Studies of extreme-ultraviolet emission from Rydberg series ofH2by electron impact

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Abstract
Electron excitation cross sections have been measured for the following two Rydberg series of H2: Σu+1 1sσnpσ (B, B′, and B′′, states with principal quantum numbers n=2, 3, and 4, respectively) and Πu1 1sσnpπ (C, D, and D′ states with principal quantum numbers n=2, 3, and 4, respectively) over the energy range from threshold to 350 eV. The cross sections for these six states account for all (>99%) of the vacuum-ultraviolet emission (78-170 nm) of the singlet states of H2. The estimated total direct-excitation cross sections for these six states at 100 eV in decreasing value are (4.02±0.60)×10−17 cm2 for BΣu+1 (3.86±0.60)×10−17 cm2 for CΠu1, (0.76±0.11)×10−17 cm2 for DΠu1, (0.76±0.11)×10−17 cm2 for B′ Σu+1, (0.30±0.06)×10−17 cm2 for D′Πu1, and (0.23±0.05)×10−17 cm2 for B′′Σu+1 and, additionally, (0.43±0.10)×10−17 cm2 for E,FΣg+1 which populates the BΣu+1 state through radiative cascade transitions. We estimate the predissociation (autoionization is weak) and emission yields of the vibrational levels of the D, D′, and B′′ states whose band systems exhibit strong "breaking off in emission" for wavelengths below 85 nm. Furthermore, we report the first direct measurement of the dissociative excitation cross section for production of Lyman-β of (8.9±3.0)×10−19 cm2 at 100 eV. In particular, it is shown that the high-lying Rydberg states (n=3 and 4) make a substantial contribution to the observed emission below 110 nm while above 110 nm the Lyman bands (BΣu+1→XΣg+1) and Werner bands (CΠu1→XΣg+1), the first members of the Rydberg series, dominate the spectrum. As a result of these measurements and spectroscopic models the ultraviolet (UV) spectrum from H2 by electron impact can serve as an intensity calibration standard from 80 to 170 nm.