Radiative and predissociative rates for NO A 2Σ+v′=0–5 and D 2Σ+v′=0–3

Abstract

Two-photon laser-induced fluorescence of the NO A–X(0–5,0) and D–X(0–3,0) bands is used to obtain collision-free lifetimes of the A v′=0–5 and D v′=0–3 levels in a flow cell. The vibrational trend from NO A v′=0 to v′=3 is small, with radiative lifetimes between 205±7 to 184±8 ns, respectively, in agreement with recent calculations derived from branching ratio measurements and ab initio calculations. Rotationally resolved measurements in the NO A 2Σ+ and D 2Σ+ states reveal very different lifetime behavior for these nearly isoenergetic levels above the NO X 2Π state dissociation limit. The rotational lifetimes of A 2Σ+v′=4,5 and D 2Σ+v′=0 are basically constant with rotation. However, D 2Σ+v′=1, 2, and 3 have a strong rotational dependence, decreasing nearly threefold between N′=0 and 20. These observations suggest weak coupling with the ground-state continuum to be the most likely predissociation mechanism for the high A 2Σ+ levels, and rotational on heterogeneous (gyroscopic) predissociation by the C 2Π is the dominant mechanism for the D 2Σ+ state above v′=0. The collision-free lifetime of E 2Σ+v′=0 is 40±3 ns, and that for F 2Δv′=0 is 27±3 ns.