Energy transfer in collisions of rare gas atoms with CS2: Translational excitation of internal degrees of freedom

Abstract

Angle and recoil velocity distributions have been measured for the scattering of CS2 molecules from a crossed beam of rare gas atoms. Both beams were expanded through nozzles and seeded with H2 in order to provide rotational cooling of the CS2 molecules and narrow collision energy distributions, ranging from E=8.1 to 30.7 kJ/mole for Xe+CS2. Angular distributions of CS2 scattered in the plane of the crossed beams were measured by rotating the beam sources about a stationary mass spectrometer. The scattered beam was modulated with a pseudorandom binary sequence and the flight-time distribution of the scattered molecules was calculated from the cross correlated signal with an on-line computer. The measured velocity–angle contour plots show that the translational to vibrational excitation cross section is small compared with the translational to rotational excitation cross section and that the latter is strongly peaked in the forward direction. The CS2 angle and recoil velocity distributions are strongly coupled and the rotational excitation reaches a maximum at intermediate values of the c.m. scattering angle; this, together with the strong forward peaking of the cross section, suggests that a direct mechanism is responsible for the observed energy transfer. Further analysis of the data using only the most probable scattering energy shows that a substantial amount of energy, ranging from maxima of ∼18% for Ne to ∼45–50% for Ar, Kr, and Xe, is transferred from relative translation to rotational excitation of the CS2 molecules; that the fractional energy losses −ΔE/E for Xe+CS2 are comparable for E=17.2 and 30.7 kJ/mole, but considerably less at 8.1 kJ/mole; that both the rotational excitation cross sections and the fractional energy losses at comparable collision energies follow the relations Kr?Xe≳Ar≳Ne; and that the scattering angle for maximum energy loss is smallest for Kr and largest for Ne and Ar and increases somewhat with increasing collision energy.