Graduation date: 2007
The central purpose of this dissertation was to explore and expand the use of
an uniform electric field for orientation and for subsequent spectroscopic investigation
of asymmetric polar molecules in the gas phase. The systems of study were
nitrobenzene (NB) and the nitrotoluene (NT) isomers. We were able to quantitatively
determine the direction of the electronic transition dipole relative to the permanent
dipole for these molecules, thus providing quantitative information on the symmetry
of the second and third singlet excited electronic states. Transition to the second
singlet excited state (S2) was shown to have a dipole predominantly perpendicular to
the permanent dipole, consistent with a localized excitation of the -NO2 moiety. The
transition dipole to the S3 state for the meta and para isomers was almost completely
parallel to the permanent dipole, opposite to that observed for the S2 transition. The
success of these experiments has demonstrated the ability of the Brute Force
Orientation technique to quantitatively characterize the transition dipole properties of
large molecules. The importance of this technique lies in the fact that most large
molecules undergo rapid internal conversion and slow dissociation after absorption,
therefore information on the symmetry properties of these systems is otherwise
unattainable.
In addition to the determination of the transition dipole direction, we were able
to characterize many details of the dissociation process by analyzing the internal
energy distribution of the Nitric Oxide (NO) photofragment. The Resonantly
Enhanced Multi Photon Ionization spectrum of NO has revealed that the methyl group
causes significant perturbation in the dissociation process, while it seems to have
minimal effect on the transition dipole direction among NB and NT. All NT isomers
showed significantly more vibrational excitation than previously reported for NB.
Additionally, the meta and para isomers were observed to have a vibrational inversion
behavior for v” ≥ 3, with the higher vibrational levels having larger populations. The
higher vibrational levels were also found to have higher degree of rotational
excitation. A bimodal behavior was also observed in the rotational distribution of
several vibrational bands. A preference was observed for the π lobe being in the plane
of rotation of the fragment. After a thorough comparison with relevant literature
reports, and based on our experimental results, we present a model for the dissociation
of NT isomers. We believe the bimodal rotational distribution and vibrational
inversion indicate at least two dissociation channels in which isomerization of the
nitro group and an NO2 photofragment play essential roles.