Nondipolar electron angular distributions from fixed-in-space molecules

The first indication of nondipole effects in the azimuthal dependence of photoelectron angular distributions emitted from fixed-in-space molecules is demonstrated in N (2). Comparison of the results with angular distributions observed for randomly oriented molecules and theoretical derivations for the nondipole correction first order in photon momentum suggests that higher orders will be needed to describe distributions measured in the molecular frame.     

Jet-like structures in photoelectron angular distributions

The photoelectron angular distributions (PADs) of hydrogen atoms in an intense laser field of linear polarization are studied using the S-matrix theory in the length gauge. The PADs show main lobes along the laser polarization and jet-like structures sticking from the waist of main lobes. Our previous prediction, based on a nonperturbative scattering theory of photoionization developed by Guo et al, showing that the number of jets on one side of PADs may increase by one, three, or other odd numbers and may decrease by one when one more photon is absorbed, is confirmed by this treatment. Within the strong-field approximation, good agreement is obtained between these two quite different treatments. We further study the influence of the Coulomb attraction to PADs, by taking a Coulomb--Volkov state as the continuum state of photoelectrons. We find that under the influence of the Coulomb attraction, the PADs change greatly but the predicted phenomena still appear. This study verifies that the jet-like structures have no relation with the angular momentum of photoelectrons.     

The variation of photoelectron angular distributions with energy

Photoelectron angular distributions for the ionization of the outerp-shell of the rare gases Ar, Kr, and Xe, and of the outerS- andd shell of Hg, are measured at various photon energies obtained from rare gas resonance radiation. The distributions are found to agree with the theoretically predicted form. Values of the anisotropy parameter β are determined, and their dependence on electron energy is compared with available numerical results, as well as with model calculations performed in this work. In the case of Hg the possible influence of spin-orbit coupling on the β-values is discussed.     

Theoretical analysis of photoelectron angular distributions in hydrogen chloride

Multiple-scattering calculations for the ≈X² II and ≈A²Σ⁺ ionizations of HCl are analysed by a partitioning procedure to explain the angular distributions around the Cooper minima. A formal analogy to resonances is pointed out. The Cooper minima in the valence ionizations of Cl₂ are discussed qualitatively.     

Surface analysis and angular distributions in x-ray photoelectron spectroscopy

A theoretical and experimental study of the application of x-ray-photoelectron angular distribution measurements to quantitative surface characterizations is presented. The basic theoretical model that has been used previously to analyze such angular distributions from flat surfaces in the absence of electron-diffraction (channeling) effects is discussed, including certain new generalizations and special cases pertinent to surface analysis. Previous experimental work is reviewed. The predictions of this model are also found to be consistent with new experimental data obtained from gold specimens with carbon-containing surface layers and from aluminum specimens with successive oxide- and carbon-containing layers. An order of magnitude increase in surface-layer relative intensities is observed at low electron escape angles relative to the surface. Also, effects due to x-ray refraction and reflection are found for very low angles of incidence, and these lead to approximately a four-fold increase in surface-layer relative intensities. Extensions of the theory to include the effects of non-uniform x-ray flux, a more realistic spectrometer acceptance function, non-uniformity of surface layers, and surface roughness are also considered, and numerical calculations for the specific case of a sinusoidally rough surface are presented. It is shown that rough-surface intensities will equal flat-surface intensities provided that both surfaces are clean and that no x-ray shading occurs. If surface layers are present, however, rough-surface angular distributions are predicted to deviate markedly from flat-surface distributions. By means of angular distribution measurements, it thus appears possible to selectively enhance near-surface contributions to photoelectron spectra, as well as to obtain information concerning electron mean free paths, surface layer thicknesses and uniformity, and perhaps surface roughnesses.     

X-ray photoelectron angular distributions with dispersion-compensating x-ray and electron optics

A general model for predicting instrumental effects on intensifies and line-shapes in angle-resolved X-ray photoemission experiments is presented. This     

How to measure the internuclear vector from photoelectron angular distributions

This paper uses a nonperturbative scattering theory to study photoelectron angular distributions of homonuclear diatomic molecules irradiated by circularly polarized laser fields. This study shows that the nonisotropic feature of photoelectron angular distributions is not due to the polarization of the laser field but the internuclear vector of the molecules. It suggests a method to measure the molecular orientation and the internuclear distance of molecules through the measurement of photoelectron angular distributions.     

Strong field modifications of photoelectron angular distributions in resonant two-photon ionization

The two symmetry parameters β₂ and β₄, which govern completely the photoelectron angular distribution in resonant two-photon ionization, are found to depend strongly on the electric field strength of the light pulse.     

