Photodissociation of o-nitrotoluene between 220 and 250 nm in a uniform electric field

Measurements of the NO product from photodissociation of o-nitrotoluene between 220 and 250 nm were performed using one laser for dissociation and detection. The NO fragment was observed in nu = 0-2 vibrational states with an average vibrational energy of 1760 cm(-1). The rotational distributions were non-Boltzmann, with average rotational energies of 2050, 1900, and 460 cm(-1) for the nu = 0, 1, and 2 vibrational levels. Although the quantitative results should be treated with caution due to the variation in the dissociation energy inherent to this type of one laser experiment, this work reveals some significant similarities and differences between nitrotoluene and other nitro compounds, such as nitrobenzene. The direction of the transition dipole moment was determined by orienting gas phase molecules with a strong, uniform electric field prior to dissociation with linearly polarized light. For all of the rotational transitions studied, a 46% enhancement in the NO signal was observed when the photolysis beam was polarized perpendicular rather than parallel to the orientation field. This indicated a predominantly perpendicular relationship between the transition dipole and the permanent dipole of the molecule. The degree of enhancement was less than that predicted of a pure perpendicular transition; thus we propose that a second potential energy surface was simultaneously accessed through a parallel transition. This transition was found to contribute about 15% to the overall oscillator strength.