Chang-hua Zhang

Electron-ion interaction effects in attosecond time-resolved photoelectron spectra

Photoionization by attosecond extreme ultraviolet (xuv) pulses into the laser-dressed continuum of the ionized atom is commonly described in strong-field approximation, neglecting the Coulomb interaction between the emitted photoelectron (PE) and the residual ion. By solving the time-dependent Schroedinger equation, we identify a temporal shift {delta}{tau} in streaked PE spectra, which becomes significant at low PE energies. Within an eikonal approximation, we trace this shift to the combined action of Coulomb and laser forces on the released PE, suggesting the experimental and theoretical scrutiny of their coupling in streaked PE spectra. Further, we examined the effect of initial state polarization by the laser pulse on the xuv streaked spectrum.

Probing dielectric-response effects with attosecond time-resolved streaked photoelectron spectroscopy of metal surfaces

The release of conduction-band electrons from a metal surface by a subfemtosecond extreme ultraviolet (XUV) pulse and their propagation through the solid provoke a dielectric response in the solid that acts back on the photoelectron wave packet. We calculated the (wake) potential associated with this photoelectron self-interaction in terms of bulk and surface plasmon excitations and show that it induces a considerable, XUV-frequency-dependent temporal shift in laser-streaked XUV photoemission spectra, suggesting the observation of the ultrafast solid-state dielectric response in contemporary streaked photoemission experiments.

Attosecond time-resolved autoionization

Autoionization in argon atoms was studied by transient absorption spectroscopy with isolated attosecond XUV pulses. The peak position, line shape and population of the resonant states were modified by intense near infrared laser pulses.

Time-resolved IR laser-assisted XUV photoelectron spectroscopy of metal surfaces

Photoemission of localized and delocalized electrons from an (adsorbate-covered) metal surface by an XUV pulse of length τX into the field of a delayed IR laser pulse with carrier period TL allows for the time-resolved observation of surface and adsorbate electronic processes. For τX ≪ TL, the energy of the emitted photoelectrons (PEs) oscillates with period TL as a function of the XUV-IR pulse delay, leading to streaked PE spectra. In contrast, for τX TL, the PE spectrum is characterized by a satellite structure of sideband peaks located at integer multiples of the IR photon energy from the main photoemission peak. We present a theoretical model that allows us to discuss both, streaked and sideband photoemission spectra in comparison with recent experiments.

Attosecond time-resolved photoelectron spectroscopy of surfaces

We discuss streaking time delays for photoemission from solid model surfaces as a function of the degree of the initial-wave-function localization and including the dynamical plasmon response to the motion of the photoelectron.

Attosecond photoelectron spectroscopy of metal surfaces

Recent attosecond-streaking spectroscopy experiments [A. L. Cavalieri, Nature (London) 449, 1029 (2007)10.1038/nature06229] using copropagating extreme ultraviolet (XUV) and infrared (IR) pulses of variable relative delay have measured a delay of approximately 100 attoseconds between photoelectrons emitted by a single XUV photon from localized core states and delocalized conduction-band states of a tungsten surface. We analyze the underlying XUV-photoemission-IR-streaking mechanism by combining a perturbative description of the XUV-photoemission process and the subsequent nonperturbative IR streaking of the photoelectrons. Our calculated time-resolved photoelectron spectra agree with the experiments of Cavalieri et al. and demonstrate that the observed temporal shift is caused by the interference of core-level photoelectrons that originate in different layers of the solid.