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Physical Review A

Angle-dependent time delay in two-color XUV+IR photoemission of He and Ne

Igor A. Ivanov, Anatoli S. Kheifets

Published Jul 11, 2017• 53 citations
Publisher

Abstract

We solve the time-dependent Schr\"odinger equation for a noble gas atom (He and Ne) driven by an ionizing XUV and dressing IR fields. From this solution we deduce an angular dependence of the photoemission time delay as measured by the RABBITT (reconstruction of attosecond beating by interference of two-photon transitions) technique. We use a recent angle-resolved RABBITT measurement on helium [S. Heuser et al., Phys. Rev. A 94, 063409 (2016)] to test and calibrate our theoretical model. Based on this calibration, we find no significant difference between the time delay in He measured in the angle-integrated RABBITT experiments [C. Palatchi et al., J. Phys. B 47, 245003 (2014) and D. Gu\'enot et al., J. Phys. B 47, 245602 (2014)] and measured or calculated in the polarization axis direction. The angular dependence of the photoemission time delay of Ne is shown to be qualitatively different from He because of the different orbital character of the valence $2p$ orbital. The angular momentum projection dependence of the time delay in Ne is also investigated.

DOI
10.1103/physreva.96.013408

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We solve the time-dependent Schr"odinger equation for a noble gas atom (He and Ne) driven by an ionizing XUV and dressing IR fields. From this solution we deduce an angular dependence of the photoemission time delay as measured by the RABBITT (reconstruction of attosecond beating by interference of two-photon transitions) technique. We use a recent angle-resolved RABBITT measurement on helium [S. Heuser et al., Phys. Rev. A 94, 063409 (2016)] to test and calibrate our theoretical model. Based on this calibration, we find no significant difference between the time delay in He measured in the angle-integrated RABBITT experiments [C. Palatchi et al., J. Phys. B 47, 245003 (2014) and D. Gu'enot et al., J. Phys. B 47, 245602 (2014)] and measured or calculated in the polarization axis direction. The angular dependence of the photoemission time delay of Ne is shown to be qualitatively different from He because of the different orbital character of the valence 2p2p2p orbital. The angular momentum projection dependence of the time delay in Ne is also investigated.
Abstract The mutual angle formed by the non-collinear polarization axes of two laser pulses is used to control two-photon extreme ultraviolet and infrared ionization of noble gas atoms in the process of reconstruction of attosecond bursts by beating of two-photon transitions (RABBITT). The magnitude and the phase of this beating can be controlled very efficiently by the mutual polarization angle. The mechanism of this control can be understood within the lowest order perturbation theory and the soft photon approximation. We offer a very sensitive test on the polarization control of the angular dependent RABBITT process which validates our numerical simulations. We apply this test to the recent theoretical and experimental results of polarization controlled RABBITT on hydrogen and helium by Boll et al (2023 Phys. Rev. A 107 043113) and heavier noble gases by Jiang et al (2022 Nat. Commun. 13 5072).
The reconstruction of attosecond beating by interference of two-photon transitions (RABBITT) setup is theoretically studied for various combinations of extreme ultraviolet and infrared (IR) field components polarization: ``linear+linear\mathrm{linear}+\mathrm{linear}linear+linear,'' ``linear+circular\mathrm{linear}+\mathrm{circular}linear+circular'' with crossed propagation directions, and ``circular+circular\mathrm{circular}+\mathrm{circular}circular+circular'' with parallel propagation directions. We examine the general properties of photoelectron angular distributions and their response to the variation of the IR pulse delay. Numerical simulations are performed for the neon valence shell ionization into the structureless continuum using two approaches: time-dependent perturbation theory and the solution of amplitude rate equations. To distinguish between ``geometrical'' governed by fields polarization and spectroscopic features, we provide an additional analysis for the case of sss-shell ionization.
