Article

Photoelectron angular distributions for the two-photon ionization of helium by ultrashort extreme ultraviolet free-electron laser pulses

Journal of Physics B Atomic Molecular and Optical Physics (Impact Factor: 2.03). 08/2013; 46:164018. DOI: 10.1088/0953-4075/46/16/164018
Source: arXiv

ABSTRACT The two-photon ionization of helium atoms by ultrashort extreme-ultraviolet free-electron laser pulses, produced by the SPring-8 Compact SASE Source test accelerator, was investigated at photon energies of 20.3, 21.3, 23.0 and 24.3 eV. The angular distribution of photoelectrons generated by two-photon ionization is obtained using a velocity map imaging spectrometer. The phase-shift differences and amplitude ratios of the outgoing s and d continuum wave packets are extracted from the photoelectron angular distributions. The obtained values of the phase-shift differences are distinct from scattering phase-shift differences when the photon energy is tuned to a resonance with an excited level or Rydberg manifold. The difference stems from the co-presence of resonant and non-resonant path contributions in the two-photon ionization by femtosecond pulses. Since the relative contribution of both paths can be controlled in principle by the pulse shape, these results illustrate a new way to tailor the continuum wave packet. (Some figures may appear in colour only in the online journal)

3 Bookmarks
 · 
76 Views
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We theoretically study the photoelectron angular distribution (PAD) from the two-photon single ionization of H and He by femtosecond and attosecond extreme-ultraviolet pulses, based on the time-dependent perturbation theory and simulations with the full time-dependent Schrödinger equation. The PAD is formed by the interference of the s and d continuum wave packets, and, thus, contains the information on the relative phase δ and amplitude ratio between them. We find that, when a spectrally broadened femtosecond pulse is resonant with an excited level, the PAD substantially changes with pulse width, since the competition between resonant and nonresonant ionization paths, leading to δ distinct from the scattering phase shift difference, changes with it. In contrast, when the Rydberg manifold is excited, and for the case of above-threshold two-photon ionization, δ and the PAD do not depend much on pulse width, except for the attosecond region. Thus, the Rydberg manifold and the continuum behave similarly in this respect. For a high-harmonic pulse composed of multiple harmonic orders, while the value of δ is different from that for a single-component pulse, the PAD still rapidly varies with pulse width. The present results illustrate a new way to tailor the continuum wave packet.
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: An era of exploring the interactions of high-intensity, hard X-rays with matter has begun with the start-up of a hard-X-ray free-electron laser, the Linac Coherent Light Source (LCLS). Understanding how electrons in matter respond to ultra-intense X-ray radiation is essential for all applications. Here we reveal the nature of the electronic response in a free atom to unprecedented high-intensity, short-wavelength, high-fluence radiation (respectively 10(18) W cm(-2), 1.5-0.6 nm, approximately 10(5) X-ray photons per A(2)). At this fluence, the neon target inevitably changes during the course of a single femtosecond-duration X-ray pulse-by sequentially ejecting electrons-to produce fully-stripped neon through absorption of six photons. Rapid photoejection of inner-shell electrons produces 'hollow' atoms and an intensity-induced X-ray transparency. Such transparency, due to the presence of inner-shell vacancies, can be induced in all atomic, molecular and condensed matter systems at high intensity. Quantitative comparison with theory allows us to extract LCLS fluence and pulse duration. Our successful modelling of X-ray/atom interactions using a straightforward rate equation approach augurs favourably for extension to complex systems.
    Nature 07/2010; 466(7302):56-61. · 38.60 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: An overview of the recent experimental and theoretical works on photoinduced cascades of Auger and fluorescence transitions in atoms is presented. We concentrate on the angular correlation and polarization phenomena in such cascades and how they are related to the alignment and orientation of the core-excited or core-ionized atomic states produced in the photoabsorption. During the decay, the alignment and orientation are then transferred to the energetically lower atomic states, causing an anisotropy and polarization of the subsequent Auger or fluorescence emission. Special attention is paid to the practically important case of overlapping resonances and intermediate states. Coherent excitation of the resonances and the coherence transfer in the decay strongly influence the angular and polarization characteristics of the decay products. The studies on the angular correlations and polarization of the emitted particles provide detailed spectroscopic and dynamic information about the transitions involved. In some cases, moreover, a so-called 'complete' experiment, i.e. the experimental determination of the transition amplitudes including their relative phases, becomes possible.
    Physics Reports 01/2007; 45180(32). · 22.93 Impact Factor

Full-text (2 Sources)

View
41 Downloads
Available from
May 31, 2014