Absorption and fluorescence spectra of gaseous Se2

Abstract

The identification of the transitions in new resonance fluorescence series in ⁷⁸Se₂ and ⁸⁰Se₂ required some extension of the rotational analysis of the system B³∑^–_u–X³∑^–_g of ⁷⁸Se₂ to higher values of v″. Improved constants have been obtained for the ground state and these have been used to calculate Franck-Condon factors for the B—X system. A detailed study of the perturbations in B³∑^–_u has been made: some perturbing levels have been recognized which seem to interact both with the 0⁺_u and 1_u components. The analysis supports the value of the 1_g—0⁺_u separation, 367 cm^–1, obtained from an earlier study of the absorption spectrum in the Schumann region. Moreover, if this value of the 1_g—0⁺_g separation is accepted, then the “high” value of the dissociation energy of Se₂ is eliminated, and D^°₀(Se₂)= 25 518 ± 20 cm^–1. Two relatively weak fluorescence series have been observed with a lower state vibrational spacing about 15 % smaller than in the ground state. The upper state may tentatively be identified with one of the relatively weakly bound states which perturb B³sum;^–_u: if so, v= 0 of the lower state of the new series lies not more than about 4000 cm^–1 above v= 0 of X0⁺_g.