Rotational and vibrational relaxation of He2a3Σ +u

Abstract

The formation of He₂a³Σ⁺_u, during and following an intense microwave-pulse discharge, was investigated by kinetic absorption spectroscopy. At 77 K, He₂a³Σ⁺_u(v= 0) in high K states was formed both during and at the termination of the pulse. He₂a³Σ⁺_u(v= 0 and l), in a Boltzmann distribution, appeared only at the pulse termination. The overall decay of He₂a³Σ⁺_u was second order.

An abnormally slow rate of rotational relaxation of He₂a³Σ⁺_u(v= 0) was observed, corresponding to ∼10⁶ gas kinetic collisions at 77 K. This was attributed to a dominance of the inter-molecular repulsion by the almost spherical 2s orbital of He₂a³Σ⁺_u. Rotational relaxation of ³He ⁴He a³Σ⁺ was faster by about 10 fold. The vibrational relaxation rate of He₂a³Σ⁺_u(v= 1) was also abnormally slow, indicating a “soft” intermolecular repulsion.