The dominant pumping mechanism of the 441.6 nm Cd II line in a three-colour hollow-cathode discharge laser

The three most probable processes for pumping of the upper lasing level of the 441.6 nm Cd II line under hollow-cathode discharge conditions, namely Penning ionization, `direct' and `stepwise' electron excitation, have been analysed. Atomic ionization and excitation cross sections and the electron distribution function in the energy interval up to 300 eV have been calculated. Radial dependencies of pumping rates of laser transition for a cylindrical cathode cavity and an integral pumping rate have been determined versus the cadmium vapour and helium pressures. It has been shown that the contributions of Penning ionization and direct and stepwise excitations to the pumping rate are strongly dependent on the pressures of discharge components. Contributions of these processes are found to reach up to about 70, 30 and less than 1%, respectively, under optimal conditions for the laser discharge. The comparison of calculated radial pumping rate distributions with experimental data confirmed that the Penning ionization plays the dominant role.