Rotational and Vibrational Cooling in Pulsed High-Pressure Molecular Beam Expansions from 3 bar into the Supercritical Regime

Abstract

Experiments employing pulsed high-pressure (< or = 90 bar) supersonic jet expansions into a laser REMPI time-of-flight mass spectrometer are presented. Due to the very short opening time of the high-pressure valve, a compact arrangement with moderate pumping speed requirements is sufficient, which can be used up to 50 Hz repetition frequency, still maintaining an operating pressure in the expansion chamber of ca. 10(-4) mbar. p-Fluorotoluene was used as a rotational and vibrational "molecular thermometer" to characterize the cooling capabilities of high-pressure pulsed argon and CO(2) expansions into vacuum, spanning a wide range of stagnation pressures into the supercritical regime. Rotational and torsional temperatures were deduced from the contour of the S(0) --> S(1)(0-0) transition by an asymmetric-top/free-internal-rotor simulation and compared with the results of a simpler rigid-rotor asymmetric-top fit. The experiments show that average rotational temperatures of ca. 1-2 K in argon and 13-15 K in CO(2) can be reached at the highest pressures studied. The population of higher |m| torsional levels of p-fluorotoluene is clearly demonstrated by the appearance of characteristic features on the blue edge of the contours, which are more pronounced in the warmer CO(2) expansions. While the rotational temperatures in argon expansions compare well with estimated gas dynamical terminal translational temperatures, there are considerable differences in the case of CO(2). Still, the degree of internal cooling reached with CO(2) is sufficiently low, so that a setup of this kind might provide good opportunities for future studies of thermally labile low-volatile molecules to cool them to cryogenic temperatures low enough to achieve a sufficient simplification of their spectra.