Laser-assisted Photoionization Research Articles

Laser-assisted photoionization beyond the dipole approximation

We present a theoretical study of atomic laser-assisted photoionization emission (LAPE) beyond the dipole approximation. By considering the non-relativistic non-dipole strong-field approximation (non-dipole Gordon-Volkov wave function), we analyze the different contributions to the photoelectron spectrum (PES), which can be written in terms of intra- and intercycle factors. We find that not only does our non-dipole approach exhibit asymmetric emission in the direction of light propagation, but also allows emission in dipole-forbidden directions. The former feature can be rooted both in intra- and intercycle interference processes, whilst the latter stems from a dependence of the sideband energy on the emission angle with respect to the propagation direction. Our theoretical scheme, presented here for He atoms in the 1s quantum state, is general enough to be applied to other atomic species and field configurations.

Thomas–Reiche–Kuhn correction for truncated configuration-interaction spaces: Case of laser-assisted dynamical interference

The Thomas--Reiche--Kuhn sum rule is used to form an effective potential that is added to the time-dependent configuration-interaction singles (TDCIS) equations of motion in velocity gauge. The purpose of the effective potential is to include virtual coupling from singles to doubles, which is required for size-consistent velocity-gauge TDCIS results. The proposed method is compared to length-gauge TDCIS results for laser-assisted photoionization. Finally, a dynamical interference effect controlled by two-color fields is predicted for atomic targets.

Analytical model for attosecond time delays and Fano's propensity rules in the continuum

Extracting single photoionization time delays associated with atomic (or molecular) species from attosecond timescale two-photon experiments usually relies on the theoretical description of continuum-continuum transitions. The available models for those processes predict a universal phase contribution, independent of the angular quantum numbers of final states. However, a recent experimental-theoretical study [Fuchs et al., Optica 7, 154 (2020)] determined a sizable time-delay dependence on the angular momentum of near-threshold photoelectrons. In this paper, we present an analytical model for the two-photon two-color transition matrix amplitudes that reproduces the phase dependence on the angular quantum number of final states. Finally, we show that our analytical model can also describe the generalized Fano's propensity rules [Busto et al., Phys. Rev. Lett. 123, 133201 (2019)] for laser-assisted photoionization.

Multiphoton interaction phase shifts in attosecond science

Ab initio simulations of a range of interferometric experiments are used to identify a strong dependence on multiphoton phase shifts in above-threshold ionization. A simple rule of thumb for interaction phase shifts is derived to explain both the conservation of photoelectron yield and its absolute CEP-dependence. For instance, it is found that interferometric above-threshold ionization experiments are shifted by $\pi/4$ relative to RABBIT experiments, and that there is no RABBIT-term in a laser-assisted photoionization experiment with odd and even harmonics. Thus, our work helps to resolve the issues of interpretation of quantum dynamics in attosecond and free-electron laser sciences.

The Coulomb effects in laser-assisted single photoionization

Attosecond photoionization time delays reveal information about an outgoing electron getting away from the Coulomb potential. In this study, we improve the eikonal approximation model that overestimates the laser-Coulomb coupling effect in the laser-assisted photoionization of atoms. The improved eikonal approximation model can be accurate on about the 2 asec level relative to the time-dependent Schrödinger equation results. The improvement of photoionization time delays demonstrates that the electrons ionized by extreme ultraviolet attosecond pulse possesses momentum larger than the asymptotical momentum due to the attractive Coulomb potential of the parent ion. These results will give a deeper insight into understanding the laser-assisted single photoionization.

An eikonal approximation model for two-color two-photon attosecond interferometric spectrum

