18 research outputs found
Probing time-ordering in two-photon double ionization of helium on the attosecond time scale
We show that time ordering underlying time-dependent quantum dynamics is a
physical observable accessible by attosecond streaking. We demonstrate the
extraction of time ordering for the prototypical case of time-resolved
two-photon double ionization (TPDI) of helium by an attosecond XUV pulse. The
Eisenbud-Wigner-Smith time delay for the emission of a two-electron wavepacket
and the time interval between subsequent emission events can be unambiguously
determined by attosecond streaking. The delay between the two emission events
sensitively depends on the energy, pulse duration, and angular distribution of
the emitted electron pair. Our fully-dimensional ab-initio quantum mechanical
simulations provide benchmark data for experimentally accessible observables.Comment: 8 pages, 5 figures; revised version, added appendi
Photoionization of helium by attosecond pulses: extraction of spectra from correlated wave functions
We investigate the photoionization spectrum of helium by attosecond XUV
pulses both in the spectral region of doubly excited resonances as well as
above the double ionization threshold. In order to probe for convergence, we
compare three techniques to extract photoelectron spectra from the wavepacket
resulting from the integration of the time-dependent Schroedinger equation in
a finite-element discrete variable representation basis. These techniques are:
projection on products of hydrogenic bound and continuum states, projection
onto multi-channel scattering states computed in a B-spline close-coupling
basis, and a technique based on exterior complex scaling (ECS) implemented in
the same basis used for the time propagation. These methods allow to monitor
the population of continuum states in wavepackets created with ultrashort
pulses in different regimes. Applications include photo cross sections and
anisotropy parameters in the spectral region of doubly excited resonances,
time-resolved photoexcitation of autoionizing resonances in an attosecond
pump-probe setting, and the energy and angular distribution of correlated
wavepackets for two-photon double ionization.Comment: 19 pages, 12 figure
Attosecond streaking of Cohen-Fano interferences in the photoionization of H
We present the first numerical simulation of the time delay in the
photoionization of the simplest diatomic molecule H as observed by
attosecond streaking. We show that the strong variation of the
Eisenbud-Wigner-Smith time delay as a function of energy and emission angle
becomes observable in the streaking time shift provided laser field-induced
components are accounted for. The strongly enhanced photoemission time shifts
are traced to destructive Cohen-Fano (or two-center) interferences. Signatures
of these interferences in the streaking trace are shown to be enhanced when the
ionic fragments are detected in coincidence
Attosecond streaking of correlated two-electron transitions in helium
We present fully ab initio simulations of attosecond streaking for ionization
of helium accompanied by shake-up of the second electron. This process
represents a prototypical case for strongly correlated electron dynamics on the
attosecond timescale. We show that streaking spectroscopy can provide detailed
information on the Eisenbud-Wigner-Smith time delay as well as on the infrared
field dressing of both bound and continuum states. We find a novel contribution
to the streaking delay that stems from the interplay of electron-electron and
infrared-field interactions in the exit channel. We quantify all the
contributions with attosecond precision and provide a benchmark for future
experiments.Comment: 5 pages, 4 figure
Ionization delays in few-cycle-pulse multiphoton quantum-beat spectroscopy in helium
We explore quantum beats in the photoelectron signal produced when a bound electron wave packet created by an isolated attosecond pulse is ionized by a delayed, few-cycle infrared pulse. Our calculations for helium atoms show that the broad bandwidth of the few-cycle pulse creates spectrally overlapping photoelectron peaks that result from one-, two-, or three-photon ionization processes. The beat signals can, in principle, be interferometrically resolved with high resolution, giving access to the relative phase between different multiphoton ionization pathways. For few-cycle near-infrared fields the relative spectral phases can be extracted over a large energy region, and dynamical information becomes available. We find that multiphoton ionization is temporally shifted with respect to one-photon ionization by several hundred attoseconds. Our results also reveal the impact of depletion and resonant pathways on the phase of the quantum beats