Knowledge Bank

A recent $analysisI$ of the spectrum of CO-Ar in the $4.7\mu m$ region was based on two complementary techniques: (1) a pulsed slit-jet supersonic source used with a tunable diode laser spectrometer, and (2) a long-path low-temperature static absorption cell used with a Bomem Fourier transform spectrometer. We have now extended this study by using a different combination of resources, namely the long-path cell with a diode laser spectrometer. This gives higher resolution and sensitivity than the FT spectrometer, and higher temperatures (and thus J- and K-values) than the supersonic jet. We used a diode laser manufactured by M""{u}tek, mounted in a conventional Laser Analytics cold head. The laser beam made 32 traversals of a 5.5 m cell, for a total path of 176 m at a temperature of 77 K. The total gas pressure was about 1 torr, with a Ar:CO ratio of about 10:I. The CO-Ar absorption was detected by frequency modulation of the diode followed by phase-sensitive demodulation of the transmitted signal at 2f. Wave number calibration was made by means of simultaneous recording of signals from an absorption cell containing $N2O$ and a temperature-stabilized solid Ge etalon The new data enable some significant extensions to be made in the assignments of the spectrum; for example, in the $Ka=2\leftarrow 1$ subband, the maximum J-value is raised from 19 to 28 in the Q-branch, and from 21 to 27 in the R-branch. Furthermore, the original analysis involved subbands with Ka-values of 3 or less, and we now find some evidence (also present in the FT spectra) for subbands with $Ka=6\leftarrow 5,5\leftarrow 4$, and $4\leftarrow 3$. But there appear to be some perturbations which make it difficult to be absolutely certain about these assignments.