THE VISIBLE PHOTOACOUSTIC SPECTRA OF VAPOR-PHASE METHANE AND DEUTERATED METHANES AT 77 K

Abstract

We have constructed a longitudinally resonant photoacoustic cell which can be cooled to liquid nitrogen temperature. This cell has been used intracavity in our automated dye laser $spectrometer^{1}$ to obtain low temperature, Doppler-limited spectra in the visible. Working at 77 k has two advantages: first, the Doppler width is 1/2 of its room temperature value; second, the rotational partition function is about 8 times smaller (for nonlinear molecules). Both of these features improve normally congested spectra. We have taken spectra of the C-H stretching overtones of methane and of deuterated methanes in the visible. The low temperature spectrum of $CH_{4}$ shows Doppler-width features, although the spectrum is still very congested. We have also studied $CD_{3}H$ at both room temperature and at 77 K. At room temperature, the rotational analysis is complicated by overlapping bands, and by incompletely resolved K-structure. We will present a rotational analysis of the low temperature spectrumof $CD_{3}H$ which shows completely resolved structures. $^{1}$K.K. Lehmann, G.J. Scherer and W. Klemperer, J. Chem. Phys. 77. 2853 (1982).