Radiative lifetimes of NO A Σ2+(v′=,1,2) and the electronic transition moment of the A Σ2+−X Π2 system

Abstract

Improved measurements of the radiative lifetimes of NO A Σ2+(v′=0,1,2) are presented and used to update the absolute electronic transition moment for the NO γ bands. The pressure-dependent fluorescence decay rate was measured in a low-pressure, room-temperature, flow cell containing dilute mixtures of NO in N2 using time-resolved laser-induced fluorescence excited with a picosecond laser and detected with a microchannel-plate photomultiplier tube. Fluorescence decay rates were determined using an analysis procedure that accounted for the electronic response of the detection system and measurement noise. Radiative lifetimes were determined from an extrapolation of the measured decay rates to zero pressure. In comparison with prior measurements of these radiative lifetimes, the improved experimental approach and analysis procedure result in a significant improvement in the measurement precision. The accuracy of the fluorescence decay-rate measurements was confirmed by independent measurements using time-correlated single-photon counting and time-resolved probing of laser-excited population in A Σ2+ using 266 nm photoionization and charge detection. The measured radiative lifetimes are 192.6±0.2 ns for v′=0, 186.2±0.4 ns for v′=1, and 179.4±0.7 ns for v′=2. The measured lifetimes are shown to be in outstanding agreement with those predicted by an electronic transition moment that is identical in form to the function recommended by Luque and Crosley [J. Chem. Phys. 111, 7405 (1999)] after appropriate rescaling. This rescaling does not affect the agreement of the transition moment function with the previously reported vibrational branching ratios and improves agreement with previously reported absolute oscillator strength measurements. Based on the rescaled transition moment, updated values of absolute transition probabilities in the NO A Σ2+−X Π2 system are presented.