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.