Christian Naulin

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.

C2 Radicals in a supersonic molecular beam. Radiative lifetime of the d 3Πg state measured by laser-induced fluorescence

The combined techniques of pulsed supersonic molecular beams and radical generation by laser vaporisation of the corresponding solid target are used to produce C2 radicals seeded in a rare gas carrier beam. Extreme rotational cooling in the expansion, together with collision-free beam conditions, are shown to be particularly well suited for actual lifetime measurements. Analysis of laser-induced fluorescence decays of C2(d3Πg) species yields the following lifetime values for the first three vibrational levels of d state: τν=O = 101.8 ± 4.2 ns, τν=1 = 96.7 ± 5.2 ns, and τν=2 104 ± 17 ns.

Crossed-beam study of the Al()+O2() → AlO()+O() reaction at low and very low kinetic energies

Abstract The title reaction is revisited at kinetic energies between 255 and 6 meV with a new crossed-beam apparatus. The AlO dissociation energy is evaluated to be D0(AlO)=5.27±0.01 eV. Al( 2 P 1/2 ) and Al( 2 P 3/2 ) spin–orbit reactivities are shown to be different. The relative cross-section of Al( 2 P 1/2 )+O2 is found to increase monotonically when decreasing kinetic energy. The variation of the calculated thermal rate constant is in agreement with kinetic measurements performed between 1083 and 23 K. Good agreement is also found with theoretical treatment based on the adiabatic capture centrifugal sudden approximation.

Rate coefficients and cross-sections for the reactions of C) atoms with methylacetylene and allene

Abstract The reactivity of C ( 3 P J ) atoms towards methylacetylene and allene has been investigated using complementary experimental methods. Kinetic measurements performed over the range T=15–295 K have yielded thermal rate coefficients: (2.7±0.6)×10 −10 (T/298 K ) −(0.11±0.07) and (3.5±0.8)×10 −10 (T/298 K ) −(0.01±0.12) cm 3 molecule −1 s −1 , respectively. Crossed beam experiments conducted in the range of relative translational energies e tr =4.9–282 meV have yielded integral cross-sections for H ( 2 S 1/2 ) production that are proportional to ( e tr ) −0.56±0.02 and ( e tr ) −0.55±0.01 , respectively, in excellent agreement with the temperature dependencies of the rate coefficients.

The dissociation energy of the SiN radical determined from a crossed molecular beam study of the Si+N2O→SiN+NO reaction

Abstract The Si( 3 P J )+N 2 O(X 1 Σ + →SiN(X 2 Σ + )+NO(X 2 Π r ) reaction is studied using pulsed, crossed, supersonic molecular The SiN(X 2 Σ + ) product is probed by laser-induced fluorescence. The reaction exhibits a translational energy threshold between 0.2 and 0.3 eV. The determination of the SiN population limit just above the threshold leads to a reaction endoergicity: 0.12⩽Δe 0 ⩽0.38 eV, which combined with D 0 (N−NO)=4.93±0.01 eV yields: 4.54⩽ D 0 (SiN)⩽4.82 eV. This value is in agreemen with a recent ab initio result from Curtiss, Raghavachari, Trucks and Pople (4.58 eV) derived from the so-called Gaussian-2 theory. The validity of the experimental approach is also discussed.

The C (3PJ) + NO(X 2Πr)→ CN (X 2Σ+) + O (3PJ) reaction dynamics studied at 0.06 and 0.23 eV relative translational energy in pulsed crossed supersonic molecular beams

Abstract The C ( 3 P J ) + NO (X 2 Π r )→CN (X 2 Σ + ) + O ( 3 P J ) reaction is studied with pulsed, crossed, supersonic, molec translational energies of 0.06 and 0.23 eV. The rovibrational distributions of the CN (X 2 Σ + ) product are determined by laser-induced fluorescence. The experimental evidence strongly suggests that the reaction proceeds via a pseudo-direct mechanism. The rovibrational distributions differ from the prior outcome. At lower collision energy, the fraction of available energy channelled into vibration and rotation is 〈ƒ V 〉 = 0.30 and 〈ƒ R 〉 = 0.15, respectively. At higher collision energy, experiments show that the additional translational energy is converted largely into CN product rotation (55%) and vibration (28%). Evidence is also found for forward scattering. The agreement with theoretical results is discussed.

