Vacuum-Ultraviolet Emission from Proton-Excited Helium Gas

Vacuum-ultraviolet (vuv) studies were made of the emission from helium gas excited by 4-MeV protons, under conditions where the gas purity was monitored by concomitant total-ionization measurements. The relative intensity of the 584-Å resonance line was studied from 0.01 to 100 Torr. Three continua, near 600, 675, and 800 Å, respectively, were investigated from a few Torr to 800 Torr. Our studies of intensity as a function of pressure are consistent with the interpretation that the 675-Å continuum is the bound-continuum transition $D{}_{}{}^1{}_{}{}^{}\Sigma _u^{+}{}_{}{}^{}\to X{}_{}{}^1{}_{}{}^{}\Sigma _g^{+}{}_{}{}^{}$ in the helium diatomic molecule, where the $D$ state is formed by three-body collisions involving the atomic $2{}_{}{}^1{}_{}{}^{}P$ state. Some of the 800-Å continuum, as well as the narrow continuum near 600 Å, is produced, most likely, from a similar process involving emission from the $A{}_{}{}^1{}_{}{}^{}\Sigma _u^{+}{}_{}{}^{}$ molecular state, created from three-body conversion of a $2{}_{}{}^1{}_{}{}^{}S$ atomic state. Certain other details are given on the energy pathways following charged-particle excitation of helium gas, and comments are made on some of the recently published emission data for liquid helium.