Orientation of asymmetric top molecules in a uniform electric field: Calculations for species without symmetry axes

Calculations of orientation effects of polar molecules in a uniform electric field are presented for the most general scenario, an asymmetric top molecule with a permanent dipole not parallel to a principal axis. In addition to details of the calculation procedure, including matrix elements of the Hamiltonian, three different treatments of the population distribution of the Stark levels in an electric field are discussed. The adiabatic approach assumes the noncrossing rule for all energy levels as the orientation field increases, the nonadiabatic approach searches for the level with the most similar wave function under field-free conditions to find the population of the Stark level in the field, and the thermal calculation assumes thermal distribution for all of the Stark levels. Among these, the thermal calculation results in the highest degree of orientation, and in high fields, it shows the best agreement with available experimental data in terms of polarization ratios (the ratios of overall excitation probabilities under two perpendicular polarization directions of the laser). By use of cytosine at a rotational temperature of 5 K and adenine at 2 K as model compounds, the thermal calculation suggests that in a field of 50 kV/cm, more than 30% of the molecules should be confined within a 45 degrees cone surrounding the direction of the orientation field, and that if a transition dipole is perpendicular to the permanent dipole, the excitation probability can be enhanced by 50% when the polarization direction of the laser is perpendicular; rather than parallel, to the orientation field. The adiabatic and nonadiabatic calculations yield similar distribution functions of the permanent dipole, both predicting weaker orientation than that of the thermal calculation. According to comparisons of spectroscopic details between the calculations and experiment using the pi* <-- n transition in pyrimidine, however, all three calculations agree with the experimental spectra. Further experimental evidence with higher quality spectra is needed for a conclusive statement. Orientation using a uniform electric field is particularly suitable for studies of large systems with small rotational constants: the orientation effect is proven to be determined by the size of the permanent dipole, essentially independent of the orientation of the permanent dipole in the molecular frame. For small molecules, however, this type or orientation is unfavorable, and the resulting orientation is sensitive to the molecular parameters, such as the rotational constants, and the size and direction of the permanent dipole in the molecular frame.