Quenching ofH2Vibrations in Ultracold3Heand4HeCollisions

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 ${10}^{-3\mathrm{}}$ K. The rate coefficients increase by 3 orders of magnitude when the initial vibrational level of the molecule is raised from $v=1\mathrm{}$ to $v=10\mathrm{}$. 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.