Vibrationally (and rotationally) inelastic scattering characteristics for the He+I₂ system

Abstract

An analysis is provided for the state-resolved vibrationally inelastic scattering cross sections σ(Δv) for He interacting with I 2 B0u+ molecules in either v′ = 15, 25, or 35. The collision energy for these crossed molecular beam data is 720 cm -1 (89 meV), whereas the local I2* vibrational quantum size varies from about 100 to 60 cm-1. The σ(Δv) encompass scattering events with Δv ranging to ± 3 for v′ = 15 and to ± 7 for v′ = 35. The sets of σ(Δv) for each initial v′ scale with an exponential energy gap law, and the scaling is identical for all initial v′ levels. Additionally, σ(Δv) values for conjugate T→V and V→T transitions (i.e., pairs of Δv = ±n for UP vs DOWN transitions) are nearly equal so that the single scaling law σ(Δv) ∝ exp( -|ΔEvib|/110 cm -1) describes the entire set of data. The scaling for the He target beam is identical to that for D2 but different from H2 indicating that the pattern of vibrational energy flow is determined mainly by the mass of the target gas and collision energy as opposed to subtle details of the interaction potential. 1D and 3D classical trajectory calculations replicate the principal characteristics of the scattering, particularly the common exponential scaling and UP-DOWN symmetry of conjugate σ(Δv), but fail to account quantitatively for processes with large Δv. The vibrational flow pattern is not markedly influenced by big variations in the rotational energy content of the initial v′ level. The competition between rotationally and vibrationally inelastic scattering is about the same for each initial v′. The rotational cross section is only about 2.5 X larger than σ( Δv = -1), the largest vibrational cross section. The total vibrational cross section, however, actually equals or exceeds that for pure rotationally inelastic scattering for all initial v′ levels. Comparisons are made with the vibrational and rotational energy transfer characteristics observed in 300 K bulb experiments. © 1989 American Institute of Physics.