Abstract Ionization of laser-dressed atomic helium is investigated with focus on photoelectron angular distributions stemming from two-color multi-photon excited states. The experiment combines extreme ultraviolet (XUV) with infrared (IR) radiation, while the relative polarization and the temporal delay between the pulses can be varied. By means of an XUV photon energy scan over several electronvolts, we get access to excited states in the dressed atom exhibiting various binding energies, angular momenta, and magnetic quantum numbers. Furthermore, varying the relative polarization is employed as a handle to switch on and off the population of certain states that are only accessible by two-photon excitation. In this way, photoemission can be suppressed for specific XUV photon energies. Additionally, we investigate the dependence of the photoelectron angular distributions on the IR laser intensity. At our higher IR intensities, we start leaving the simple multi-photon ionization regime. The interpretation of the experimental results is supported by numerically solving the time-dependent Schrödinger equation in a single-active-electron approximation. Graphic abstract
The age of attosecond physics was ushered in by the invention of methods for probing phenomena on a time scale less than femtoseconds [F. Krausz, M. Ivanov, Rev. Mod. Phys. 81, 163 (2009)]. In photoemission, the Eisenbud-Wigner-Smith (EWS) time delay [R. Pazourek, S. Nagele, J. Burgdorfer, Rev. Mod. Phys. 87, 765 (2015)] is the interval between photon annihilation and emission of the electron, usually measured in attoseconds (10−1810^{-18}10−18 seconds), and varies with the kinetic energy and the angle of emission of the electron wave packet. We present a new interferometric method of measurement, using two phase-locked Extreme Ultraviolet pulses of frequency ω\omegaω and 2ω2\omega2ω, from a Free-Electron Laser. Phase differences between one- and two-photon ionization channels are extracted, and used to derive the relative EWS delay for one- and two-photon ionization of neon as a function of emission angle. Agreement with theory is excellent. Competing techniques require the use of an IR field, which necessitates corrections, but our method is correction-free. This new quantum clock is a powerful tool for investigating ultrafast processes in atoms and molecules.
Early investigations of the photoionization time delay focused solely on dipole contributions at low-energy. Later experimental progress demonstrated that non-dipole time delays are also measurable. This motivated us to initiate a theoretical program aimed at studying detailed quadrupole time delays in the noble gasses Ne and Ar and assessing their physical significance. In this work, we examined the angular dependence of the quadrupole photoemission time delay for Ne and Ar using the Relativistic-Random-Phase Approximation (RRPA) method. Our approach began with identifying the relevant transitions for each sub-shell and obtaining the corresponding expression for the transition amplitude. From these amplitudes, we extracted the phase and computed the associated non-dipole time delay as a function of emission angle and energy for each sub-shell. Where applicable, we evaluated the time delay across both low- and high-energy regions and compared subshells that share the same principal quantum number n. These comparisons reveal the substantial influence of relativistic interactions between different subshells and atomic species. Currently, we have obtained results for Ne and Ar for the subs-hells ns1/2, np1/2, np3/2. Future work will extend these calculations to additional atomic systems, including heavier noble gasses, various atomic ions, and confined atomic environments.
Attosecond chronoscopy enables the exploration of correlated electron dynamics in real time. One key observable of attosecond physics is the determination of "time zero" of photoionization, the time delay with which the wave packet of the ionized electron departs from the ionic core. This observable has become accessible by experimental advances in attosecond streaking and reconstruction of attosecond beating by interference of two-photon transitions (RABBIT) techniques. In this Letter, we explore photoionization time delays by strong extreme ultraviolet fields beyond the linear-response limit. We identify novel signatures in time delays signifying strong coupling between atoms and light fields and the light-field dressing of the ion. As a prototypical case, we study the interelectronic coherence and entanglement in helium driven by a strong extreme ultraviolet field. By the numerical solution of the time-dependent Schrödinger equation in its full dimensionality, we show that the time delay of the photoionized electron allows one to monitor the ultrafast variations of coherence dynamics and entanglement in real time.