Reduction of 1 + 1 REMPI spectra to population distributions: saturation and intermediate state alignment effects

A two-step methodology is presented for extracting ground state population distributions and alignment factors from 1 + 1 resonance-enhanced multiphoton ionization (REMPI) spectra. In the first step the ion signal is corrected for variation with laser intensity on a shot-to-shot basis, generating an isopower spectrum. In the second step populations and alignments are derived from the isopower spectrum by correcting for the interdependent effects of saturation and intermediate state alignment. Both classical and quantum-mechanical models are presented and their attributes are noted. This analysis procedure is applied to a room temperature thermal distribution of nitric oxide using the 1 + 1 REMPI process in which lines of the NO A2Σ+—X2Π (0,0) band constitute the resonant transition. The present treatment is able to recover the known rovibrational population distribution, independent of branch choice, over a wide range of practical operating conditions.