M. Garay

Collision energy and product polarization effects in the Ca*(1D2) + HCL → CaCl*(A, B) + H reaction

The collision energy dependence of the Ca(1D2)+ HCl → CaCl(A)+ H reaction cross-section has been measured over the low-energy range using the time-of-flight technique under crossed-beam conditions. The excitation function shows a step-like functionality which seems to be of a non-classical nature. These steps could be associated with the opening of internal transition states that become available as the total energy increases.In addition beam–gas experiments have been carried out to measure the polarization of the CaCl*(B) emission to the ground electronic state. The collision-energy effects and the deviation from the kinematic limit of the product polarization have been also measured and they show a strong product rotational momentum alignment. The reaction stereodynamics are discussed with the aid of a dynamical model for kinematically constrained reactions. Finally a comparison of the present (full-collision) cross-beam reaction with the van der Waals (half-collision) photoinitiated reaction is also made.

Reaction cross-section and product polarization in the Ca(1D2) + HBr → CaBr(A,B) + H reaction☆

Abstract The Ca ( 1 D 2 ) + HBr → CaBr(A,B) + H reaction has been studied by measuring its chemiluminescence at an average collision energy of ET = 0.16 eV under beam-gas conditions. Absolute values of the reaction cross-section for both chemiluminescence channels (e.g., A and B electronic states) were found to be σ A = 2.2 ± 0.9 A 2 and σ B = 0.52 ± 0.24 A 2 . In addition, the product polarization of the chemiluminescence emission was also measured, indicating a strong product rotational angular momentum alignment. The results are discussed in the light of a recent stereodynamical model for these, heavy + heavy-light, kinematically constrained reactions.

Influence of the radical size on the Ca∗ + ROH → CaOH∗ + R (RCH3, C2H5 and C3H7) reaction cross section

Abstract Absolute values of the reaction cross section, σ R , for the title reaction family have been measured by chemiluminescence emission under beam-gas conditions. The σ R values diminish as the radical group R increases in size, showing an important overall steric effect. These results are discussed in the light of simple models for reaction stereodynamics.

Determination of D00 (Ca…HBr) from full and half collision studies. Van der Waals and charge transfer contributions to the formation of Ca…HX (X = Cl, Br) complexes

Abstract The dissociation energy of the weakly bound complex Ca…HBr in it ground state D 0 0 (Ca…HBr), has been determined using energy balance arguments. Byu combining collisional information from molecular beam studies on the Ca ( 3 P ) + HBr → CaBr(A) 2 II ) + h reaction and spectroscopic infomration from the emission of the product ogf the reaction Ca + HBr → Ca…HBr → hv CaBr (A 2 II) + H, we find D 0 0 (Ca…HBr) ≤ 270 ± 50 meV. The measured bond enery is duiscussed as due to the combined effect of interactions of van der Waals and ionic type. An empirical description of both these contributions accounts for the results obtained for Ca…HBr, as well as for those previously reported for Ca…HCl.

Reaction dynamics of the (R CH3, C2H5 and n-C3 H7) system: chemiluminescence, photon yield, electronic branching ratio and influence of the radical group

Abstract The Ca ( 1 D 2 , 3 P J ) + RI ( R  CH 3 , C 2 H 5 , n - C 3 H 7) → CaI ( A,B ) + R reaction system has been studied by measuring its chemiluminescence under beam gas conditions. Absolute values of the reaction cross-sections were determined at low collision energy E T ≤ 0.20 eV . In addition, the electronic branching ratio, photon yield and influence of the radical size group on the reaction cross-section have been estimated. The results are discussed in the light of different reaction mechanisms involving the attacking Ca atom into the Cz.sbnd;I bond.

