High‐Resolution Laser Techniques Applied To Reaction Dynamics

Abstract

In this paper, using the particular case of the Cs(7P) + H2 → CsH + H reaction, it is shown how the application of high‐resolution continuous‐wave (CW) laser techniques to crossed‐beam experiments can yield information about the mechanism of reactive collisions. Firstly, CW laser techniques allow the preparation of Cs atoms in given fine and hyperfine structure levels; for this reaction, measurements of hyperfine cross‐sections vs. different orientations of the atomic orbital show how the molecular symmetry of the reagents controls the efficiency of the harpooning process involved. Secondly, the laser‐induced fluorescence (LIF) technique of detection allows the characterization of CsH products in given rotational levels. The high‐precision scanning of the CW laser frequency over absorption profiles of CsH molecules results in the determination of the Doppler shifts, which are associated with the recoil velocity of the products; two laser beam arrangements are used to provide the product velocity and angular scattering probability. A “forward” peaking is discovered, which depends on collision energy but not on product rotation. Finally, the technique of saturated absorption is applied for the first time to reactive experiments; it could lead to methods for selecting products which scatter in a given plane or in given directions.

Experimental investigation of weakly bound B(2p,3s)–H2/D2 complexes through laser fluorescence excitation spectroscopy

“…Metal atom-hydrogen interactions have been probed through studies of bimolecular nonreactive 1-4 ͑electronic-to-vibrational energy transfer͒ and reactive collisions, [5][6][7][8][9] ''half-collision'' experiments through far-wing absorption on atomic transitions, 10,11 reactions and energy transfer within metal atom-hydrogen van der Waals complexes, [12][13][14] electronic absorption spectroscopy of metal atoms embedded in a cryogenic hydrogen matrix, 15 and infrared absorption spectroscopy of metal atoms and hydrogen codeposited in rare gas matrices. 16 In the overwhelming majority of systems investigated, these experiments have involved group 1, 2, or 12 atoms, for which the ground electronic state has ns or ns 2 electron occupancy and electronic excitation involves promotion to a P state.…”

Laser studies of reactive collisions

“…Such experiments give information about the excited potential energy surface for the reactions, their separations, crossings and symmetries. Spin-orbit and hyperfine effects in chemical reactions have recently been reviewed by Dagdigian (1987) and Vetter (1989).…”

“…Recently, Doppler methods have been used to obtain differential cross sections for the reactions F+12 (Billy et a1 1990, Girard et a1 1990) and Cs*+ H2 (Vetter 1989). In these crossed molecular beam experiments, two geometries are employed.…”