Observations are reported for the first time of significant nondipole effects in the photoionization of the outer-valence orbitals of diatomic molecules. Measured nondipole angular-distribution parameters for the $3{\sigma }_g$, $1{\pi }_u$, and $2{\sigma }_u$ shells of ${\mathrm{N}}_2$ exhibit spectral variations with incident photon energies from thresholds to $\sim 200\mathrm{eV}$ which are attributed via concomitant calculations to particular final-state symmetry waves arising from $(E1)\otimes (M1,E2)$ radiation-matter interactions first-order in photon momentum. Comparisons with previously reported $K$-edge studies in ${\mathrm{N}}_2$ verify linear scaling with photon momentum, accounting in part for the significantly enhanced nondipole behavior observed in inner-shell ionization at correspondingly higher momentum values in this molecule.

• Received 2 June 2006

DOI:https://doi.org/10.1103/PhysRevLett.97.103006