Three- and two-photon absorption spectroscopy: REMPI of HCl and HBr

Approximation expressions for absorption line strengths due to Wʹ = 0,1,2,3 ¬ W" = 0 (S, P, D, and F ¬ S) transitions for three-, two-, and one-photon absorptions are summarized. Those for three- and two-photon absorptions are used to analyse room temperature (3+1) and (2+1) REMPI (resonance enhanced multiphoton ionization) spectra due to transitions to Rydberg states in HCl and HBr to give spectroscopic parameters. A mechanism of the three-photon excitation process for the (E1 S+) ¬ X (1S+), (0,0) transition in HCl is proposed. A Rydberg state, not observed in single- or two-photon absorption, with a band origin at 80 167 cm-1 was identified in (3+1) REMPI of HBr and analysed for the first time. It was assigned to the l(3(F(Wʹ = 3)) ((s2p3)5dd) state, (0,0) band. Use of the three-photon absorption line strength expressions for deriving rotational population distributions in ground-state HBr is demonstrated.