Direction of the transition dipole moment of nitrobenzene determined from oriented molecules in a uniform electric field

The direction of the transition dipole moment of nitrobenzene between 230 and 250 nm was determined by orienting gas-phase molecules in a strong, uniform electric field. Oriented nitrobenzene was photodissociated with linearly polarized light, and the NO fragments were detected by resonantly enhanced multiphoton ionization (REMPI). When the polarization direction of the photolysis laser was perpendicular (rather than parallel) to the orientation field, a 44% enhancement in the NO signal was observed. This implies a predominantly perpendicular relationship between the transition dipole and the permanent dipole. However, the experimentally observed enhancement falls below that expected of a pure perpendicular transition, indicating the presence of a second potential-energy surface that is simultaneously accessed through a parallel transition. Quantitative analysis indicates that the parallel transition contributes 20% of the overall oscillator strength. (C) 2000 American Institute of Physics. [S0021-9606(00)00828-X].