INTRACAVITY-LASER-ABSORPTION SPECTROSCOPY OF THE VISIBLE OVERTONE TRANSITION OF METHANE IN A SUPERSONICALLY COOLED JET

We report the CH-stretching overtone spectrum of jet-cooled methane in the visible range. Due to the very weak intensity of the transition, the intracavity-laser-absorption-spectroscopy technique (ICLAS) was used with a free jet of methane from a slit orifice placed inside a dye-laser cavity. In this way, an equivalent pathlength through the jet of 800 m was achieved. The rotational cooling drastically reduces the congestion of the spectrum: whereas the room-temperature spectrum is extremely congested and complicated, the jet spectrum is sparse and reduced to well-isolated lines. These results show that rotational congestion is mainly responsible for the overtone band shape of methane and that, in consequence, the contribution of the background vibrational states is not predominant. The experimental method developed in this work can be extended to study the overtone transitions of a wide class of jet-cooled compounds.