On the electronic energy disposal of calcium excited atom reactions with halogen-containing compounds: Electronic branching ratio and spin–orbit state populations

Abstract This paper reports on the electronic energy disposal of the Ca( 1 D 2 )+RX → CaX ∗ +R reaction system in which R=CH3, H and X=Cl, Br and I. The chemiluminescence yield was collected from either beam–gas or beam–beam reactions resolving the A and B electronically excited CaX states as well as the 2 Π 3/2, 1/2 spin–orbit states. The experimentally observed branching ratio, i.e. Γ(A/B) deviates significantly from the statistical value of 2 and hardly changes with collision energy. This deviation has been attributed to an increased role of Σ→Π transitions, i.e. the lack of orbital following as the impact parameter increases. In relation to the spin–orbit state population the overall behaviour is a nearly equally state population which, in principle, seems to be due to strong state mixing in these excited alkaline-earth halide reaction products. These findings are discussed in light of dynamical models currently used to interpret the reaction dynamics of excited species.

Reaction Dynamics of Electronically Excited Calcium Atom

Absolute values of the total chemiluminescence cross-section for the beam-gas Ca(3P, 1D) + Cl4C → CaCl(A, B) + Cl3C and Ca(3P, 1D) + SF6 → CaF(A, B) + SF5 reactions have been measured at low collision energy, E T = 0.15 and 0.14eV, respectively. Both metastable atomic calcium states Ca(3P, 1D) were produced under low voltage dc-discharge conditions. By changing the discharge conditions, different metastable concentrations were produced to measure the state-to-state cross-section for both 3P and 1D reactions. The following values for the total chemiluminescence cross-sections were obtained: σ D 1 = 1.77 A and σ P 3 = 0.25 A for the Ca(3P, 1D) + Cl4C → CaCl(A, B) + Cl3C reaction. σ D 1 = 0.59 A2 and σ P 3 = 0.56 A2 for the Ca(3P, 1D) + SF6 → CaF(A) + SF5 reaction. σ D 1 = 0.04 A2 and σ P 3 = 0.12 A for the Ca(3P, 1D) + SF6 → CaF(B) + SF5 reaction.In addition, beam-beam experiments were carried out at the same average low collision energy that of the beam-gas, and therefore, normalization between both experiments was possible. This procedure allowed us to obtain the excitation function of the Ca(1D) + SF6 reaction in absolute values over the 0.15–0.60eV collision energy range.On the other hand, by simulation, the ratio of CaCl(B-X/A-X) emissions intensities was found to be of 0.15. The variation of this ratio with the relative concentration of 1D/3P in a Broida oven leads to the conclusion that this state favours the formation of the B state in the chemiluminescent Ca(3P, 1D) + CH3CHCl2 → CaCl(A, B) + CH3CHCl reaction.

Crossed‐beam (full collision) versus van der Waals (half collision) studies. Application to the determination of the D00(Ca...HCl)

The dissociation energy of the van der Waals molecule Ca...HCl in its ground state, D00(Ca...HCl), has been determined using energy balance arguments. By combining collisional information from the Ca(3P)+HCl→CaCl(A 2Π)+H reaction with spectroscopic information from the Ca...HCl+hν→CaCl(A 2Π)+H reaction we found D00(Ca...HCl)≤150 meV.

Reply to comment on “influence of the radical size on the Ca∗ + ROH → CaOH∗ + R (R = CH3, C2H5and C3H7) reaction cross section”

Abstract The argument given by Mestdagh et al. in their Comment is discussed in the light of previous work carried out by the same authors. In our Reply we emphasize that in addition to reaction energetics, dynamical and stereodynamical effects could also decide the onset of a chemical reaction.

Research Activity Using Molecular Beam and Laser Techniques: A Progress Report

We shall briefly describe the main research areas, now in progress in our molecular beam and laser group. The current activity includes our traditional molecular beam work as well as other experiments carried out in the new labs of our Institute. Essentially we present here the current progress on Crossed-beam and Beam-gas Chemilumine scence, Laser-induced Beam Surface Reaction, Laser Analytical Chemistry, Laser-initiated Reactions Studies by Chemiluminescence and Laser-induced Fluorescence, Spectroscopy and Structure of van der Waals molecules. For each section recent results are presented. Due to the basic scope of the present progress report the emphasis is made on the new experimental developments as well as on the capabilities of both basic and applied research techniques now operative in our group.