SYMMETRY SELECTIVE COOLING IN THE SUPERSONIC JET EXPANSION OF METHANE

The rotational temperatures of methane in the supersonic jet expansion seeded in argon have been measured precisely by a diode laser spectrometer in relation to the distance from the nozzle. A remarkable difference has been observed between the ground state populations of the two J = 2 levels monitored by the ν4 rovibrational transitions near 1300 cm-1; one level is of symmetry E and the other of symmetry F2, with measured rotational excitation temperatures of about 20 and 8 K, respectively. This fact furnishes direct evidence that the nuclear spin modification is conserved during the collisional process, whereas the rotational relaxation is fast. © 1991.