Angle-energy distributions of Penning ions in crossed molecular beams. IV. He^*(2 ¹S,2 ³S) + H₂He + H + e^–

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Relative doubly differential cross sections for the Penning ionization of H₂ by spin-state-selected metastable He (1s2s) are reported at center-of-mass collision energies E of 3.1 and 4.2 kcal/mol in a crossed supersonic beam experiment employing a rotatable mass spectrometer detector. The measurements are sufficiently dense in velocity space as to avoid having to functionalize the differential cross sections in order to transform the intensities into the c.m. The H product is scattered sharply forward, c.m. <10° half-width at half-maximum, with respect to the incident direction of H₂ at both energies for both spin states. On the average the products have lost energy upon recoil, mean recoil energy E<E, and E– coupling is significant and increasing with E. Extensive comparison is made with infinite-order-sudden quantum scattering calculations based on previous experimental He^* + H₂ optical potential surfaces and a recent ab initio He + H exit channel surface. Aside from predicted even sharper forward scattering, agreement is fair to good for both spin states. The calculations allow an assessment of the possibility of observing quantum effects in the differential reactive scattering in these systems. ©2004 American Institute of Physics.