Quenching and excitation-transfer processes in then=4helium sublevels in a low-pressure glow discharge

Collisional and radiative processes in a glow discharge leading to quenching and excitation transfer in the $n=4\mathrm{}$ helium sublevels are investigated by means of a laser-perturbation method. Laser-induced population perturbations are solutions of coupled rate equations, the coefficients of which are determined by an accurate numerical method of data analysis (the identification method) previously developed, so as to minimize the difference between experimental curves and those calculated from the model. In the pressure and current-intensity ranges investigated, only the radiative and atom-atom collision processes contribute to quenching and excitation transfer in the $n=4\mathrm{}$ sublevels. Numerical identification of the $n=4\mathrm{}$ experiments provides a nearly complete set of thermally averaged cross sections and in particular shows that the singlet-triplet transfers are mainly due to stepwise collisional processes via the $4F$ state.