INFRARED SPECTRUM OF Co-He COMPLEXES IN THE 4.7 $\mu$m REGION
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Date
1992
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Ohio State University
Abstract
Well-resolved spectra of the very weakly bound complex Co-He have been observed in the region of the CO fundamental band around $2140 cm^{-1}$ using a long-path low-temperature gas cell and a Bomem FT spectrometer. Typical experimental conditions were similar to those used $earlier^{1}$ for $Co-H_{2}$, namely: path=84 m, temperature=48 K, CO pressure=0.25 Torr, He pressure=6 Torr, and resolution$=0.009 cm^{-1}$. Results were obtained using both $^{4}He$ and $^{3}He$. The observed spectra are weak but very simple, and can be assigned in terms of only 13 bound rotational levels for $CO-^{4}$ He. In terms of conventional semirigid molecule notation ($v_{2}$, J, $K_{a}$, these are the ground state levels (0,J,0) with $J=0$ to 4, the two sets of $K=1$ levels (0,J,1) with $J=1$ to 3, and the excited bending levels (1,J,0) with $J=0$ and 1. For $CO-^{3}He$, there are only 9 levels. In contrast to $Co-Ar,^{2}$ parallel ($\Delta K=0$) transitions are observed in addition to the perpendicular ($\Delta K=1$) ones expected for a T-shaped complex. It is perhaps more appropriate to use free intgernal rotor quantum numbers, (J,j,l), where j signifies the CO rotation and l the end-over-end rotation of the complex. The strongest transitions are then those that correlate with the CO monomer P and R lines, namely those with $\Delta j=1$ and $\Delta l=0$. All the observed levels have $j=0$ or 1; evidently levels of the complex with $j=2$ and higher are too short-lived to give detectable transitions. An analysis of the spectrum directly in terms of the CO-He intermolecular potential energy surface has been performed by Chuaqui and Le Roy. Their calculated line positions and intensities were used to assign the weaker lines in the spectrum, and the resulting potentials provide a precise a precise description of the forces between CO and He in the region o the attractive well for both the $v=0$ and 1 vibrational states of Co.
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1.A.R.W. McKellar, Chem. Phys. Lett. 186 58 (1991). 2. A.R.W. McKellar, Y.P. Zena, S.W. Sharpe, C. Witting, and R.A. Beaudet, J. Mol. Spectrose. 153 , to be published (1992).
Author Institution: Herzberg Institute of Astrophysics, National Research Council of Canada
Author Institution: Herzberg Institute of Astrophysics, National Research Council of Canada