Quantitative Determination of HD Internal State Distributions via (2+1) REMPI

The relationship between quantum state populations and ion signals in (2+1) resonance‐enhanced multiphoton ionization (REMPI) detection of HD is investigated both experimentally, by measurements of hot‐nozzle population distributions, and theoretically, by calculations of the two‐photon E,F 1Σg+—X1Σg+ transition moments. The experimental and theoretical relative rotational cross‐sections are in good agreement for v″ = 0 and v″ = 1, but differ for v″ = 2 and for the relative vibrational cross‐sections. Arguments are presented that saturation of the E,F‐X transition is the main cause of these discrepancies. In spite of the presence of saturation, this calibration gives confidence in determining the relative internal state populations of HD from its (2+1) REMPI spectrum.