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 electri c field are presented for the most general scenario, an asymmetric top mole cule with a permanent dipole not parallel to a principal axis. In addition to details of the calculation procedure, including matrix elements of the H amiltonian, three different treatments of the population distribution of th e Stark levels in an electric field are discussed. The adiabatic approach a ssumes the noncrossing rule for all energy levels as the orientation field increases, the nonadiabatic approach searches for the level with the most s imilar wave function under field-free conditions to find the population of the Stark level in the field, and the thermal calculation assumes thermal d istribution for all of the Stark levels. Among these, the thermal calculati on results in the highest degree of orientation, and in high fields, it sho ws the best agreement with available experimental data in terms of polariza tion ratios (the ratios of overall excitation probabilities under two perpe ndicular polarization directions of the laser). By use of cytosine at a rot ational temperature of 5 K and adenine at 2 K as model compounds, the therm al calculation suggests that in a field of 50 kV/cm, more than 30% of the m olecules should be confined within a 45 degrees cone surrounding the direct ion of the orientation field, and that if a transition dipole is perpendicu lar to the permanent dipole, the excitation probability can be enhanced by 50% when the polarization direction of the laser is perpendicular; rather t han parallel, to the orientation field. The adiabatic and nonadiabatic calc ulations yield similar distribution functions of the permanent dipole, both predicting weaker orientation than that of the thermal calculation. Accord ing to comparisons of spectroscopic details between the calculations and ex periment using the pi* <-- n transition in pyrimidine, however, all three c alculations agree with the experimental spectra. Further experimental evide nce with higher quality spectra is needed for a conclusive statement. Orien tation 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 i ndependent of the orientation of the permanent dipole in the molecular fram e. For small molecules, however, this type or orientation is unfavorable, a nd the resulting orientation is sensitive to the molecular parameters, such as the rotational constants, and the size and direction of the permanent d ipole in the molecular frame.