Gas-Phase Reactions and Energy Transfer at Very Low Temperatures

Abstract

Experimental studies of gas-phase chemical reactions and molecular energy transfer at very low temperatures and between electrically neutral species are reviewed. Although work of collisionally induced vibrational and rotational transfer is described, emphasis is placed on very recent results on the rates of free radical reactions obtained by applying the pulsed laser photolysis (PLP)-laser-induced fluorescence (LIF) technique in a CRESU (Cinétique de Réactions en Ecoulement Supersonique Uniforme) apparatus at temperatures as low as 13 K. These measurements demonstrate that quite a wide variety of reactions-including those between two radicals, those between radicals and unsaturated molecules, and even some of those between radicals and saturated molecules-remain rapid at very low temperatures. Theoretical efforts to explain some of these results are described, as is their impact on attempts to model the synthesis of molecules in interstellar clouds.

Ultracold molecules and ultracold chemistry

“…[53,58]) in which a large diameter Laval nozzle is used to obtain genuine thermal equilibrium within a supersonic beam at temperatures as low as 7 K, but more typically down to 15 K. Reactive species are generated either by pulsed laser photolysis or photoionization within a helium beam using a high backing pressure and high densities (∼10 16 molecules cm −3 ). Reactive collisions in the jet occur in the 100-500 µs during which the flow remains uniform.…”

Dynamics of gas phase Ne* + NH3 and Ne* + ND3 Penning ionisation at low temperatures

“…10 Here, one reactant and a precursor to the second one are co-expanded in a uniform expansion through a Laval nozzle. The reaction is induced by the formation of the second reactant by electron bombardment or with a laser.…”

Fractional abundances of molecules in dense interstellar clouds: A compendium of recent model results

“…Recent laboratory work (Sims & Smith 1995) has shown that certain classes of neutralneutral reactions can occur rapidly at very low temperature, but the extent of this phenomenon is still not determined. In general, the inclusion of rapid neutral-neutral reactions tends to depress the calculated early-time abundances of complex molecules, sometimes catastrophically (Herbst et al 1994).…”