First measurements of macroscopic drag currents under the action of photon flux

Until recently, it was believed that photoelectron angular distributions of atoms and molecules at photon energies below 1 keV is well described by the dipole approximation. While this is true for the angle-integrated total and partial cross sections to which only the squares of the transition matrix elements contribute, recent calculations and measurements [1] have shown that nondipole effects can strongly influence the differential cross sections. These nondipole effects lead to a forward-backward asymmetry in photoelectron emission resulting in a net flux of electrons parallel or anti-parallel to the photon propagation direction. The integrated flux can be measured as a macroscopic current called drag current [2]. With our new instrument we were able to measure these drag currents on a variety of gases such as Ne, Xe, and N$_{2}$ in the photonenergy range between 80 and 240 eV. Our results show that drag currents are non-negligible electron transport processes even at low photon energies. This may be of importance to fields such as astrophysics or physics of the upper planetary atmospheres. [1] O. Hemmers, R. Guillemin, D.W. Lindle, Rad. Phys. Chem. \textbf{70}, 123 (2004). [2] M.Ya. Amusia, A.S. Baltenkov, L.V. Chernysheva, Z. Felfli, A.Z. Msezane, and J. Nordgren, Phys. Rev. A \textbf{63}, 052512 (2001).