Ian R. Sims

A KINETIC DATABASE FOR ASTROCHEMISTRY (KIDA)

We present a novel chemical database for gas-phase astrochemistry. Named the KInetic Database for Astrochemistry (KIDA), this database consists of gas-phase reactions with rate coefficients and uncertainties that will be vetted to the greatest extent possible. Submissions of measured and calculated rate coefficients are welcome, and will be studied by experts before inclusion into the database. Besides providing kinetic information for the interstellar medium, KIDA is planned to contain such data for planetary atmospheres and for circumstellar envelopes. Each year, a subset of the reactions in the database (kida.uva) will be provided as a network for the simulation of the chemistry of dense interstellar clouds with temperatures between 10 K and 300 K. We also provide a code, named Nahoon, to study the time-dependent gas-phase chemistry of zero-dimensional and one-dimensional interstellar sources.

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.

Ultralow temperature kinetics of neutral–neutral reactions. The technique and results for the reactions CN+O2 down to 13 K and CN+NH3 down to 25 K

An entirely new experimental method is described which enables the rate constants of neutral–neutral gas‐phase reactions to be measured at ultralow temperatures. The measurements are made by applying the pulsed laser photolysis (PLP), laser‐induced fluorescence (LIF) technique of studying the kinetics of free radical reactions in the ultracold environment provided by the gas flow in a Cinetique de Reaction en Ecoulement Supersonique Uniforme (CRESU) apparatus. The experimental method is described in some detail and its application and limitations are discussed. Results are reported for the reactions of CN radicals with O2 and NH3. For reaction (1) between CN and O2 data are reported for the temperature range T=13–295 K and the rate constants are well‐matched by the expression k1(T)=(2.49±0.17)×10−11 (T/298)(−0.63±0.04) cm3 molecule−1 s−1. For reaction (2) between CN and NH3, rate constants in the temperature range T=25–295 K fit the expression k2(T)=(2.77±0.67)×10−11 (T/298)(−1.14±0.15) cm3 molecule−1 s−1...

The 2014 KIDA network for interstellar chemistry

Chemical models used to study the chemical composition of the gas and the ices in the interstellar medium are based on a network of chemical reactions and associated rate coefficients. These reacti ...

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.

Neutral–neutral reactions at the temperatures of interstellar clouds Rate coefficients for reactions of C2H radicals with O2, C2H2, C2H4 and C3H6 down to 15 K

A CRESU (Cine′tique de Re′action en Ecoulement Supersonique Uniforme) apparatus has been used to measure rate constants for the reactions of the ethynyl radical (C2H) with O2, C2H2, C2H4 and C3H6 at temperatures from 295 down to 15 K. C2H radicals are generated by photolysis of C2H2 at 193 nm using an ArF excimer laser and reaction rates are determined by observing the chemiluminescence from CH(A2Δ) which is generated in a minor channel of the reaction between C2H radicals and O2. The rate constants for all four reactions increase as the temperature is lowered, and those for reactions with the unsaturated hydrocarbons exceed 10-10 cm3 molecule-1 s-1 at all temperatures below 100 K. The results confirm that C2H radicals, like CN radicals, react rapidly with unsaturated hydrocarbons at very low temperatures and, in both cases, the radical replaces an H atom in the unsaturated molecule. It therefore seems likely that combinations of these reactions play a major role in forming species with long C chains and in synthesising the cyanopolyynes in dense interstellar clouds.

Kinetics and Dynamics of the S(1D2) + H2 → SH + H Reaction at Very Low Temperatures and Collision Energies

We report combined studies on the prototypical S(1D2) + H2 insertion reaction. Kinetics and crossed-beam experiments are performed in experimental conditions approaching the cold energy regime, yielding absolute rate coefficients down to 5.8 K and relative integral cross sections to collision energies as low as 0.68 meV. They are supported by quantum calculations on a potential energy surface treating long-range interactions accurately. All results are consistent and the excitation function behavior is explained in terms of the cumulative contribution of various partial waves.

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 ,