Na Cherepkov

On a complete experiment on photoionization of atoms

There are seven experimentally measurable parameters characterizing the photoionization of a closed shell atom in the electric dipole approximation, namely, the cross section, four parameters describing the photoelectron angular distribution and spin polarization, and two parameters describing the alignment and orientation of the residual ion. They are defined by five theoretical values, three dipole matrix elements and two phase shift differences. As a consequence, these seven experimental parameters are not mutually independent, and there must be two equations connecting them. One of these equations connecting the photoionization parameters was found earlier, and the second one for the particular case of photoionization of an np3/2 subshell of a closed shell atom is derived here. It connects the photoelectron parameters with the alignment and orientation of the residual ion. We also checked that there are no other independent relations between the parameters. From these facts it follows that a complete photoionization experiment is possible, and it must include the measurements of the parameters of both the photoelectron and the residual ion.

Angular distributions and dichroism of photoelectrons ejected from fixed-in-space molecules of definite symmetry: Application to the C2v symmetry group

A theoretical investigation of the angular distributions of photoelectrons (ADP) ejected from fixed-in-space molecules or clusters having C2v symmetry is presented. The consideration is based on an expansion of the photoelectron wave functions in symmetry-adapted spherical harmonics. The ADP is derived for linearly polarized, circularly polarized, and unpolarized light in the case when the light beam is parallel to the C2 axis of the molecule. The twofold symmetry of the target reveals itself most clearly for circularly polarized or unpolarized light if the dependence on the azimuthal angle is studied for a fixed polar angle. The circular dichroism in the angular distribution (CDAD) is investigated for the general case of an arbitrary direction of the photon beam. It is shown on model examples that the CDAD has characteristic zeros in the mirror planes of the molecule that may be used to determine the symmetry of the initial state. For linearly polarized light the photoelectron angular distributions have ...

Angular distributions of photoelectrons ejected from fixed-in-space molecules of C3v symmetry group

The angular distributions of photoelectrons from fixed-in-space molecules of C3v symmetry group are studied for the particular case when the light beam is incoming parallel to the C3 rotational axis of the molecule. The consideration is based on the expansion of photoelectron wave functions in symmetry-adapted spherical harmonics. The analytical equations for the angular distributions of photoelectrons ejected by linearly polarized, circularly polarized, and unpolarized light have been derived. The threefold symmetry of the target reveals itself most clearly for circularly polarized and unpolarized light when the dependence on the azimuthal angle is studied for a fixed polar angle. For linearly polarized light the photoelectron angular distributions have a more complicated dependence on the azimuthal angle. On the other hand, with linearly polarized light one can easily distinguish between the a1 and a2 initial states by observing the zeroes of photoelectron intensities in the symmetry plane ejected by li...

Auger decay of fixed-in-space linear molecules

Abstract Photoionization and subsequent Auger decay of fixed-in-space diatomic molecules is considered using a two-step model. A general expression for the angular distribution of Auger electrons and photoelectrons detected in coincidence is obtained. Some particular cases of experimental geometries whith only Auger electrons detected, are considered in more detail.

Determination of phase differences of transition matrix elements from Pt(110) by means of spin-resolved photoemission with circularly and linearly polarized radiation

For Pt(110)-(1 x 2), the phase difference between the complex transition dipole matrix elements M-parallel to((3)) and M-parallel to((4)), i.e. the dipole matrix elements for transitions from initial states of double-group symmetry Sigma(5) with spatial parts Sigma 3/5 and Sigma 4/5 of single-group symmetry Sigma(3) and Sigma(4), respectively, to final states of double-group symmetry Sigma(5) with the spatial part Sigma 1/5 of single-group symmetry Sigma(1), has been determined quantitatively by means of spin-resolved photoemission spectroscopy. The measurements were performed at the BESSY 6.5 m NIM with circularly and linearly polarized synchrotron radiation using a photon energy of 11.8 eV in the highly symmetrical set-up of normal incidence and normal emission. The experimental results obtained with the linearly polarized radiation show very good agreement with a recent theoretical prediction of Henk and Feder [Europhys. Lett. 28 (1994) 609]. The phase difference determined is interpreted to be the physical origin of the non-vanishing electron spin polarization in the experiments with linearly polarized radiation. Within an atomic model, the alignment of the atoms on the surface is shown to be the physical origin of the effect