Combined crossed-beam studies of C(3PJ)+C2H4→C3H3+H reaction dynamics between 0.49 and 30.8 kJ mol−1

The reaction C(3PJ)+C2H4(X 1A1)→C3H3+H(2S1/2) has been studied using complementary crossed molecular beam techniques. Integral cross sections have been obtained in the range of relative translational energies ET=0.49–24.9 kJ mol−1 in experiments conducted with pulsed supersonic beams coupled with laser-induced fluorescence detection of H(2S1/2) atoms. The major reaction pathway leading to HCCCH2 (propargyl)+H has been found without any barrier, with relative integral cross sections that are proportional to (ET)−0.60±0.03 below 8 kJ mol−1. Threshold for a minor pathway, leading also to H formation, occurs around 6 kJ mol−1; the relative importance of this second pathway increases with relative translational energy. Differential cross sections have been obtained at three relative translational energies: ET=9.1, 17.2, and 30.8 kJ mol−1 in experiments conducted with continuous supersonic molecular beams coupled with universal mass spectrometric detection and time-of-flight analysis. At the lowest ET of 9.1 kJ...

The dissociation energy of TiO determined from a crossed-beam study of the Ti + NO → TiO + N reaction

Abstract The Ti + NO → TiO + N reaction has been studied with pulsed, crossed, supersonic molecular beams, at relative translational energies between 0.2 and 0.6 eV, with the Ti, NO reactants and the TiO product being probed by laser-induced fluorescence. The reactive channel Ti ( 3 F J ) + NO ( X 2 Π r ) → TiO ( a 1 Δ) + N ( 4 S 3 2 ) has been identified through the detection of the β( c 1 Φ ← a 1 Δ) Δv = 0 transitions of TiO. Analysis of the TiO ( a 1 Δ) rovibrational energy close to the threshold for production of v = 1 leads to a reaction exoergicity, Δe 0 , of −0.37 −0.05 +0.07 eV for the production of ground-state products TiO ( X 3 Δ) + N ( 4 S 3 2 ) and to a TiO bond-dissociation energy, D 0 (TiO), equal to 6.87 −0.05 +0.07 eV.

Kinematic Effects on Laser-induced Fluorescence Measurements Performed in Reactive Crossed Beam Experiments

A simple, realistic model is developed to take into account kinematic effects on laser-induced fluorescence (LIF) measurements in crossed beam reactive scattering experiments. The conversion factor from nascent populations to measured densities (which are proportional to the LIF intensity) is calculated for several cases of practical interest. The density-flux transformation proposed by Zare and coworkers arises from the model as a limiting case. Results concerning the C

Experimental search for scattering resonances in near cold molecular collisions

We review the experimental findings on scattering resonances in molecular collisions occurring in the energy domain equivalent to temperatures of a few Kelvin. We discuss the conditions required to observe shape (or orbiting) and Feshbach resonances, the crucial factor being in all cases the number of partial waves involved in the process. This implies operating with supersonic beams with matched and low velocities, high velocity resolution and low beam-intersection angle. This is exemplified for elastic, inelastic and reactive collisions, where different strategies, thence experimental arrangements, are used. We review the first observation of resonances in elastic collisions from the 1970s and the recent results in inelastic and reactive collisions. The most striking result is obtained for O2 + para-H2 inelastic collisions for which rotational excitation occurs through a pure quantum process, yielding a well-resolved structure in the integral cross section assigned to definite well-separated partial waves.