Multiple collision rotational rainbows: Theory and experiment for Xe–CO2

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Abstract
In a crossed molecular beam experiment differential energy loss spectra have been measured for Xe+CO2 collisions at energies of 0.2, 0.58, 1.0, and 1.6 eV. Nearly the complete angular range from 40° to 180° in the center-of-mass system was covered. At large deflection angles and small energy transfers (Δ E/E≂0.1) the spectra exhibit a large intensity peak which cannot be explained by usual rotational rainbow theory. Quantum and classical calculations in the centrifugal sudden approximation demonstrate that this effect is a multiple collision rotational rainbow. In the first collision the kinetic energy is nearly completely transferred to rotational energy of the CO2 molecule. Since the heavy Xe atom leaves the interaction region very slowly, a second collision occurs and the rotational motion is deaccelerated. The classical excitation function J (γi), which relates the final angular momentum with the orientation angle of the molecule, has three extrema, two of which give rise to the multiple collision rotational rainbow. Various test calculations show that the effect depends strongly on the reduced mass, on the anisotropy and, in contrast to the normal rotational rainbow, also on the slope of the repulsive part of the interaction potential. Exact three-dimensional classical trajectory calculations at E=1.0 eV based on a realistic model potential agreed satisfactorily with the experimental results.