ANGLE-ENERGY DISTRIBUTIONS OF PENNING IONS IN CROSSED MOLECULAR-BEAMS.3. ANGLE-ENERGY COUPLING IN HE-ASTERISK(2(1)S,2(3)S)-]HE+AR++E(-)(AR)

Relative doubly differential cross sections for the Penning ionization of Ar by spin-state-selected metastable He (1s2s) are reported at fou r center-of-mass collision energies covering the range E=1.8-6.3 kcal/ mol in a crossed supersonic beam experiment employing a rotatable mass spectrometer detector. The measurements are sufficiently dense in vel ocity space as to avoid having to functionalize the differential cross sections in order to transform the intensities into the c.m. The Arproduct is scattered sharply forward with respect to the incident dire ction of Ar at all energies for both spin states, increasingly so at h igher E, but with a persistent backward-hemisphere component. On the a verage the products have lost energy upon recoil, mean recoil energy ( E') < E, except for S-1 at high E, but E' is found to depend strongly on c.m. recoil angle theta, with forward scattering yielding (E'(theta )) approximate to E and sideways up to 50% energy loss. Extensive comp arison is made with predictions based on previously determined He+Ar optical potentials and spectroscopic ion exit channel potentials. Agre ement is generally good for both spin states, except for a significant discrepancy in backscattered intensity for S-1 that grows from a fact or of 2-6 from lowest to highest E, theory falling below experiment. A n alteration to the repulsive part of the S-1 potential suggested by p revious work fails to improve the comparison, but does show that the a ngle-energy coupling is uniquely sensitive to the form of the repulsio n. The results appear to indicate nonexponential behavior of the discr ete-continuum coupling (imaginary part of the optical potential) at sh ort range. (C) 1997 American Institute of Physics.