Ian W. M. Smith

Rate constants for the reactions of CN with hydrocarbons at low and ultra-low temperatures

Abstract The pulsed laser-photolysis (PLP), time-resolved laser-induced fluorescence (LIF) technique has been used to study the reactions of the CN radical with CH 4 , C 2 H 6 , C 2 H 4 , C 3 H 6 and C 2 H 2 at low and ultra-low temperatures. Using a cryogenically cooled cell, rate constants have been determined for all five reactions at temperatures down to 160 K. The PLP-LIF method has also been implemented in a CRESU (cinetique de reaction en ecoulement supersonique uniforme) apparatus providing rate constants for the reactions of CN with C 2 H 6 , C 2 H 4 and C 2 H 2 at temperatures down to 25 K. The rate constants for CN+C 2 H 4 and CN+C 2 H 2 increase monotonically as the temperature is lowered from room temperature to 44 K but the values at 25 K are lower than those at 44 K. Remarkably, the rate constant for CN+C 2 H 6 increases below 75 K, reaching its largest value at 25 K. It is tentatively suggested that this behaviour may reflect the transient formation of an energised van der Waals complex which facilitates the subsequent abstraction of an H atom.

GAS-PHASE REACTIONS AND ENERGY TRANSFER AT VERY LOW TEMPERATURES

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.

Ultra‐low temperature kinetics of neutral–neutral reactions: The reaction CN+O2 down to 26 K

A new method is described which enables the measurement for the first time of the rates of neutral–neutral gas‐phase reactions at temperatures down to 26 K (and in the future, below). Results for the reaction of CN radicals with O2 are presented and discussed in terms of current theoretical treatments.

Low temperature rate coefficients for the reactions of CN and C2H radicals with allene (CH2CCH2) and methyl acetylene (CH3CCH)

Abstract Using a continuous flow CRESU (Cinetique de Reaction en Ecoulement Supersonique Uniforme or Reaction Kinetics in Uniform Supersonic Flow) apparatus, rate coefficients have been measured for the reactions of the cyanogen (CN) and ethynyl (C 2 H) radicals with allene (CH 2 CCH 2 ) and methyl acetylene (CH 3 CCH) at temperatures from 295 down to 15 K for the reactions of CN and down to 63 K for those of C 2 H. All four reactions occur at rates close to the collision-determined limit. The results are compared with those obtained earlier for the reactions of other alkenes and alkynes, and, in the accompanying Letter by Vakhtin et al., with results for C 2 H+CH 2 CCH 2 and C 2 H+CH 3 CCH obtained at 103 K using a pulsed Laval apparatus. The implications of these latest results for the chemistry of interstellar clouds and planetary atmospheres are discussed.

Rate coefficients for the reactions of C(

Rate coefficients for the reactions of ground state carbon atoms, C( 3 P J ), with the four unsaturated hydrocarbons C 2 H 2 , C 2 H 4 ,

\mathsf{^3}

{\rm CH_3C\equiv CH}

P

and

\mathsf{{\it _J}}

{\rm H_2C=C=CH_2}

) atoms with C

have been measured at temperatures down to 15 K. The experiments have been performed in a continuous flow CRESU (Cinetique de Reaction en Ecoulement Supersonique Uniforme) apparatus using pulsed laser photolysis of C 3 O 2 to generate C( 3 P J ) atoms and laser-induced fluorescence in the vacuum ultraviolet to observe the kinetic decays of the atoms and hence determine the rate coefficients. All four reactions are found to occur at rates close to the collision-determined limit and the rates show at most a very mild dependence on temperature. It is argued that normally these reactions proceed along a reaction path with no barrier to form a strongly bound energised adduct, which subsequently eliminates an H atom to form the reaction products. The rates of the reactions of C( 3 P J ) atoms with larger unsaturated hydrocarbons are discussed. Finally, the relevance of the results for the chemistry of dense interstellar clouds is considered.

\mathsf{_2}

Infrared-ultraviolet double resonance (IRUVDR) experiments have been implemented in the ultra-cold environment provided by a CRESU (Cinetique de Reaction en Ecoulement Supersonique Uniforme) apparatus. With this technique rate coefficients of two kinds have been measured for rotational energy transfer in collisions between NO and He, Ar and N2: (a) rate coefficients for total removal from specific states of NO(X 2Π1/2; v=3; J=0.5, 3.5 or 6.5) and (b) state-to-state rate coefficients for rotational energy transfer from these levels to specific final states. Using different Laval nozzles, results have been obtained at several different temperatures: for He as collision partner, 295, 149, 63, 27, 15 and 7 K; for Ar, 139, 53, 44 and 27 K; and for N2, 86 and 47 K. The thermally averaged cross-sections for total removal show remarkably little variation, either with temperature or with initial rotational state. The variation of state-to-state rate coefficients with ΔJ shows three general features: (i) a decrease...