Spin-polarization in HBr: Comparison between theory and experiment

Measurements of the electron spin polarization parameter A have been performed for HBr in the energy region between the two ionization thresholds, 2Π3/2 and 2Π1/2. Using the multichannel quantum defect theory and formulas derived for the spin parameters in intermediate coupling between Hund’s cases (a) and (e), calculations for both the cross section and the A parameter have been performed. It is shown that the values of A for each Rydberg resonance change with n, following the change from Hund’s case (a) to case (e). In the approximations used in this paper, no simple relation between the sign of A and the value of J+, the rotational angular momentum of the ion core, was found. Values for the calculated angular asymmetry parameter β and the spin polarization parameter ξ are also presented. Since our calculations reproduced only part of the observed resonances, and since the measurements of the A parameter have been performed in relatively narrow energy regions, one could not perform an extensive comparis...

Spin polarization of zero kinetic energy electrons from HBr

The range of observables in zero kinetic energy (ZEKE) electron spectroscopy of molecules, previously restricted to the total electron intensity, was extended by measuring the integral spin polarization of ZEKE electrons at the HBr Ω+=3/2 thresholds after single-photon excitation with narrow-band circularly polarized light. A comparison with calculated values from multichannel quantum defect theory underlines the importance of autoionization for the decay dynamics.

Fixed-molecule photoelectron angular distributions with defined spin polarisation

A general expression for the angular distribution of photoelectrons with defined spin polarisation ejected from oriented molecules is derived. For linear molecules it is shown that in Hunds cases (a) and (b) photoelectrons may be polarised only parallel to the molecular axis, while in Hunds case (c) all three mutually perpendicular components of the polarisation vector are different from zero.

Photoionization of fixed-in-space molecules by partially polarized light.

A general equation for the three-dimensional angular distribution of photoelectrons ejected from fixed-in-space molecules of any symmetry by light of arbitrary polarization is derived. The state of the light polarization is described by the Stokes parameters. The equation is also valid for photoionization of polarized atoms and aligned or oriented rotating molecules. In the particular case of linear molecules the three-dimensional angular distribution of photoelectrons is fully characterized by five two-dimensional angular distributions. Simple ways to determine experimentally these two-dimensional functions are mentioned. The application of general equations is illustrated by a numerical example of photoionization of the C K-shell of CO molecule in the region of the σ∗ shape resonance.

Resonance enhancement of non-dipole effects in spin polarization of atomic photoelectrons

Resonances in quadrupole channels corresponding to the np → ef transitions are investigated in atoms preceding Ce (Z = 59) and Pa (Z = 91), where the 4f and 5f shells, respectively, start to be filled. These resonances lead to a prominent enhancement of all non-dipole effects in the angular distribution and spin polarization of photoelectrons at low photon energies of the order of 100–200 eV. Since in atoms between Cd and Xe the 4p1/2 photoline is absent from the photoelectron spectrum, we investigate these effects in more detail in Hg where both 5p1/2 and 5p3/2 lines are present. We demonstrate that near these resonances the non-dipole contribution to the degree of polarization of photoelectrons in Hg can be of the order of 5%. For the absorption of linearly polarized light the degree of polarization due to the non-dipole contribution tends towards 100%, but the photoelectron intensity in that case tends towards zero. Corrections to the photoelectron intensities in the angular distributions at some angles can exceed 10%. The similar resonance effects in Rn are expected to be substantially stronger.