Luminescence from excited mercury vapour. Quantum yield and lifetime of the ultraviolet reservoir states and mechanisms for the termolecular formation of excited dimers

Abstract

Mixture of Hg vapour with low pressures of N₂ and an excess of added inert gas have been excited with resonance radiation, both pulsed and c.w., to determine the quantum yield and mechanism of the ultraviolet emission which arises in the combination of metastable and ground-state mercury atoms.

The time-resolved measurements demonstrate that the u.v. band is supplied from a higher energy reservoir than the A 0^±_g first excited states of the dimer. By estimating the ratio of the partition function of the reservoir to that of the first excited states from the effective emission rates, it is adduced that the C 0_u^– dimer is the sole reservoir state and is in equilibrium with the D 1_u carrier with pressures of Ar > 300 Torr; there is little deactivation of the coupled ungerade states to the lower gerade states by Ar at up to one atmosphere pressure. Addition of He, however, does quench the ultraviolet emission. A set of rate coefficients for deactivation of the C 0^–_u state is reported.

The quantum yield for formation of the C 0^–_u state, relative to the total formation of excited dimers, has been measured by comparing the u.v. intensity with that of the green trimer emission, after allowance for non-radiative decay. Only one combination occurs into the vibrationally relaxed C 0^–_u state per six or seven combinations into A 0^±_g. It is proposed that the low quantum yield of the C 0^–_u dimers is due to collisional coupling of pairs of states in high vibrational levels, where the potential energy functions merge, which preferentially drains the excitation into the state of lower energy.