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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.

The emission spectrum of air that is excited by an ArF laser has been investigated experimentally and theoretically to determine the conditions under which fluorescence from O_2 can be used for the measurement of temperature in aerodynamic flows. In addition to the expected fluorescence from O_2, the spectrum from excitation with an intense laser beam is shown to contain significant contributions from the near-resonant Raman fundamental and overtone bands, the four-photon fluorescence excitation of C produced from ambient CO_2, and possibly the three-photon excitation of O_2^+. The nature of the radiative interactions contributing to these additional features is described.