Predissociation lifetimes of OH A 2Σ+ (v′=3) obtained from optical–optical double-resonance linewidth measurements

The excitation of rapidly predissociating v′=3 levels in the A 2Σ+ state of OH has recently been suggested as a means of avoiding collisional quenching corrections when performing laser-induced fluorescence (LIF) measurements in flames. We report measurements of predissociation rates of several rotational levels in v′=3, obtained from linewidths in optical–optical double-resonance (OODR) spectra. The magnitude of the observed predissociation rates places an upper limit on the pressures at which OH LIF can be considered ‘‘quench-free.’’ In addition, analysis of spectral line shapes suggests that rotational transfer can significantly affect LIF quantum yields at pressures of 1 atm and higher. We conclude that variations of the predissociation rate with rotational level in v′=3 must be included in determinations of rotational temperatures from LIF.