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The versatility of the brute force orientation of polar asymmetric top molecules in a molecular beam has been investigated. In symmetric top molecules the electric field only mixes free rotor basis functions with different J but equal K and M values, but in asymmetric top molecules the mixing includes K, because of the asymmetry, in addition to field-induced J-mixing. This distinction is important with respect to the orientation behavior. For asymmetric top molecules all Stark curves for different J-states and different K-values, but equal M, in the corresponding symmetric eigenbasis, feature avoided crossings. Dependent on the velocity with which the molecules pass through the orientation field, these avoided crossings will be traversed adiabatically or nonadiabatically. For near-symmetric top molecules, such as iodobenzene, the crossings will in general be nonadiabatic, and, as expected, the behavior is similar to that of the corresponding symmetric top. If the crossings are adiabatic, the orientation behavior can be drastically different from the behavior of the corresponding symmetric top molecule. A strong asymmetry need not always be prohibitive in attaining a perceptible degree of orientation, as is demonstrated by the case of water.

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° 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 π* ← 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.

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The energies and eigenfunctions of hyperfine levels belonging to the J=‖K‖=1 rotational manifold of the methylhalides have been calculated as a function of the strength of an applied electric (dc) field. From the eigenfunctions, the molecular orientational distribution function P(cos ϑ), where ϑ is the angle of the principal axis relative to the electric field, has been obtained. In the case of interest (J=1), the Legendre expansion of P(ρ), with ρ≡cos ϑ, can be characterized by two moments only: 〈P1〉 the orientation and 〈P2〉 the alignment. The calculated moments have been compared with data available from different molecular beam experiments, without recourse to a specific collision model. For the passage of the molecules through different electric fields, the adiabatic approximation has been assumed to apply, except for curve crossings with extremely small gaps. For methylfluoride and methylchloride, the calculated orientations are in agreement with the measured steric effects of the reactive collisions with Ca atoms, down to a field strength of a few volts per centimeter. At lower field strengths the orientation falls off more rapidly than predicted by the theory. This behavior is tentatively attributed to the variation in the direction of the electric field, and hence in the axis of quantization, across the reaction volume, at low field strengths. This may, in addition, cause a breakdown of the adiabatic approximation. In the case of methyliodide, quantitative agreement between the theoretical and the experimental asymmetry parameters of photodissociation is obtained if the latter ones are provisionally corrected for systematic errors. Contrary to earlier preliminary conclusions, the results show that within experimental error the adiabatic approximation is applicable. Up until now, no experimental data has been available for methylbromide. The calculations clearly demonstrate the inadequacy of restricting theoretical calculations to limiting cases, i.e., weak field and strong field, particularly because the Stark curves reveal the presence and the precise position of avoided crossings. Dependent on the field strength used for state selection and focusing, these crossings may, in principle, have a marked effect on the orientation obtained, given that the final orientation field at the reaction site remains the same. In addition, knowledge of the detailed Stark curves suggests a novel and feasible way to reverse the orientation of the molecules without interference with the detection system, which may be particularly useful if ionic products are formed.