Cascade contribution to theH2Lyman band system from electron impact

The intensity of UV resonance transitions excited by electron impact is determined by both direct excitation and cascade processes. The lifetime for decay by spontaneous emission from a resonant state is short (typically 10 ns). Using a pulsed electron gun technique established to separate these two effects we report a laboratory study of the UV spectrum of ${\mathrm{H}}_{2}$ attributed to cascade. We utilize the longer lifetimes (g 30 ns) for cascade channels to measure the intense spectrum of the Lyman band system $(B{}^{1}{\ensuremath{\Sigma}}_{u}^{+}--X{}^{1}{\ensuremath{\Sigma}}_{g}^{+})$ of ${\mathrm{H}}_{2}$ populated by cascade from higher lying $({\mathrm{E}\mathrm{F},\mathrm{G}\mathrm{K},\mathrm{H}}^{1}{\ensuremath{\Sigma}}_{g}^{+})$ states.