4.6 Article

Two-photon finite-pulse model for resonant transitions in attosecond experiments

Journal

PHYSICAL REVIEW A
Volume 93, Issue 2, Pages -

Publisher

AMER PHYSICAL SOC
DOI: 10.1103/PhysRevA.93.023429

Keywords

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Funding

  1. European Research Council under the European Union's Seventh Framework Programme (FP7)/ERC Grant [XCHEM 290853]
  2. MINECO Project [FIS2013-42002-R]
  3. ERA-Chemistry Project [PIM2010EEC-00751]
  4. European grant MC-ITN CORINF
  5. European COST Action [XLIC CM1204]

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We present an analytical model capable of describing two-photon ionization of atoms with attosecond pulses in the presence of intermediate and final isolated autoionizing states. The model is based on the finite-pulse formulation of second-order time-dependent perturbation theory. It approximates the intermediate and final states with Fano's theory for resonant continua, and it depends on a small set of atomic parameters that can either be obtained from separate ab initio calculations or be extracted from a few selected experiments. We use the model to compute the two-photon resonant photoelectron spectrum of helium below the N = 2 threshold for the RABITT (reconstruction of attosecond beating by interference of two-photon transitions) pump-probe scheme, in which an XUV attosecond pulse train is used in association with a weak IR probe, obtaining results in quantitative agreement with those from accurate ab initio simulations. In particular, we show that (i) the use of finite pulses results in a homogeneous redshift of the RABITT beating frequency, as well as a resonant modulation of the beating frequency in proximity to intermediate autoionizing states; (ii) the phase of resonant two-photon amplitudes generally experiences a continuous excursion as a function of the intermediate detuning, with either zero or 2 pi overall variation.

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