Quenching of H₂ vibrations in ultracold ³He and ⁴He collisions

We present results of quantum mechanical scattering calculations including full rovibrational coupling on the relaxation of vibrationally excited H₂ molecules in collision with ultracold ³He and ⁴He 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^-3 K. The rate coefficients increase by 3 orders of magnitude when the initial vibrational level of the molecule is raised from v = 1 to v = 10. The results suggest that quasibound resonance states of ³HeH₂ and ⁴HeH₂ exist, associated with each excited vibrational state of H2, and that both the triatomic species ³HeH₂ and ⁴HeH₂ have one bound vibrational state.