Circular dichroism in the angular distribution of photoelectrons from oriented CO molecules

Abstract

Theoretical predictions of a new phenomenon arising in photoelectron emission from oriented molecules by circularly polarized light have been experimentally verified for CO. For a special geometry photoelectron-intensity differences occur upon reversal of photon helicity. The measured asymmetries (up to 80%) show good agreement with an ab initio calculation at photon energies between 20 and 40 eV. Theoretically, the new manifestation of circular dichroism is already obtained in the pure electric dipole approximation without inclusion of spin-orbit interaction.PACS numbers: 33.60. Cv, 33.55.Ad, 33.80.Eh In recent years several theoretical investigations have predicted circular dichroism (CD) in photoemission, a phenomenon leading to different photoelectron intensities when the ionizing light is either left-or rightcircularly polarized. Regarding its outward appearance, this effect shows complete analogy to the "classical" CD in photoabsorption, 6 well known for optically active molecules without a plane or center of symmetry. Classical CD is described by an interference of electric and magnetic dipole transitions. Consequently, the measured asymmetries are relatively weak. 7 For obvious reasons, CD with chiral molecules should be visible in photoemission as well. Ritchie x first pointed out that nonchiral molecules also might show a circular dichroism in the angular distribution (CDAD) of photoelectrons if they are fixed in space (we will call this spatially oriented). Cherepkov 2 found that for all fixed molecules CDAD occurs in the electric dipole approximation. Proceeding from this important result he predicted that CDAD asymmetries are of the same order of magnitude as the differential cross section itself and hence should be easily observable in a photoemission experiment.A numerical model calculation of CDAD in photoemission from the 4<r orbital of spatially oriented CO was performed by Dubs, Dixit, and McKoy. 3 The calculation showed pronounced CDAD features with huge maxima of almost 100% asymmetry. It is the purpose of our Letter to report the experimental verification of the general predictions of this new effect and to present a quantitative comparison between ab initio theory and experiment.Possible ways to obtain a molecular orientation or alignment in a photoemission experiment are the use of an external electrostatic hexapole field 8 or optical pumping to an excited state with polarized laser light. 9 The first technique especially is rather involved and yields only relatively low target densities and an incomplete orientation. In resonant two-photon photoionization of NO molecules a CDAD due to partial alignment of the intermediate state has been reported recently. 9 We chose to fix CO molecules by adsorption on a singlecrystal metal surface, which yields a perfectly oriented ensemble of molecules with the carbon end pointing towards the metal. 10 Thus it is possible to obtain a very high target density, about 2 orders of magnitude higher than in the gas phase, resulting in huge photoelectron in...

dxz/yz orbital subband structures and chiral orbital angular momentum in the (001) surface states of SrTiO3

“…Before discussing the usefulness of the CD-ARPES technique, we discuss the origin of the CD-ARPES signal as there are different views on the origin of the CD signal 32,[38][39][40] . CD signal refers to the difference in the reaction to right and left circularly polarized light.…”

Circular dichroism and bilayer splitting in the normal state of underdoped(Pb,Bi)2Sr2(Борисенко1, Kordyuk2, Legner3 et al. 2004Phys. Rev. B

“…An interference effect, known in the literature as circular dichroism in the angular distribution (CDAD), can be observed in nonchiral systems only if the experimental setup has a definite chirality. 21 As mentioned above, our experimental arrangement is such that all three vectors (incidence, emission and normal to the surface) are always kept within the same plane. This rules out a purely geometric origin of the dichroism.…”

Real-space Green's function approach to angle-resolved resonant photoemission: Spin polarization and circular dichroism in itinerant magnets

“…This is expected since for magnetic circular dichroism, spin orbit (SO) coupling is necessary, but here such coupling is neglected in va- lence and continuum states. Circular dichroism in the angular distribution (CDAD) [53][54][55][56][57][58][59][60] effects are also absent, as the geometrical set up is not chiral. The RPES signal, however, shows a large dichroism, which is maximum for the strongest enhancement of the signal and of opposite sign for the two spin channels.…”