Time resolved fluorescence from parity mixed rotational energy levels: Collisions vs electric field effects

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Abstract
We describe time-resolved and spectrally resolved laser-induced fluorescence measurements from the parity or Λ doublet levels of the A 1Π state of BCl radicals formed by CO2 laser multiphoton dissociation of BCl3 molecules. We show how the effects of collisional mixing can be discerned from the effects of electric field mixing. A set of rate equations for population transfer which include the effects of both field and collisional mixing under conditions of incoherent excitation are derived and compared to the more general theory of Alexander. Zero-field measurements of collisional mixing rates yield parity-changing rate constants kef whose rotational quantum number J dependence is consistent with a dipole–induced-dipole collision model: kef=3.4, 2.6, 1.9, and 1.4×106 s−1 Torr−1 for J=3, 5, 11, and 18, respectively. Measurement of time-resolved fluorescence from Stark mixed parity levels in the presence of a strong electric field indicates that the levels are prepared coherently. Time integrated measurements as a function of the applied field are used in generating empirical calibration curves for determining local field magnitudes in discharges containing BCl radicals.