Photoelectron angular distributions from two-photon ionization of atoms near ionization threshold

Photoelectron angular distributions(PADs) from two-photon ionization of atoms in linearly polarized strong laser fields are obtained in accordance with the nonperturbative quantum scattering theory.We also study the influence of laser wavelength on PADs.For two-photon ionization very close to the ionization threshold,most of the ionized electrons are vertically ejected to the laser polarization.PADs from twophoton ionization of atoms are determined by the second order generalized phased Bessel function at which the ponderomotive parameter plays a key role.In terms of dependence of PADs on laser wavelength,corresponding variations for the ponderomotive parameter are demonstrated.     

Photoelectron angular distributions in resonant multiphonon ionization of atoms

Conclusions It has been demonstrated that photoelectron angular distributions in resonant multiphoton ionization depend critically on the angular momentum quantum numbers of the intermediate state. This should help to interpret the spectra of atoms more complex than alkalies. In addition, information about the matrix elements for the bound-free transition can be obtained which is not available if the angle-integrated ionization cross section is measured. Reliable cross sections for photoionization out of excited atomic states are very important for astrophysics, but so far only a few laboratory data are available. The interpretation of some of these data is not straightforward, however, because of lack of knowledge about the degree of alignment of the atoms in the excited state. This problem can be solved either by optically pumping the atom into only one magnetic sublevel [24] or by measuring the photoelectron angular distribution in addition to the absolute cross section.Sektion Physik der Universitat München, D-8046 Garching, FRG. JILA Visiting Fellow, 1980–1981. Joint Institute for Laboratory Astrophysics, National Bureau of Standards and University of Colorado, Boulder, Colorado 80309. Published in Primenenie Lazerov v Atomnoi, Molekulyarnoi i Yadernoi Fiziki-Trudy II Vsesoyuznoi Shkoly, pp. 12–23, 1981.     

Photoelectron angular distributions at the ionization of atoms by intense sub-one-cycle laser pulses

The phase sensitivity of the photoelectron angular distributions by intense sub-one-cycle linearly polarized laser pulses has been discussed within the analytic Landau-Dykhne approximation. In both cases of sine and cosine laser pulses most of the electrons are ejected along the polarization axis of the laser field. Nevertheless the electron yield and the electron kinetic energies are much larger for the cosine waveform pulse.     

Photoelectron angular distributions from above threshold ionization of hydrogen atoms in strong laser fields

We apply a scattering theory of nonperturbative quantum electrodynamics to study the photoelectron angular distributions (PADs) of a hydrogen atom irradiated by linearly polarized laser light. The calculated PADs show main lobes and jetlike structure. Previous experimental studies reveal that in a set of above-threshold-ionization peaks when the absorbed-photon number increases by one, the jet number also increases by one. Our study confirms this experimental observation. Our calculations further predict that in some cases three more jets may appear with just one-more-photon absorption. With consideration of laser-frequency change, one less jet may also appear with one-more-photon absorption. The jetlike structure of PADs is due to the maxima of generalized phased Bessel functions, not an indication of the quantum number of photoelectron angular momentum states.     

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.     

Asymmetric angular distributions in two-frequency multiphoton ionization of hydrogen atoms

Summary We report on calculated angular distributions of electrons produced in the ionization of hydrogen atoms by a linearly polarized soft X-ray and a circularly polarized laser light, with both the electric fields vibrating on the same plane. Their shape exhibits asymmetries which are similar to those observed in multiphoton ionization of noble gases by singlefrequency elliptically polarized light. An explanation of these asymmetries is given in terms of lowering of symmetry caused by the simultaneous action of the two fields during the ionization of the atom. The case at hand is another example which allows a useful insight into a new class of physical situations which deserve to be further explored both experimentally and theoretically.

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Riassunto In questo lavoro si calcolano le distribuzioni angolari di elettroni prodotti nella ionizzazione di atomi di idrogeno da raggi X molli e da luce laser polarizzata circolarmente, i cui campi elettrici vibrano nello stesso punto piano. La loro forma mostra asimmetrie che sono simili a quelle osservate nella ionizzazione multifotonica di gas nobili prodotta da un laser polarizzato ellitticamente. La spiegazione di questa asimmetria è data in termini di riduzione di simmetria causata dalla simultanea azione dei due campi durante la ionizzazione dell'atomo. Il presente caso è un esempio di una nuova calsse di situazioni fisiche che meritano di essere esplorate sia sperimentalmente che teoricamente.

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Резюме Приводятся вычисленные угловые распределения электронов, образованных при ионизации водорода линейно поляризованным мягким рентгеновским излучением и диркулярно поляризованным лазерным излучением, причем оба электрических поля колеблются в той же плоскости. Форма распределения обнаруживает асимметрии, которые аналогичны асимметриям, обнаруженным при многофотонной ионизации инертных газов под действием монохроматического эллиптически поляризованного света. Предлагается объяяснение этих асимметрий в терминах понижения симметрии вследствие одновременного действия двух полей во время ионизации атома. Рассматривается другой пример, который позволяет правильно понять новый класс физических ситуаций, которые заслуживают дальнейшего теоретического и эксрериментального исследований.

