Time-resolved observation of vibrationally excited 1,3-butadiene in laser induced photolysis of bromocyclobutane and cyclohexene

Abstract

The pulsed CO₂ laser induced SF₆ sensitized photodissociation of bromocyclobutane (BCB) and cyclohexene (CHN) is investigated. BCB has two dissociation channels from its ground electronic state, HBr elimination yielding 1,3-butadiene is the major channel (>90%) whereas, the ring cleavage producing vinyl bromide and ethylene is the minor one. The time-resolved formation of 1,3-butadiene is monitored by UV absorption from which unimolecular rate constants for BCB and CHN dissociation are found to be 8 × 10⁵ s^-1 and 1 × 10⁶ s^-1, respectively. A red-shift of 38 nm and 50 nm observed in the UV spectra of butadiene produced respectively from BCB and CHN, suggests that vibrationally hot butadiene molecules are formed. The effects of laser fluence, pressure of SF₆ and the no. of laser pulses on dissociation yield and the branching ratio in BCB are also investigated.