Quenching ofH2Vibrations in Ultracold3Heand4HeCollisions

Abstract

We present results of quantum mechanical scattering calculations including full rovibrational coupling on the relaxation of vibrationally excited ${\mathrm{H}}_{2}$ molecules in collision with ultracold ${}^{3}\mathrm{He}$ and ${}^{4}\mathrm{He}$ atoms. The exothermic vibrational deactivation cross sections exhibit minima at translational temperatures near 10 K and show an inverse velocity dependence in accordance with Wigner's threshold law in the limit of zero kinetic energy. The corresponding rate coefficients exhibit minima before attaining constant values below ${10}^{\ensuremath{-}3}$ K. The rate coefficients increase by 3 orders of magnitude when the initial vibrational level of the molecule is raised from $v\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1$ to $v\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}10$. The results suggest that quasibound resonance states of ${}^{3}{\mathrm{HeH}}_{2}$ and ${}^{4}{\mathrm{HeH}}_{2}$ exist, associated with each excited vibrational state of ${\mathrm{H}}_{2}$, and that both the triatomic species ${}^{3}{\mathrm{HeH}}_{2}$ and ${}^{4}{\mathrm{HeH}}_{2}$ have one bound vibrational state.