Photon-momentum transfer in molecular photoionization

In most models and theoretical calculations describing multiphoton ionization by infrared light, the dipole approximation is used. This is equivalent to setting the very small photon momentum to zero. Using numerical solutions of the (nondipole) three-dimensional time-dependent Schr\"odinger equation for one electron in a ${\mathrm{H}}_{2}{}^{+}$ molecular ion we investigate the effect the photon-momentum transfer to the photoelectron in an ${\mathrm{H}}_{2}{}^{+}$ ion in various regimes. We find that the photon-momentum transfer in a molecule is very different from the transfer in atoms due to two-center interference effects. The photon-momentum transfer is very sensitive to the symmetry of the initial electronic state and is strongly dependent on the internuclear distance and on the ellipticity of the laser.