n sigma* and pi sigma* excited states in aryl halide photochemistry: a comprehensive study of the UV photodissociation dynamics of iodobenzene

A recent review (Ashfold et al., Phys. Chem. Chem. Phys., 2010, 12, 1218) highlighted the important role of dissociative excited states formed by electron promotion to sigma* orbitals in establishing the photochemistry of many molecular hydrides. Here we extend such considerations to molecular halides, with a particular focus on iodobenzene. Two experimental techniques (velocity mapped ion imaging (VMI) and time resolved infrared (IR) diode laser absorption) and electronic structure calculations have been employed in a comprehensive study of the near ultraviolet (UV) photodissociation of gas phase iodobenzene molecules. The VMI studies yield the speeds and angular distributions of the I(P-2(3/2)) and I*(P-2(1/2)) photofragments formed by photolysis in the wavelength range 330 >= lambda >= 206 nm. Four distinct dissociation channels are observed for the I(P-2(3/2)) atom products, and a further three channels for the I*(P-2(1/2)) fragments. The phenyl (Ph) radical partners formed via one particular I* product channel following excitation at wavelengths 305 >= lambda >= 250 nm are distributed over a sufficiently select sub-set of vibrational (nu) states that the images allow resolution of specific I* + Ph(v) channels, identification of the active product mode (nu(10), an in-plane ring breathing mode), and a refined determination of D-0(Ph-I) = 23 390 +/- 50 cm(-1). The time-resolved IR absorption studies allow determination of the spin-orbit branching ratio in the iodine atom products formed at lambda = 248 nm (phi(I*) = [I*]/([I] + [I*]) = 0.28 +/- 0.04) and at 266 nm (phi(I*) = 0.32 +/- 0.05). The complementary high-level, spin-orbit resolved ab initio calculations of sections (along the C-I bond coordinate) through the ground and first 19 excited state potential energy surfaces (PESs) reveal numerous excited states in the energy range of current interest. Except at the very shortest wavelength, however, all of the observed I and I* products display limiting or near limiting parallel recoil anisotropy. This encourages discussion of the fragmentation dynamics in terms of excitation to states of A(1) total symmetry and dissociation on the 2A(1) and 4A(1) (sigma* <- n/pi) PESs to yield, respectively, I and I* products, or via non-adiabatic coupling to other sigma* <- n/pi PESs that correlate to these respective limits. Similarities (and differences) with the available UV photochemical data for the other aryl halides, and with the simpler (and more thoroughly studied) iodides HI and CH3I, are summarised.