3+1 Resonance enhanced multiphoton ionisation spectroscopy of HBr

This paper extends the knowledge of highly excited Rydberg states of HBr by presenting detailed measurements of the 3 + 1 resonance enhanced multiphot on ionisation (REMPI) spectrum of (HBr)-Br-81 molecules obtained following excitation in the wavelength range 366-316 nm, i.e. at energies up to the f irst 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 (v' = 0) and 5 as new member s of vibrational progressions (v' not equal 0) originating from the promoti on of a pi electron into a sigma, pi or delta Rydberg orbital and correlati ng with the ground (XII)-I-2 state of the HBr+ ion core. Symmetry assignmen ts in terms of either (Lambda, S) or (Omega, omega) coupling schemes for th e various identified excited states are offered, based on band contour simu lation and quantum defect considerations. The main result of the present wo rk is the observation of new states and bands involving vibrationally excit ed levels which were not accessible in previous one and two-colour spectra. In particular, the three-photon selection rules allow observation of the v ' = 0 and 1 levels of the L (1)Phi (3) state arising from the d delta Rydbe rg manifold, as well as three other Ohm = 3 states within the d pi and d de lta series which are reported here for the first time.