Time‐resolved Electron Impact Study of Excitation of H 2 Singlet‐ Gerade States from Cascade Emission in the Vacuum Ultraviolet Region

Abstract

Medium-resolution (full width at half-maximum Δλ = 0.5 A) vacuum ultraviolet (VUV) emission spectra of H2 from forbidden excitation to the excited singlet-gerade states and subsequent cascade to the singlet-ungerade states have been studied with a time-resolved electron-impact technique. Emissions from the B 1Σ, C 1Πu, B' 1Σ, and D 1Πu states that are entirely due to cascade from the singlet-gerade states have been observed. Lyman band emission intensities dominate over the entire 940-1650 A wavelength region, though Werner band emission intensities are also significant. In addition, very weak cascade emissions from the vj = 0 and 1 levels of the B' 1Σ and D 1Πu states are also evident. In contrast to previous assumption, the cascade excitation is observed to take place over all the vibrational levels of the B 1Σ and C 1Πu states, while contributing heavily to low vj levels of the B 1Σ state. Synthetic spectra based on the calculated transition probabilities of the EF 1Σ, GK 1Σ, H 1Σ, I 1Πg, J 1Δg, O 1Σ and P 1ΣB 1Σ, C 1Πu, B' 1Σ, and D 1Πu band systems and Franck-Condon factors of the EF 1Σ-X 1Σ, GK 1Σ-X 1Σ, H 1Σ-X 1Σ, I 1Πg-X 1Σ, J 1Δg-X 1Σ, O 1Σ-X 1Σ, and P 1Σ-X 1Σ are in good agreement with the observed Lyman and Werner band intensities. The present study shows that a number of singlet-gerade and singlet-ungerade states are involved in the cascade excitation process of H2. The cascade from these states in the VUV provide diagnostic information on astrophysical sources for H2 emission.