574 research outputs found
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
(RPAE). 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 RABITT 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.Comment: 36 pages, 10 figure
The Multi-Configurational Hartree-Fock close-coupling ansatz: application to Argon photoionization cross section and delays
We present a robust, ab initio method for addressing atom-light interactions
and apply it to photoionization of argon. We use a close-coupling ansatz
constructed on a multi-configurational Hartree-Fock description of localized
states and B-spline expansions of the electron radial wave functions. In this
implementation, the general many-electron problem can be tackled thanks to the
use of the ATSP2K libraries [CPC 176 (2007) 559]. In the present contribution,
we combine this method with exterior complex scaling, thereby allowing for the
computation of the complex partial amplitudes that encode the whole dynamics of
the photoionization process. The method is validated on the 3s3p6np series of
resonances converging to the 3s extraction. Then, it is used for computing the
energy dependent differential atomic delay between 3p and 3s photoemission, and
agreement is found with the measurements of Gu\'enot et al. [PRA 85 (2012)
053424]. The effect of the presence of resonances in the one-photon spectrum on
photoionization delay measurements is studied.Comment: 15 pages, 8 figures, 4 table
Theory of attosecond delays in laser-assisted photoionization
We study the temporal aspects of laser-assisted extreme ultraviolet (XUV)
photoionization using attosecond pulses of harmonic radiation. The aim of this
paper is to establish the general form of the phase of the relevant transition
amplitudes and to make the connection with the time-delays that have been
recently measured in experiments. We find that the overall phase contains two
distinct types of contributions: one is expressed in terms of the phase-shifts
of the photoelectron continuum wavefunction while the other is linked to
continuum--continuum transitions induced by the infrared (IR) laser probe. Our
formalism applies to both kinds of measurements reported so far, namely the
ones using attosecond pulse trains of XUV harmonics and the others based on the
use of isolated attosecond pulses (streaking). The connection between the
phases and the time-delays is established with the help of finite difference
approximations to the energy derivatives of the phases. This makes clear that
the observed time-delays is a sum of two components: a one-photon Wigner-like
delay and an universal delay that originates from the probing process itself.Comment: 15 pages, 10 figures, special issue 'Attosecond spectroscopy' Chem.
Phy
Phase Measurement of Resonant Two-Photon Ionization in Helium
We study resonant two-color two-photon ionization of Helium via the 1s3p 1P1
state. The first color is the 15th harmonic of a tunable titanium sapphire
laser, while the second color is the fundamental laser radiation. Our method
uses phase-locked high-order harmonics to determine the {\it phase} of the
two-photon process by interferometry. The measurement of the two-photon
ionization phase variation as a function of detuning from the resonance and
intensity of the dressing field allows us to determine the intensity dependence
of the transition energy.Comment: 4 pages, 5 figures, under consideratio
Probing single-photon ionization on the attosecond time scale
We study photoionization of argon atoms excited by attosecond pulses using an
interferometric measurement technique. We measure the difference in time delays
between electrons emitted from the and from the shell, at
different excitation energies ranging from 32 to 42 eV. The determination of
single photoemission time delays requires to take into account the measurement
process, involving the interaction with a probing infrared field. This
contribution can be estimated using an universal formula and is found to
account for a substantial fraction of the measured delay.Comment: 4 pages, 4 figures, under consideratio
Photoionization in the time and frequency domain
Ultrafast processes in matter, such as the electron emission following light
absorption, can now be studied using ultrashort light pulses of attosecond
duration (s) in the extreme ultraviolet spectral range. The lack of
spectral resolution due to the use of short light pulses may raise serious
issues in the interpretation of the experimental results and the comparison
with detailed theoretical calculations. Here, we determine photoionization time
delays in neon atoms over a 40 eV energy range with an interferometric
technique combining high temporal and spectral resolution. We spectrally
disentangle direct ionization from ionization with shake up, where a second
electron is left in an excited state, thus obtaining excellent agreement with
theoretical calculations and thereby solving a puzzle raised by seven-year-old
measurements. Our experimental approach does not have conceptual limits,
allowing us to foresee, with the help of upcoming laser technology, ultra-high
resolution time-frequency studies from the visible to the x-ray range.Comment: 5 pages, 4 figure
Photoionization time delays
International audienceThe material presented in this chapter is based on important advances realized in " attophysics " which make feasible to follow the motion of electrons in atoms and molecules with attosecond-level time resolution. In this context, time-delays have been recently determined in the process of photoionization by extreme-ultraviolet (xuv) pulses and the question of the significance of these measured delays arises. As we shall outline here, numerical experiments show that they are intimately related to the structure of the ionized species' continuous spectrum. Another point addressed here is that, in experiments, the measurements have the common characteristic to be performed in the presence of an auxiliary infra-red (IR) field, used to " clock " the timing of the process. This implies to adapt the theory treatment to handle such " two-color " photoionization processes. We review a systematic analysis of these features that are characteristic of this class of electronic transitions, when viewed in the time domain
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