  Recent observations of considerable spin polarization in photoemission from
metal surfaces through monolayers of chiral molecules were followed by several
efforts to rationalize the results as the effect of spin-orbit interaction that
accompanies electronic motion on helical, or more generally strongly curved,
potential surfaces. In this paper we (a) argue, using simple models, that
motion in curved force-fields with the typical energies used and the
characteristic geometry of DNA cannot account for such observations; (b)
introduce the concept of induced spin filtering, whereupon selectivity in the
transmission of the electron orbital angular momentum can induce spin
selectivity in the transmission process provided there is strong spin-orbit
coupling in the substrate; and (c) show that the spin polarizability in the
tunneling current as well as the photoemission current from gold covered by
helical adsorbates can be of the observed order of magnitude. Our results can
account for most of the published observations that involved gold and silver
substrates, however recent results obtained with an aluminum substrate can be
rationalized within the present model only if strong spin-orbit coupling is
caused by the built-in electric field at the molecule-metal interface.
