Quantum-resolved dynamics of excited states. Part 1.—Predissociation in the B3Π(0+) state of BrF

Abstract

Pulses from a narrow-band tunable dye laser (5–10 ns in duration; 0.04 cm–1 band width) have been used to excite fluorescence from the vibrational levels v′= 7 and 8 of the excited B3Π(0+) states of 79BrF and 81BrF. Individual rotational levels were readily excited, and the corresponding decay lifetimes were measured.The lower rotational levels (J′⩽ 23) of v′= 7 were stable, with an extrapolated zero-pressure lifetime τ0=(22.4 ± 3.3)µs for 21 J′ 4. Onset of predissociation was found near J′= 28. Above this level, τ0 shortened systematically with increase in rotational energy, giving τ0=(0.37 ± 0.10)µs for the highest rotational level studied (J′= 34).All rotational levels of the v′= 8 state were predissociated, and τ0 values were found to shorten systematically from (1.74 ± 0.15)µs for J′= 1 to 3 down to (0.11 ± 0.06)µs for J′= 31. A linear dependence of 1/τ0 upon rotational energy [found previously for the natural predissociation in B3Π(0+u)] was not observed for either the levels v′= 8, or for v′= 7 in the B3Π(O+) state of 79BrF. An improved upper limit for the dissociation energy of 79BrF is reported.Non-exponential fluorescence decay curves were seen only for excitation of rotational levels of BrF (B, v′= 7) whose energy is either above, or somewhat below, the minimum predissocation energy. These decay curves are interpreted in terms of near collisional rates of upward and downward rotational energy (R–T) transfer of excited BrF(B) with helium gas at pressures between 50 and 400 mTorr. The results indicate that collisional rotational relaxation of 79BrF, B3Π(0+), occurs with a high probability of multi-quantum transitions (ΔJ⩽± 10). Vibrational relaxation is slow and has an appreciable effect only on the fluorescence decay curves following excitation in the 8–0 band at pressures in the 500 mTorr range.