The emission time of photoelectrons from atoms, molecules and solids can be accurately measured on an attosecond scale by using two-color two-photon attosecond interferometric spectroscopy, which helps us to understand the ultrafast electronic dynamics in laser-assisted single photoionization. Understanding the photoelectron emission time depends on the physical model, and the relevant theoretical model provides a better physical explanation and numerical prediction for the photoemission time delay. Although the numerical solution of the time-dependent Schrödinger equation can accurately predict the photoelectron emission time, but it cannot provide a physical explanation. Although some other current theoretical models can provide a more reasonable corresponding physical process, the quantitative prediction of the photoemission time delay has a large deviation. Therefore, we improve the exisating eikonal approximation model. Comparing with the existing eikonal approximation model, we use a more accurate final state wave function and calculate the photoelectron trajectory more accurately when calculating the phase accumulated in the photoelectron propagation process, so we can predict the photoemission time delay more accurately. By comparing our numerical simulation results, we find that when the final kinetic energy of photoelectron is low, the calculated results from the existing theoretical model are greatly different from those from the time-dependent Schrödinger equation, reaching tens of attoseconds. The resultsfrom the existing theoretical model are closer to those from the time-dependent Schrödinger equation with the increase of final kinetic energy of photoelectron. However, no matter what the final kinetic energy of the photoelectron is, the difference between the calculation result from the improved eikonal approximation model and that from the time-dependent Schrödinger equation is always very small. Therefore, our improved eikonal approximation model is closer to the results from the time-dependent Schrödinger equation than the existing theoretical model, which greatly deeps our understanding of the ultra-fast process of photoelectron emission.

Laser-assisted photoionization: Streaking, sideband, and pulse-train cases

We present a theoretical study of atomic laser-assisted photoionization emission (LAPE). We consider an atom driven by a linearly polarized XUV laser in two different scenarios: i) a single attosecond pulse (in both the streaking and sideband regimes) and ii) an attosecond pulse train. The process takes place assisted by a linearly polarized infrared (IR) laser field. In all these cases the energy and angle-resolved photoelectron spectrum (PES) is determined by a leading contribution, related to the intracycle factor [Gramajo et al., J. Phys. B 51, 055603 (2018)], complemented by other ones, derived from the periodicity and symmetry properties of the dipole transition matrix with respect to the IR field. Each of these terms imprint particular features in the PES that can be straightforwardly understood in terms of generalized energy conservation laws. We investigate in detail these PES structures, in particular, for the case of argon initially in the 3s quantum state. Our theoretical scheme, based on the strong-field approximation (SFA), can be applied, however, to other atomic species and field configurations as well.

Few-photon absorption and emission in the laser-assisted single photoionization

We investigate the few-photon absorption and emission in the laser-assisted single photoionization of hydrogen (H) atom. The results show that the electron wave ionized by the ultraviolet (UV) field can absorb or emit more than one infrared (IR) field photon. With the IR photon increases, the intensity of corresponding photoionization peak is getting weak and the distribution of them in time gets narrower. Furthermore, the central peak of photoelectron spectrum without absorbing or emitting IR photon shows a valley, which minimum decreases with the increase of IR intensity and wavelength. The sideband peaks of photoelectron spectrum absorbing or emitting one or two IR photon show a peak, which maximum increases with the increase of IR intensity and wavelength. Through analysis, we present that the changes in the photoionization peak are decided by the delay-dependent Bessel function.

Propensity rules and interference effects in laser-assisted photoionization of helium and neon

We investigate the angle-resolved photoelectron spectra from laser-assisted photoionization for helium and neon atoms using an ab initio method based on time-dependent surface flux and configuration interaction singles. We find that the shape of the distributions can be interpreted using a propensity rule, an intrinsic difference in the absorption and the emission processes, as well as interference effects between multiple paths to the final angular momentum state. The difference between absorption and emission is hidden in the first sideband in neon due to the multiple competing m channels, while in the second sideband a qualitative difference between absorption and emission is observed. In the m-resolved case, a clear difference is observed already in the first sideband, as exemplified in helium and m-resolved neon.

Fano's Propensity Rule in Angle-Resolved Attosecond Pump-Probe Photoionization.

In a seminal article, Fano predicts that absorption of light occurs preferably with increase of angular momentum. We generalize Fano's propensity rule to laser-assisted photoionization, consisting of absorption of an extreme-ultraviolet photon followed by absorption or emission of an infrared photon. The predicted asymmetry between absorption and emission leads to incomplete quantum interference in attosecond photoelectron interferometry. It explains both the angular dependence of the photoionization time delays and the delay dependence of the photoelectron angular distributions. Our theory is verified by experimental results in Ar in the 20-40eV range.

A dressed harmonic study in laser-assisted photoionization of water molecules by attopulses

SynopsisWe study theoretically the laser-assisted photoionization of water molecules by attopulses by using a separable Coulomb-Volkov model [1, 2, 3]. We analyze the effect of varying the delay between the attopulse and the assistant laser field on the dressed harmonics lines.

