J. L. Queffelec

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...

Laser induced fluorescence and vacuum ultraviolet spectroscopic studies of H‐atom production in the dissociative recombination of some protonated ions

The flowing afterglow technique, coupled with laser induced fluorescence (LIF) and vacuum ultraviolet (vuv) absorption spectroscopy, has been used to determine the fractional H‐atom contributions, fH, to the product distributions for the dissociative recombination of a series of protonated ions (N2H+, HCO+, HCO+2, N2OH+, OCSH+, H2CN+, H3O+, H3S+, NH+4, and CH+5 ) with electrons. The measurements were made at 300 K in two separate ways in two laboratories by (i) directly determining the H‐atom number density using vuv absorption spectroscopy at the Lα (121.6 nm) wavelength and (ii) converting the H atoms to OH radicals using the reaction H+NO2→OH+NO followed by LIF to determine the OH number density. The agreement between the two techniques is excellent and values of fH varying from ∼0.2 (for OCSH+ ) to 1.2 (for CH+5 ) have been obtained showing that in some of the cases recombination can lead to the ejection of two separate H atoms. Comparison of the oxygen/sulphur analogs, HCO+2/OCSH+ and H3O+/H3S+ showe...

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.

The dissociative recombination of N2+(v=0,1) as a source of metastable atoms in planetary atmospheres

Abstract The yield of metastable nitrogen atoms in dissociative recombination of N2+ (v = 0, 1)ions has been tudied for different experimental conditions. In a first experiment, the branching ratio for N(2D) production was directly measured as being higher than 1.85; for N2+ (v = 0) this implies that 2D + 2D is the main reaction channel; for N2+ (v = 1) a minor channel could be 2P + 2D, 2P being then quenched toward 2D by electrons. In a second experiment, at higher electron densities, the influence of superelastic collisions was studied; a steady state analysis yields the quenching rate coefficient k4, of 2D towards 4S equal to 2.4 × 10−10 cm3 s−1for Te = 3900 K and shows that 2D + 2D is always the major channel of the reaction for N2+ (v = 1), 2D + 2P being a minor channel. All these results are in good agreement with thermospheric models but imply that N2+ dissociative recombination is a less important source for nitrogen escape of Mars.

TEMPERATURE DEPENDENCE OF THE RATE CONSTANT FOR THE CL-+CH3BR REACTION DOWN TO 23 K

Rate constants measured for the Cl−+CH3Br→CH3Cl+Br− reaction increase by over two orders of magnitude when the temperature is reduced from 300 to 23 K. Calculations done with the rotating bond approximation provide a simple explanation of this unusually strong negative temperature dependence of the rate constant.

The yield of oxygen and hydrogen atoms through dissociative recombination of H2O+ ions with electrons

The branching ratios of the dissociative recombination of excited H2O+ ions with electrons were studied using a plasma flow tube experiment. The total oxygen atom channel accounts for 45% of the recombination with more than 24% in O+H+H. The remaining channel is OH+H for 55%. These results are discussed in comparison with the few available theories and with photodissociation data.

Further measurements of the H+3(v=0,1,2) dissociative recombination rate coefficient

A new flowing afterglow apparatus that utilizes a Langmuir probe/mass spectrometer to monitor both electron and ion decay in a hydrogen plasma has been used to measure the dissociative recombination rate coefficient of H+3 at two different electron temperatures. At 300 K a rate coefficient of 1.5×10−7 cm3 s−1 was found for H+3 ions with a low degree of vibrational excitation (v≤2). The rate coefficient for ground state ions H+3(v=0) was measured as 1.1×10−7 cm3 s−1 at 650 K. A discussion is given of the excitation states of H+3 ions in the afterglow in the light of slow deexcitation rates for low vibrational states. A new model for the recombination of H+3 is presented.

Experimental studies of cold electron attachment to SF6, CF3Br, and CCl2F2

A new technique has been used for the measurement of electron attachment rate coefficients for the molecules, SF6, CF3Br, and CCl2F2 at temperatures between 48 and 170 K. The results demonstrate very clearly the strong effect that internal vibrational energy of the molecules has on the attachment process.

The yield of metastable atoms through dissociative recombination of O+2 ions with electrons

The yield of metastable oxygen atoms through dissociative recombination of O+2 ions with electrons has been studied using a plasma flow tube experiment. For O+2 with high vibrational excitation (around v=9) it was found that half of the oxygen atoms are formed on the O(1D) state and that the branching ratio toward O(1S) is large (∼0.4). Using Xe+ instead of Ar+ as precursor ions, it was shown that the O(1S) yield is much less for ions with low vibrational excitation. However, the present experimental results are not compatible with the extremely low theoretical value of this yield which was reported recently for O+2 (v=0).

Flowing Afterglow Langmuir Probe measurement of the N+2(v=0) dissociative recombination rate coefficient

A measurement of the dissociative recombination coefficient α is presented for reactions of electrons with ground state N+2(X 2Σ+g,v=0) ions at 300 K. The measurement has been made under truly thermalized conditions using a Flowing Afterglow Langmuir Probe (FALP). The rate constant was determined to be α[N+2(v=0)]=2.6×10−7 cm3 s−1 which is in good agreement with previous data except for the latest merged beam experiment.