Photodissociation of laser aligned iodobenzene: Towards selective photoexcitation

Iodobenzene molecules, having their symmetry axis aligned to the polarization direction of a strong, linearly polarized nanosecond laser pulse, are photodissociated into phenyl and iodine radicals with a 1.5 ps long laser pulse at 266 nm. The yield of I photoproducts, detected by resonant multiphoton ionization, is enhanced up to a factor of 2.7 when the dissociation laser is polarized parallel instead of perpendicular to the alignment laser polarization. In particular, the high-velocity distribution of I products, corresponding to excitation of an (n,sigma*) repulsive surface, is enhanced by a factor of 3.3, when comparing parallel and perpendicular polarizations, whereas the low-velocity distribution of I products, corresponding to excitation of predissociative (pi,pi*) surfaces is only enhanced by a factor of 2.2. The difference is explained by the different directions of the transition dipole moment for the two transitions. We discuss the perspectives for selective photoexcitation using three dimensionally aligned molecules. (C) 2002 American Institute of Physics.