Radiative decay rates from deperturbed v=0–7 vibrational levels of CO A 1Π measured using synchrotron radiation

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Abstract
Fluorescence decay times from single vibronic levels and from restricted groups of rotational levels are measured for CO A 1Π under selective excitation by synchrotron radiation. The largest cause of variation of decay curves with v′ and J′ quantum numbers is singlet ∼ triplet perturbations. These perturbations cause measured, rotationally averaged decay rates to differ from the deperturbed, pure A 1Π decay rate by up to 20%. Failure to take perturbation effects into account could systematically affect transition moment functions derived from radiative lifetimes, but not those from absorption oscillator strengths or fluorescence branching ratios. The dependence of the CO A 1Π–X 1Σ+ transition moment on internuclear distance is evaluated from deperturbed lifetimes of A 1Π v=0–7 vibrational levels and found, assuming linear Re vs r̄ variation within the region 1.0≤r̄v′v′′≤1.6 Å, to be Re(r̄vA′vX′′)= [7.48+0.34−0.34][1−(0.683+0.006−0.008) r̄v′v′′]D, where r̄vA′vX′′ is the r centroid and the correlated uncertainties cited correspond to three standard deviations. However, the occurrence of strong A–X fluorescence bands with r centroids near 1.46 Å implies a decidedly nonlinear form for Re(r̄). Since the v=0–7 A 1Π lifetimes sample Re(r̄) very weakly at r̄≳1.3 Å, the transition moment function derived here is valid only for 1.0≤r̄≤1.3 Å. The Re(r̄) function determined here is nearly identical to that derived by Mumma et al. [J. Chem. Phys. 5 4, 2627 (1971)] from the radiative lifetimes measured by Hesser [J. Chem. Phys. 4 8, 2518 (1968)]. This is a coincidence, not evidence that perturbation effects are inconsequential.