Photodissociation study of ethyl bromide in the ultraviolet range by the ion-velocity imaging technique

The photodissociation of ethyl bromide has been studied in the wavelength range of 231-267 nm by means of the ion velocity imaging technique coupled with a [2+1] resonance-enhanced multiphoton ionization (REMPI) scheme. The velocity distributions for the Br (²P_1/2) (denoted Br*) and Br (²P_3/2) (denoted Br) fragments are determined, and each can be well-fitted by a narrow single-peaked Gaussian curve, which suggests that the bromine fragments are generated as a result of direct dissociation via repulsive potential-energy surfaces (PES). The recoil anisotropy results show that β(Br) and β(Br*) decrease with the wavelength, and the angular distributions of Br* suggest a typical parallel transition. The product relative quantum yields at two different wavelengths are Φ_234nm(Br*) = 0.17 and Φ_267nm(Br*) = 0.31, The relative fractions of each potential surface for the bromine fragments' production at 234 and 267 nm reveal the existence of a curve crossing between the ³Q₀ and ¹Q₁ potential surfaces, and the probability of curve crossing decreases with the laser wavelength. The symmetry reduction of C₂H₅Br from C _3v to C₅ invokes a nonadiabatic coupling between the ³Q₀ and ¹Q₁ states, and with higher energy photons, the probability that crossing will take place increases.