3 + 1 Resonance enhanced multiphoton ionisation spectroscopy of HBr

Abstract

This paper extends the knowledge of highly excited Rydberg states of HBr by presenting detailed measurements of the 3 + 1 resonance enhanced multiphoton ionisation (REMPI) spectrum of H⁸¹Br molecules obtained following excitation in the wavelength range 366–316 nm, i.e. at energies up to the first ionisation energy. Spectra have been recorded using both linearly and circularly polarised laser light. 34 vibronic bands have been analyzed, of which 13 are identified as new Rydberg origins (′ = 0) and 5 as new members of vibrational progressions (′≠0) originating from the promotion of a π electron into a σ, π or δ Rydberg orbital and correlating with the ground X²Π state of the HBr⁺ ion core. Symmetry assignments in terms of either (Λ, S) or (Ω, ω) coupling schemes for the various identified excited states are offered, based on band contour simulation and quantum defect considerations. The main result of the present work is the observation of new states and bands involving vibrationally excited levels which were not accessible in previous one and two-colour spectra. In particular, the three-photon selection rules allow observation of the ′ = 0 and 1 levels of the L¹Φ₃ state arising from the dδ Rydberg manifold, as well as three other Ω = 3 states within the dπ and dδ series which are reported here for the first time.