D. Travers

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...

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.

Kinetics over a wide range of temperature (13–744 K): Rate constants for the reactions of CH(ν=0) with H2 and D2 and for the removal of CH(ν=1) by H2 and D2

Rate constants were determined for the reactions of CH(X2Π,ν=0) with H2 and D2 and for the relaxation of CH(X2Π,ν=1) by H2 and D2. The method, employing pulsed laser photolysis to generate CH radicals and laser-induced fluorescence to observe their rate of removal, was implemented between 744 and 86 K in heated and cryogenically cooled cells and from 295 to 13 K in a Cinetique de Reaction en Ecoulement Supersonique Uniforme (CRESU) apparatus. The rate constants for the reaction of CH(ν=0) with D2 were determined from 13 to 584 K and those for the removal of CH(ν=1) by H2 and D2 from 23 to 584 K. These rate constants show no dependence on total pressure and a mild negative temperature dependence, and they are clearly related closely to the rate of capture to form a strongly bound CH3* or CHD2* energized collision complex. The rate constants for the reaction of CH(ν=0) with H2 were measured from 53 to 744 K. By contrast, their values depend in a complex fashion on temperature and total pressure, the latter ...

Ultra-low temperature kinetics of neutral–neutral reactions: rate constants for the reactions of OH radicals with butenes between 295 and 23 K

The first experiments on the kinetics of reactions of the OH radical at temperatures below 80 K are reported. They have been carried out by applying the pulsed laser photolysis (PLP), laser-induced fluorescence (LIF) technique for studying the kinetics of free radical reactions in the ultra-cold environment provided by the gas flow in a CRESU (Cinetique de Reaction in Ecoulement Supersonique Uniforme) apparatus. This method has yielded rate constants for the reactions of OH with but-1-ene, (Z)-but-2-ene and (E)-but-2-ene at temperatures down to 23 K. The rate constants for all three reactions increase monotonically as the temperature is lowered and this dependence of the rate constants on temperature can be fitted to an empirical expression of the form (k/10–10 cm3 molecule–1 s–1)=a exp[b(T/298K)] with a and b equal to 5.2 and –2.8 for but-1-ene, 4.7 and –2.1 for (Z)-but-2-ene and 5.4 and –2.1 for (E)-but-2-ene.

Rate coefficients for the reactions of Si(3PJ) with C2H2 and C2H4: Experimental results down to 15 K

Rate coefficients for the reaction of ground-state silicon atoms Si(3PJ) with acetylene and ethylene 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 Si(CH3)4 to generate Si(3PJ) atoms and laser-induced fluorescence to observe the kinetic decay of the atoms and hence determine the rate coefficients. Both reactions are found to be fast, and the reaction rates show a very mild dependence on temperature. The rate coefficients match the expressions k(Si+C2H2)=(2.6±0.6)10−10(T/300)−(0.71±0.24) exp(−(29±10)/T) cm3 molecule−1 s−1 and k(Si+C2H4)=(3.7±0.3)10−10(T/300)−(0.34±0.10) exp(−(16±4)/T) cm3 molecule−1 s−1 in the temperature range 15–300 K. The nature of the products and the similarities of the carbon and silicon chemistry are discussed.

First Experimental Determination of the Absolute Gas-Phase Rate Coefficient for the Reaction of OH with 4-Hydroxy-2-Butanone (4H2B) at 294 K by Vapor Pressure Measurements of 4H2B

The reaction of the OH radicals with 4-hydroxy-2-butanone was investigated in the gas phase using an absolute rate method at room temperature and over the pressure range 10-330 Torr in He and air as diluent gases. The rate coefficients were measured using pulsed laser photolysis (PLP) of H(2)O(2) to produce OH and laser induced fluorescence (LIF) to measure the OH temporal profile. An average value of (4.8 ± 1.2) × 10(-12) cm(3) molecule(-1) s(-1) was obtained. The OH quantum yield following the 266 nm pulsed laser photolysis of 4-hydroxy-2-butanone was measured for the first time and found to be about 0.3%. The investigated kinetic study required accurate measurements of the vapor pressure of 4-hydroxy-2-butanone, which was measured using a static apparatus. The vapor pressure was found to range from 0.056 to 7.11 Torr between 254 and 323 K. This work provides the first absolute rate coefficients for the reaction of 4-hydroxy-2-butanone with OH and the first experimental saturated vapor pressures of the studied compound below 311 K. The obtained results are compared to those of the literature and the effects of the experimental conditions on the reactivity are examined. The calculated tropospheric lifetime obtained in this work suggests that once emitted into the atmosphere, 4H2B may contribute to the photochemical pollution in a local or regional scale.

Reactions of Electrons with Hydrocarbon Cations: From Linear Alkanes to Aromatic Species

In the framework of our systematic study of the recombination of hydrocarbon ions, we have already obtained results for classical aliphatic carbocations,1 small unsaturated rings2 and methyl substituted benzenic rings.3 The recombination rate constants of these ions range from 5 to 12 × 10−7 cm3s−1 and are independent of the number of carbon atoms or structure.

Pulsed injection of ions into the CRESU experiment

Abstract A new versatile experimental technique for the study of ion–molecule reactions at very low temperatures is presented. The technique is based upon the pulsed injection of ions into a uniform and isentropic supersonic expansion (CRESU experiment). Several ion/molecule reactions have been studied in order to test the setup and the reaction between CH + and CO has been investigated under low temperature conditions.

A comparison of flow velocities measured using an impact-pressure probe and electron time of flight in a supersonic flow. Implications for electron thermalization

The bulk velocity of electrons in a burst of plasma created in a uniform supersonic flow by a pulsed electron beam has been measured by a time-of-flight technique using a Langmuir probe. This velocity is compared with the neutral-species bulk velocity deduced from impact-pressure measurements. This comparison allows a determination of an upper limit of the electron drift velocity to be made, which in turn shows that electrons are well thermalized in the flow. Therefore this kind of flowing supersonic afterglow can be used for electron-attachment studies at very low temperatures.