Metastability of Rotationally Hot Dihelium at 77° K

Abstract

WE report here the detection by kinetic absorption spectroscopy of He₂ (a³Σ_u⁺(σ=0)) produced in the gas phase at 77° K with an intense microwave pulse discharge. Although vibrationally cold, at this temperature the molecules exhibit an extremely non-equilibrium distribution in the high rotational states, which show only a minor degree of relaxation during their lifetime of 10⁻⁴ sec with total helium pressures up to 100 mm. The observations appear to constitute a unique example, as yet, in which the absorption spectrum of a rotationally hot molecule yields relative reaction rates into individual rotational states. It is suggested that the observed rotational distributions of He₂ (a³Σ_u⁺) may reveal the states of He₂⁺ (X²Σ_u⁺) from which it is formed.