Knowledge Bank

High resolution (RP = 200 000 or more) spectroscopy at high S/N of the very weak high vibrational overtone and combination spectra of simple molecules is demonstrated. Spectra of the 6190 \AA band of $C\Pi 4$ and the 6450 \AA band of $NH3$ are described in some detail. Both of these transitions are observed in the spectra of Jupiter and the $CH4$ transition is prominent in all the spectra of the major planets. The efficacy of the photoacoustic method is that very small pathlengths of gas are required, compared with usual long path measurements. The amounts, in column density, are reduced by more than a factor of ten thousand. This allows excellent spectra to be obtained with path lengths of a few centimeters, with pressures of 0.01 to 1 atmospheres, for most of the overtone spectra in the visible-near-infrared region of the spectrum which is accessible with dye lasers.