It has been shown by Loesch and Remscheid, and by Friedrich and Hershbach, that applying a strong, homogeneous electric field is a versatile method for orienting polar molecules in a molecular beam. It is convenient to expand the orientation distribution function in Legendre polynomials. The Legendre moments can be calculated from the expansion of the wavefunctions in free-rotor functions, with different J, by carrying out the appropriate integrations. Here, we use an alternative method based on a Clebsch-Gordan expansion, by which explicit integration is avoided. Early quantum-mechanical perturbation treatments of rotating polar molecules in an electric field are reviewed. For the average orientation of a linear dipole in a strong electric field a simple relation is obtained which is based on a zeroth-order perturbation treatment of vibration in two dimensions. This relation is useful for estimating the average orientation of rotationally cold, linear dipoles in a strong electric field.
