M. C. Heaven

Laser-induced fluorescence of the BO and BO2 free radicals

Abstract The first observation of quantum-resolved laser-induced fluorescence of the BO free radical is described. The source of ground-state X 2 Σ + BO radicals was the reaction of BCl 3 with a mixture of O 3 P and N 4 S atoms in the absence of O 2 . Laser excitation spectra involving the vibrational levels 11 ⩾ ν′ ⩾ 0 of the A 2 Π state are reported, using excitation wavelengths varying from 425 nm down to 277 nm, from a YAG-pumped narrow-band dye laser. The radiative lifetime of BO A 2 Π (11 ⩾ ν′ ⩾ 0) was determined to be (1.7 6 ± 0.1 3 ) μs (1σ), based on fluorescence decay studied. Implications of this relatively long lifetime for lasing on the AX transition of BO are discussed. Quenching of BO A 2 Π by N 2 is relatively slow, with k N 2 −11 cm 3 molecule −1 s −1 . Laser excitation spectra of BO 2 radicals are reported from the reaction of O 3 P atoms with BCl 3 with O 2 present; no BO radicals were detected under these conditions. The radiative lifetime of the 002 vibronic level of BO 2 A 2 Π has been determined to be (182 ± 20) ns (1σ). Quenching of this excited level of BO 2 by O 2 is efficient.

Theoretical treatment of the spontaneous predissociation of Br2, B 3Π(0u+)

The spontaneous heterogeneous predissociation of the B 3Π(0u+) state of bromine is discussed. Theoretical treatment of the heterogeneous interaction is presented, with emphasis placed on the role of the Franck–Condon principle in predissociation. Numerically generated vibrational wave functions are used to calculate predissociation probabilities for the B state. From these calculations the parameters of the repulsive potential responsible for the predissociation are determined, and this potential curve is identified as the 1Π(1u) state. Good agreement with experimental results was obtained with a 1Πu potential of the form U(R) = 1.594×104/R9.384. This crosses the B state curve between v′ = 4 and 5, which is two levels higher than indicated in previous work. The new 1Πu curve is compatible with other theoretical and experimental results.

Kinetics of excited states of Br2 using laser excitation. Part 1 .—System description and rotationally-dependent lifetimes in Br2(B)

A pressure-tuned narrow-band day laser is described. This instrument was used to provide linear scans, free of mode hops, of selected parts of the Br2(B–X) band spectrum in laser-induced fluorescence. Resolution was Doppler limited.Measurements of collision-free lifetimes τ0 of Br2(B) excited molecules were made over a range of rotational states J′, for several vibrational states 23 v′ 11. A strong dependence of Γ0= 1/τ0 upon J′, was found, which was attributed to a heterogeneous predissociation of the emitting B3Π(0+u) state. The results gave a good fit to the relationship, Γ0=ΓR+kv′J′(J′+ 1). The mean value of the radiative, rotation-free lifetime (τR= 1/ΓR) was 8.1 µs for 24 v′ 11.The results show that virtually all band absorption of Br2 between 540 and 590 nm leads to formation of 2P+2P bromine atoms via predissociation.

Laser-induced fluorescence of the P radical

Abstract The PO X2Π ground-state radical has been observed, using laser-induced fluorescence with a 1 pm bandwidth, pulsed dye laser. The 0-0 band of the B2Σ†-X2Π 1 2 transition of PO was excited, giving a radiative lifetime τR = 250±10 ns (1σ) for PO B2Σ+ Quenching of PO B2Σ2 by N2 was found to be rapid.

Quantum-resolved dynamics of excited states. Part 5.—The long-lived A3Π(1u) state of Br2

Laser-induced fluorescence from resolved ro-vibrational levels (ν′, J′) of excited Br2A3Π(1u) has been observed, as the A–X transition. Excitation near 705 nm was carried out, using 10 ns pulses of narrow-band radiation (0.04 cm–1) with 250 µJ/pulse. Identification of the A–X ro-vibrational transitions was made through rotational combination differences and vibrational isotope splittings; new data for vibrational isotope splittings are reported.Lifetimes of the F– component of the (11,23) state of Br2(A) were measured as a function of pressure of Br2 and Ar as bath gases. The radiative lifetime τR of Br2(A) is reported to be (347 ± 50)µs (1σ). Vibrational energy transfer in collisions of Br2(A) with Br2(X) is very efficient, with kv 2.4 × 10–10 cm3 molecule–1 s–1 at 293 K. Vibrational-to-translational transfer in Br2(A)+ Ar collisions is about one order of magnitude slower. Electronic quenching of Br2(A) by Br2 and Ar is slow and an upper limit for quenching in Br2(A)+ Br2 collisions was determined, kQ⩽ 4˙7× 10–12 cm3 molecule–1 s–1.The interpretation of work by other workers on laser-induced fluorescence of excited IC1 A3Π(1) is discussed in the light of the present results on Br2.

Energy transfer in collisions of excited Ar(3P0.2) metastable atoms with H(2S) atoms. II. Lyman-α emission profile

Abstract Spectra of the Lyman-α emission resulting from the Ar(4s, 3 P 2.0 ) + H(Is, 2 S) interaction have been recorded. The emission line profile is essentially rectangular with a full width of 13 pm. These results show that excited H( n = 2) atoms are formed in the reaction, with nearly all the excess energy (1.34 eV) appearing as kinetic energy of the hydrogen atom. Lyman-α emission profiles also have been obtained from microwave discharge plasmas in argon and helium, containing traces of hydrogen; these profiles are compared with those from the Ar( 3 P 2.0 ) + H( 2 S) system.