Brute Force Orientation and Alignment of Pyridazine Probed by Resonantly Enhanced Multiphoton Ionization

Experimental measurements and theoretical calculations of brute force orientation in pyridazine (1,2-diazine) are discussed. The sample was prepared using supersonic expansion, and orientation was attempted via a strong uniform electric field. The electrostatic interaction between the permanent dipole moment and the electric field was sufficient to orient and align the molecular axes. Resonantly enhanced multiphoton ionization through the π* ← n transition was used to characterize the alignment of molecular axes. The permanent dipole moment of pyridazine is perpendicular to the transition dipole moment. As molecules became aligned in the orientation field, the transition dipole moments were perpendicular to the field. Enhancement of 40% in the excitation probability was observed when the polarization direction of the resonant laser was changed from parallel to perpendicular relative to the orientation field. Using a quantum mechanical approach, the experimental observation can be explained with satisfaction. Stark effects of the orientation field on energy levels, molecular axes, and electronic transitions were calculated using basis functions of symmetric tops. The calculation reproduced not only the overall variations in the absorption probability due to alignment but also the spectral shape of the electronic transition. This work demonstrates the feasibility of brute force orientation and has established the foundation for potential applications of this technique in studies of complex systems.