Nondipolar electron angular distributions from fixed-in-space molecules

Observation of Nondipole-Induced Asymmetry in the Angular Emission Distribution of Photoelectrons from Fixed-in-Space CO Molecules

We investigate experimentally and theoretically the C and O 1s photoionization of fixed-in-space CO molecules at a photon energy of 905 eV. We find a significant dependence of the photoelectron angular distributions on the direction of propagation of the ionizing radiation. It results from an interplay of nondipole effects, on one hand, and molecular effects, on the other. The nondipole effects lead to an increase of the emission probability in the forward direction along the light propagation, and the photoelectron wave being scattered by the molecular potential gives rise to a strong peak in the direction of the atom neighboring the emitter site. These effects can either conspire or extenuate each other, depending on the photoelectron emission direction and molecular orientation in space.

Stimulated Compton scattering in two-color ionization of hydrogen with keV electromagnetic fields

We present a theoretical study of two-color ionization of hydrogen with keV photons at intensities ranging from 1016 to 1018  W/cm2. We consider the atom in interaction with a superposition of two electromagnetic pulses centered around two frequencies that differ by a few atomic units and we present in detail the case of the frequencies 55 and 50 a.u. We present the electron energy spectra, angular distributions, and ionization rates based on nonperturbative and perturbative calculations. Although the ejected electron energy distribution is dominated by one-photon ionization from each pulse, we are able to identify the contribution of stimulated Compton scattering, a process in which one photon is absorbed while the other is emitted, the photon energy difference being transferred to the electron. This leads to low-energy electrons, and we show in particular that it is of crucial importance to consider the retardation effects on the ionization rates and the electron angular distributions. The relative propagation direction of the two fields also plays an important role; in the case of counterpropagating fields, the ionization by stimulated Compton scattering is dominated by A2 and competes with one-photon ionization at high intensities.

Low-energy nondipole effects in molecular nitrogen valence-shell photoionization

Observations are reported for the first time of significant nondipole effects in the photoionization of the outer-valence orbitals of diatomic molecules. Measured nondipole angular-distribution parameters for the 3sigma(g), 1pi(u), and 2sigma(u) shells of N2 exhibit spectral variations with incident photon energies from thresholds to approximately 200 eV which are attributed via concomitant calculations to particular final-state symmetry waves arising from (E1)multiply sign in circle(M1,E2) radiation-matter interactions first-order in photon momentum. Comparisons with previously reported K-edge studies in N2 verify linear scaling with photon momentum, accounting in part for the significantly enhanced nondipole behavior observed in inner-shell ionization at correspondingly higher momentum values in this molecule.

Molecular photoionization cross sections in electron propagator theory: Angular distributions beyond the dipole approximation

Corrections to dipole approximation results for angular distributions in photoionization of first-row hydrides have determined by using Dyson orbitals calculated with ab initio electron propagator theory and by considering the full multipole expansion for the incident photon representation. The relative importance of first-order corrections which consist of electric quadrupole and magnetic dipole terms and of higher-order terms has been estimated as a function of photon energy. Multipole corrections to the dipole approximation depend on photon energy and on the characteristics of the Dyson orbitals.