Photodissociation of cyclobutyl bromide at 234 nm studied using velocity map imaging

This study investigates the 234 nm photodissociation dynamics of cyclobutyl bromide using a two-dimensional photofragment velocity imaging technique. The spin - orbit ground- and excited-state Br( ²P) atoms are stateselectively detected via [2+1] resonance enhanced multiphoton ionization (REMPI), whereas the cyclobutyl radicals are ionized using 157 nm laser light. The Br( ²P _3/2) and the Br( ²P _1/2) atoms and their c-C ₄H ₇ radical cofragments evidence a single-peaked, Gaussian-shaped translational energy distribution ranging from ∼14 to ∼ 39 kcal/mol and angular distributions with significant parallel character. The Br( ²P _1/2)/ Br( ²P _3/2) spinorbit branching ratio is determined to be 0.11 ± 0.07 by momentum match between the Br( ²P) photofragments and the recoiling c-C ₄H ₇ fragments, assuming a uniform photoionization probability of the c-C ₄H ₇ radicals with an internal energy range of 10-35 kcal/mol. The REMPI line strength ratio for the detection of Br( ²P _3/2) and Br( ²P _1/2) atoms at 233.681 and 234.021 nm, respectively, is therefore derived to be 0.10 ± 0.07. The measured recoil kinetic energies of the c-C ₄H ₇ radicals, and the resulting distribution of internal energies, indicates some of the radicals are formed with total internal energies above the barrier to isomerization and subsequent dissociation, but our analysis indicates they may be stable due to the substantial fraction of the internal energy which is partitioned to rotational energy of the radicals. © 2006 American Chemical Society.