State-to-state differential cross sections for rotationally inelastic scattering of Na2 by He

Abstract

State‐to‐state differential cross sections for rotational transitions of Na₂ in collisions with He are measured in the electronic and vibrational ground state at thermal collision energies using a new laser technique. Single rotational levels j_i are labelled by modulation of their population via laser optical pumping using a dye laser. The modulation of the fluorescence induced by an Ar⁺ laser tuned to the level j_f=28 is proportional to the cross section for collisional transfer j_i→j_f and is detected at the scattering angle ϑ. A single optical fiber and a fiber bundle provide a flexible connection between the detector and the laser and photomultiplier, respectively. Transitions as large as Δj=20 are observed. At small angles elastic scattering is dominant, but rotationally inelastic processes become increasingly important at larger scattering angles. Rotational rainbow structure causing a steep onset of the cross section with the scattering angle ϑ (at fixed Δj) or a sharp cutoff with Δj (at fixed ϑ) is found. Preliminary results on rotational energy transfer in v=1 indicates that vibrational motion of the molecule favors larger rotational quantum jumps. semiclassical picture for the scattering of a hard ellipsoid gives a