1,275 research outputs found
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 of harmonics from strongly driven two-level atoms
Published versio
Stark ionization in dc and ac fields: An L2 complex-coordinate approach
This is the published version, also available here: http://dx.doi.org/10.1103/PhysRevA.27.2946.A finite-dimensional-matrix technique valid for computation of complex eigenvalues and eigenfunctions useful for discussing time evolution in both dc and ac Stark fields is presented. The complex eigenvalue parameters are those of appropriately analytically continued, time-independent Stark Hamiltonians as obtained via the complex scale transformation r→reiθ. Such a transformation distorts the continuous spectrum away from the real axis, exposing the Stark resonances, and also allowing use of finite variational expansions employing L2 basis functions chosen from a complete discrete basis. The structure of the dc and ac Stark Hamiltonians is discussed and extensive convergence studies performed in both the dc and ac cases to fully document the utility of the method. Sudden and adiabatic dc Stark time evolution is used to illustrate the power of finite-dimensional-matrix methods in describing complex, multiple-time-scale time evolution. The relationship between the ac Stark Hamiltonian used (a time-independent truncated Floquet Hamiltonian) and continued-fraction perturbation theory follows easily via use of matrix partitioning, and provides a particularly straightforward derivation of these results. Finally, some illustrative calculations of off-resonant generalized cross sections are given at low and high intensities, indicating that the method works satisfactorily at intensities the order of internal atomic field strengths. A more detailed discussion of time evolution in two-, three-, and four-photon ionization processes appears in the following paper by Holt, Raymer, and Reinhardt
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
Imaging orbitals with attosecond and Ångström resolutions: toward attochemistry? Imaging orbitals with attosecond and Ångström resolutions: toward attochemistry?
International audienceThe recently developed attosecond light sources make the investigation of ultrafast processes in matter possible with unprecedented time resolution. It has been proposed that the very mechanism underlying the attosecond emission allows the imaging of valence orbitals with Ångström space resolution. This controversial idea together with the possibility of combining attosecond and Ångström resolutions in the same measurements has become a hot topic in strong-field science. Indeed , this could provide a new way to image the evolution of the molecular electron cloud during , e. g. a chemical reaction in ' real time '. Here we review both experimental and theoretical challenges raised by the implementation of these prospects. In particular , we show how the valence orbital structure is encoded in the spectral phase of the recombination dipole moment calculated for Coulomb scattering states , which allows a tomographic reconstruction of the orbital using first-order corrections to the plane-wave approach. The possibility of disentangling multi-channel contributions to the attosecond emission is discussed as well as the necessary compromise between the temporal and spatial resolutions. (Some figures may appear in colour only in the online journal
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