Laser-excitation studies of Br2. Collisional energy transfer involving resolved quantum states of excited Br2B3Π(0u+)

Abstract

Selected ro-vibrational states (v′,J′) of excited Br₂B³Π(0_u⁺) were populated by absorption of radiation (570–600 nm) from a single-frequency c.w. dye laser, or from a 1 pm bandwidth pulsed dye laser. The rates of electronic, vibrational and rotational energy transfer within the B state of Br₂ were studied, using Br₂, Ar, He, N₂ and Cl₂ as collision partners.

Wavelength-resolved fluorescence was analysed in the c.w. excitation studied. The results showed that Ar and N₂ possessed large cross-sections for R–T transfer, whilst electronic quenching was slow. Collision efficiency of N₂ for V–V transfer in Br₂(B, ν′= 11–15) was ≈0.1, with Δv=+1 slower than Δv=–1. Minimal coupling of R–T and vibrational transfer in collisions with N₂ found. When Δv′ quantum jumps occurred in vibrational transfer with N₂, conservation of rotational quantum number J′ was observed, with a small amount of broadening of the initial rotational distribution. Collisions of Br₂(B) with Br₂(X) resulted in efficient electronic deactivation, with very little observed ro-vibrational energy transfer.

Time-resolved undispersed fluorescence was analysed in the pulsed excitation studies. The rate constant for electronic self-deactivation of Br₂(B, ν′= 14) was (4.2 ± 1.3)× 10^–10 cm³ molecule^–1 s^–1 at 298 K. Electronic deactivation is assigned to collisional predissociation, which thus has a high efficiency.

The results are interpreted in terms of a large cross-section for R–T transfer in collisions of Br₂(B) with rare gas atoms, which lead to collision-induced predissociation and thus to depletion of Br₂(B) population. Collisions with Cl₂ also were found to give rapid collisional depletion of Br₂(B). The elementary processes responsible in this case are probably a combination of electronic deactivation and rovibrational energy transfer, leading to predissociative loss of excited Br₂(B). Electronic deactivation in collisions of Br₂(B) with Ar and He is slow.