Dynamics of the H2 levels lying between the third and fourth dissociation thresholds (132 500 – 139 000 cm− 1)

Abstract

The absorption spectrum of H₂ at room temperature, was recorded at high resolution at the BESSY II synchrotron radiation storage ring in the 132 500 – 139 000 cm⁻¹ energy range, allowing the study of the dynamics of the excited levels (npσ ${}^1{\Sigma }_u^{+}$ and npπ ${}^1{\Pi }_u^{\pm }$) located between the H(n = 3) + H(n = 1) and H(n = 4) + H(n = 1) thresholds. Ionization anddissociation were found to compete for levels with n ≤ 8. In most cases, for the levels with n > 8, ionization is the only decay channel. The relative importance of the direct ionization, direct dissociation, autoionization and predissociation processes was evaluated for the whole energy range. From the different detected channels, and from the comparison with theoretical approaches, the main continua responsible for the predissociations were assigned to the V and to the $B"\overline{B}$ states and the H(3p)/H(n = 3) ratios of the dissociation fragments from the 4pπ and 5pπ ${}^1{\Pi }_u^{-}$ levels were measured. Molecular fluorescence was observed for few ${}^1{\Pi }_u^{-}$ levels and the importance of its contribution is discussed.