Photoelectron sidebands induced by a chirped laser field for shot-by-shot temporal characterization of FEL pulses

We theoretically investigate the laser-assisted photoionization of He by an extreme ultra violet (XUV) pulse in the presence of a linearly chirped intense laser pulse by solving the time-dependent Schrödinger equation within the single-active-electron approximation. Analysis based on the time-dependent perturbation theory is also carried out to provide more physical insights. A new scheme is shown to be capable of extracting the arrival time of an XUV free-electron laser (FEL) pulse relative to an external laser pulse as well as the XUV pulse duration from the photoelectron sidebands resulting from XUV ionization in the presence of a chirped laser pulse. This scheme is independent of the energy fluctuation and the timing jittering of the FEL pulse. Therefore it can be implemented in a non-invasive way to characterize FEL pulses on a shot-by-shot basis in time-resolved spectroscopy.

Many-body calculations on attosecond delays in photoionization

SynopsisWe discuss how attosecond delays in laser-assisted photoionization can be computed using perturbation theory based on two-photon matrix elements.

Asymmetric electron emission in laser-assisted photoionization of diatomic molecules

SynopsisWe study theoretically the different kinds of asymmetries that may arise in the laser-assisted photoionization of diatomic molecules.

Electron streaking in the autoionization region of H2

SynopsisWe use a UV-pump/IR-probe scheme, combining a single attosecond UV pulse and a 750 nm IR pulse, to explore laser-assisted photoionization of the hydrogen molecule in the autoionization region. The electron energy distributions exhibit unusual streaking patterns that are explored for different angles of the electron ejection with respect to the polarization vector and the molecular axis. Moreover, by controlling the time delay between the pulses, we observe that one can suppress the autoionization channel.

Fotoionización de moléculas diatómicas asistida por láser: de atto- a femto-pulsos

Estudiamos teoricamente la fotoionizacion de moleculas diatomicas sencillas mediante pulsos XUV de corta duracion, asistida por laser en el infrarrojo cercano. Ademas de los conocidos efectos de interferencia por emision coherente desde ambos centros moleculares, analizamos las interferencias que surgen por presencia del campo laser asistente. Asimismo, examinamos la transicion desde esquemas tipo streaking (donde la duracion de los pulsos XUV es de algunos cientos de attosegundos) hacia esquemas tipo Free electron laser (en los que la duracion de los mismos abarca uno o varias veces el perodo del campo laser asistente).

Photoionization of diatomic molecules: effect of coulombic interactions

We analyze the angular distribution of electrons in the laser-assisted photoionization of HeH2+ molecular targets. By means of a simple model, we obtain the observables for the reaction and we compare them with previous results obtained with more elaborated ones. The two-center interference effect and the asymmetric emission in the monochromatic ionization persist in the presence of a near infrared laser bath leading to a characteristic spectrum.

Interferences and asymmetries in laser-assisted photoionization of diatomic molecules

Fil: Boll, Diego I. R.. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Rosario. Instituto de Fisica de Rosario (i); Argentina

Statistical properties of the photoelectron energy spectrum generated by an intense laser pulse and a continuous X-ray

This study shows that the photoelectron energy spectrum generated by an intense laser pulse in the presence of a continuous X-ray has interesting and useful statistical properties. The total photoionization production is linearly proportional to the time duration of the laser pulse and the square of the beam size. The spectral double energy-integration is an intrinsic value of the laser-assisted X-ray photoionization, which linearly depends on the laser intensity and which quantitatively reflects the strengths of the laser-field modulation and the quantum interference between photoelectrons. The spectral energy width also linearly depends on the laser intensity. These linear relationships suggest new methods for the in-situ measurement of laser intensity and pulse duration with high precision.

Study of attosecond delays using perturbation diagrams and exterior complex scaling

We describe in detail how attosecond delays in laser-assisted photoionization can be computed using perturbation theory based on two-photon matrix elements. Special emphasis is laid on above-threshold ionization, where the electron interacts with an infrared field after photoionization by an extreme ultraviolet field. Correlation effects are introduced using diagrammatic many-body theory to the level of the random-phase approximation with exchange. Our aim is to provide an ab initio route to correlated multi-photon processes that are required for an accurate description of experiments on the attosecond time scale. Here, our results are focused on photoionization of the M-shell of argon atoms, where experiments have been carried out using the so-called reconstruction of attosecond beating by the two-photon interference transitions technique. An influence of autoionizing resonances in attosecond delay measurements is observed. Further, it is shown that the delay depends on both detection angle of the photoelectron and energy of the probe photon.