐state (v=1) spectroscopic constants: A0=11 863.48(56) MHz, B0=1883.915(4) MHz, C0=1613.297(4) MHz, A1=11 772.17(37) MHz, B1=1882.76(6) MHz, C1=1611.44(5) MHz, ΔJ =8.145(2) kHz, ΔJK =339.42(1) kHz, δJ =1.26(1) kHz, and δK =309.8(4) kHz when the centrifugal distortion constants were constrained to be equal in both states. Spectroscopic constants for the isotopomers (CO2, 13CO) and (CO2, C18O) have also been determined. The nature of the observed spectra, especially the systematic absences of certain transitions, and the magnitudes of the spectroscopic constants have been interpreted in favor of a dimer with a T‐shaped geometry in which the CO molecule lies along the C2 axis of CO2, with the two C atoms contiguous and r0(C⋅⋅⋅C)=3.277 Å. The intermolecular stretching force constant has the small value kσ =2.7 N m−1.