  The properties of molecule-optical elements such as lenses or prisms based on
the interaction of molecules with optical fields depend in a crucial way on the
molecular quantum state and its alignment created by the optical field.
However, in previous experimental studies, the effects of state-dependent
alignment have never been included in estimates of the optical dipole force
acting on the molecules while previous theoretical investigations took the
state-dependent molecular alignment into account only implicitly. Herein, we
consider the effects of molecular alignment explicitly and, to this end,
introduce an effective polarizability which takes proper account of molecular
alignment and is directly related to the alignment-dependent optical dipole
force. We illustrate the significance of including molecular alignment in the
optical dipole force by a trajectory study that compares previously used
approximations with the present approach. The trajectory simulations were
carried out for an ensemble of linear molecules subject to either propagating
or standing-wave optical fields for a range of temperatures and laser
intensities. The results demonstrate that the alignment-dependent effective
polarizability can serve to provide correct estimates of the optical dipole
force, on which a state-selection method applicable to nonpolar molecules could
be based. We note that an analogous analysis of the forces acting on polar
molecules subject to an inhomogeneous static electric field reveals a similarly
strong dependence on molecular orientation.
