QUENCHING OF H-2 VIBRATIONS IN ULTRACOLD HE-3 AND HE-4 COLLISIONS

We present results of quantum mechanical scattering calculations inclu ding full rovibrational coupling on the relaxation of vibrationally ex cited H-2 molecules in collision with ultracold He-3 and He-4 atoms. T he exothermic vibrational deactivation cross sections exhibit minima a t translational temperatures near 10 K and show an inverse velocity de pendence in accordance with Wigner's threshold law in the limit of zer o kinetic energy. The corresponding rate coefficients exhibit minima b efore attaining constant values below 10(-3) K. The rate coefficients increase by 3 orders of magnitude when the initial vibrational level o f the molecule is raised from v = 1 to v = 10. The results suggest tha t quasibound resonance states of (HeH2)-He-3 and (HeH2)-He-4 exist, as sociated with each excited vibrational state of H-2, and that both the triatomic species (HeH2)-He-3 and (HeH2)-He-4 have one bound vibratio nal state.