Excitation transfer due to neutral collisions in excited states of atomic hydrogen

The H alpha Balmer line emitted by a low-excitation electrical discharge at liquid nitrogen and room temperatures has been studied with a Fabry-Perot interferometer. With partial resolution of the fine structure, the line profile was observed to depend on buffer gas composition, pressure, and temperature, but not on discharge current. Interferograms computed from the known fine structure and transition probabilities, with allowance for non-thermal populations in the excited substates, matched the observed profiles quite well. The populations deduced from these fits are shown to approach thermal equilibrium with increasing gas pressure in a way which allows the collisional excitation transfer from the 3p to 3d states to be determined. Data for H-H, H-He, and H-H 2 O collisions are presented.