Time-resolved photoelectron angular distributions from strong-field ionization of rotating naphthalene molecules

A nanosecond laser pulse confines the spatial orientation of naphthalene in 1D or 3D while a femtosecond kick pulse initiates rotation of the molecular plane around the fixed long axis. Time-dependent photoelectron angular distributions (PADs), resulting from ionization by an intense femtosecond probe pulse, exhibit pronounced changes as the molecular plane rotates. Enhanced 3D alignment, occurring shortly after the kick pulse, provides strongly improved contrast in molecular-frame PADs. Calculations in the strong-field approximation show that the striking structures observed in the PADs originate from nodal planes in occupied valence orbitals.     

Internal inelastic scattering satellite probed by molecular-frame photoelectron angular distributions from CO2

The molecular-frame photoelectron angular distribution (MFPAD) of the satellite accompanying the C 1s photoline of the CO2 molecule has been measured at the C 1s(2sigmag)-->4sigmau* shape resonance, using electron-ion multicoincidence momentum spectroscopy. The observed MFPAD indicates that the conjugate satellite is excited by internal inelastic scattering. In this scenario, a photoelectron is ejected from the C 1s(2sigmag) orbital along the molecular axis and collides with an O lone-pair electron in the highest occupied molecular orbital 1pig. Then one of the colliding electrons is trapped to the lowest unoccupied molecular orbital 2piu*, while the other is emitted as a satellite photoelectron of sigmag symmetry, losing the information of the original photoelectron emission direction and parity.     

Decay channel dependence of the photoelectron angular distributions in core-level ionization of Ne dimers

For K-shell photoionization of neon dimers, we report Ne 1s photoelectron angular distributions for Ne2++Ne+ and Ne++Ne+ channels exhibiting quite different patterns. Noninversion-symmetric patterns of the former obtained by the fast interatomic Coulombic decay of Auger final states show direct evidence of core-hole localization. Dipolar patterns of the latter obtained by the slow radiative decay of the other Auger final states clearly show that the radiative process is slow enough to allow dicationic dimers to rotate many times before fragmentation.     

Observation of the phase lag in the asymmetric photoelectron angular distributions of atomic barium

We have observed and measured the phase lag in the phase-dependent variation of the asymmetric photoelectron angular distribution of atomic barium. For these measurements, we photoionize the 6s6p(1)P(1) intermediate state of barium with concurrent one-photon and two-photon (omega-2omega) interactions. The laser interactions ionize the atoms in the vicinity of the series of autoionizing states converging upon the 5d(2)D(5/2) threshold. We study the variation of the phase lag as a function of the laser frequency. The variation shows strong correlation to the location of the autoionizing resonances, with full range exceeding 2pi, confirming the critical role of these resonances.     

The photoelectron angular distributions of HBr and HI AT hν = 21.2 eV

Photoelectron angular distributions for the ²Π_{$\text{3}\text{2}$, $\text{1}\text{2}$}(pπ)^?1 and ²Σ_{$\text{1}\text{2}$}⁺(pσ)^?1 ionic states of HBr and HI have been measured at a photon energy of 21.2 eV. The asymmetry parameters for the highly localized pπ orbitals closely follow those of the “lone-pair” orbitals of the related CH₃X molecules and the outer np atomic orbitals of the corresponding united atoms. Furthermore, the asymmetry parameters for the ²Π_{$\text{3}\text{2}$} and ²Π_{$\text{1}\text{2}$} states were found to be equal within experimental uncertainty, despite the large spin—orbit splittings of these heavy molecules. The asymmetry parameters for the ²Σ⁺(pσ)^?1 ionic states are significantly smaller than for the ²Π(pπ)^?1 states, in contrast to recent predictions based on angular momentum transfer theory.     

Molecular photoelectron momentum and angular distributions of N_2 molecules by ultrashort attosecond laser pulses

The ultrafast photoionization dynamics of N_2 molecules by x-ray/XUV laser pulses is investigated.The molecular frame photoelectron momentum distributions(MF-PMDs) and the molecular frame photoelectron angular distributions(MF-PADs) are obtained by numerically solving 2 D time-dependent Schrodinger equations within the single-electron approximation(SEA) frame.The results show that the molecular photoionization diffraction appears in 5 nm laser fields.However,when the laser wavelength is 30 nm,the molecular photoionization diffraction disappears and the MF-PMDs show four-lobe pattern.The ultrafast photoionization model can be employed to describe the MF-PMDs and MF-PADs of N_2 molecules.