     

Photoelectron angular distributions and energy spectra of atomic hydrogen in above threshold ionization

Abstract

We extend our model for two-color above threshold ionization to include an infinite number of continua, which are labeled by their angular momenta. Results are presented for the photoelectron spectra and the angular photoelectron distributions.     

Photoelectron angular distributions from gold

Angular distributions of electrons photoemitted from evaporated gold have been measured for 21.2 eV and 40.8 eV photons by varying the acceptance angle of a spherical, electrostatic, electron energy analyzer. The results for primary (unscattered) electrons are shown to be consistent with the three-step model of photoelectron emission.     

Photoelectron angular distributions from fixed-in-space molecules

Since the fixed-molecule photoelectron angular distribution experiment started in the middle of 1990s, its potential in helping to solve in detail problems in the photoionization dynamics of gas-phase molecules has been recognized. In this review, which is aimed at experimentalists, general expressions of photoelectron angular distributions from fixed-in-space molecules are given for arbitrary polarization, special expressions of them are presented for practical uses, and their applications are explained along our progress on relevant studies with emphasis on the photoionization dynamics exhibited by molecular shape resonances.     

Photoelectron angular distributions of H ionization in low energy regime:Comparison between different potentials

We theoretically investigate the low energy part of the photoelectron spectra in the tunneling ionization regime by numerically solving the time-dependent Schrdinger equation for different atomic potentials at various wavelengths.We find that the shift of the first above-threshold ionization(ATI) peak is closely related to the interferences between electron wave packets,which are controlled by the laser field and largely independent of the potential.By gradually changing the short-range potential to the long-range Coulomb potential,we show that the long-range potential's effect is mainly to focus the electrons along the laser's polarization and to generate the spider structure by enhancing the rescattering process with the parent ion.In addition,we find that the intermediate transitions and the Rydberg states have important influences on the number and the shape of the lobes near the threshold.     

Intrinsic electric dipole moments of paramagnetic atoms: rubidium and cesium

The electric dipole moment (EDM) of paramagnetic atoms is sensitive to the intrinsic EDM contribution from that of its constituent electrons and a scalar-pseudoscalar (S-PS) electron-nucleus interaction. The electron EDM and the S-PS contributions to the EDMs of these atoms scale as approximately Z;{3}. Thus, the heavy paramagnetic atoms will exhibit large EDM enhancement factors. However, the sizes of the couplings are so small that they are of interest of high precision atomic experiments. In this work we have computed the EDM enhancement factors of the ground states of Rb and Cs due to both the electron EDM and the S-PS EDM using the relativistic coupled-cluster theory. The importance of determining precise ab initio enhancement factors and experimental results of atomic EDMs in deducing a reliable limit on the electron EDM is emphasized.     

Modeling of laser-induced excitation and ionization in cesium atoms

A theoretical model is presented to describe kinetics of the plasma formation in cesium undergone to resonant laser excitation (D_1,2 line). The model is based on a rate equations approach where the following populations are considered: ground state (6s level), laser excited level (6p), a series of high excited levels close to the ionization limit, and the electron density. We show temporal evolution of these populations and provide an explanation of the kinetic governing the ionization path-ways. Moreover, we compare the behavior of the electron density as a function of the laser power with the experimental data by Hunnenkens et al. This comparison for the electron density with irradiation time is proved a good agreement with the experimental results.     

Photoelectron angular distributions from strong-field coherent electronic excitation

Photoelectron Angular Distributions (PADs) resulting from nonperturbative excitation of potassium atoms using shaped femtosecond laser pulses are presented. We study control exerted by (1) the polarization of an unshaped, i.e., a bandwidth-limited light pulse, (2) shaped linearly polarized light, and (3) a combination of both degrees of freedom, i.e., polarization-shaped laser pulses. A theoretical approach to describe PADs from nonperturbative Resonance Enhanced Multi-Photon Ionization (REMPI) with ultrashort polarization-shaped laser pulses is presented and compared to experimental results. Applications of this technique to the generation and observation of atomic ring currents are discussed.     

Mid-infrared strong field ionization angular distributions

We present a study of the photoelectrons energy distribution from ionization of Argon by a linearly polarized, intense, mid-infrared laser field with special attention to the recently discovered Low Energy Structure (LES) [1] whose origin is not yet fully understood. In this paper we will go deeper in the analysis of the LES by studying its angular distribution and examine its behavior in circularly polarized light.     

Resonant two-photon single ionization of two identical atoms

Resonant two-photon single ionization in a system consisting of two spatially well-separated identical atoms is studied. Because of two-center electron-electron correlations, the ionization may also proceed through photoexcitation of both atoms with subsequent interatomic Coulombic decay. We show that this channel may qualitatively change the dependence of the photoionization on the field intensity as well as the spectra of emitted electrons.