Intracavity absorption spectroscopy of the third stretching overtone transition of jet cooled methane

The third stretching overtone transition of jet cooled methane has been recorded between 11 170 and 11 315 cm−1 by intracavity laser absorption spectroscopy associated with a supersonic slit jet expansion. By varying the methane percentage seeded in the carrier gas, we have studied the evolution of the spectrum vs the rotational temperature. The rotational cooling drastically reduces the congestion of the spectrum and only a few lines are observed in the spectrum corresponding to the lowest rotational temperature. In the 11 245–11 315 cm−1 range, the rotational structure of two superimposed bands is clearly observed. The lowest rotational levels of each band are assigned and the corresponding band origins, rotational constants and Coriolis parameters are obtained. The two bands are assigned to the two components of a Fermi dyad corresponding to the interaction of the 3ν1+ν3 (F2) stretching level carrying the oscillator strength with a near resonant vibrational state of the background. The high mixing of the vibrational wave functions is reflected by the close values of the rotational constants obtained for the two bands. In the lower energy range of the spectrum (11 170–11 245 cm−1), two weaker vibrational bands are tentatively assigned.