DIODE-LASER STUDY OF THE ROTATIONAL REDISTRIBUTION IN CH3OH DURING A SUPERSONIC JET EXPANSION

After rapid cooling in a supersonic jet expansion, the populations of the lowest rotational levels of methanol have been determined from the intensities of IR absorption transitions of the C-O stretching band a t 10 mum. A newly constructed multireflection system (White type, 16 p asses) has been employed for the detection of weak absorption lines. T he rotational temperature of methanol has been found to be lower for t he E-symmetry substate of the ground (upsilon(t) = 0) torsional state than for the A-symmetry substate: T(rot)(A-Sym) = (15.8 +/- 0.5) K and T(rot)(E-Sym) = ( 1 3.3 +/- 0.9) K. No torsional hot-band transitions have been observed in the jet spectrum, indicating an efficient cooli ng of the torsional degree of freedom. The population distribution of the probed rotational levels is strongly nonthermal, or ''normal,'' in the sense that the A/E population ratio maintains its room temperatur e value of near unity. (C) 1994 Academic Press, Inc.