Chien-Nan Liu

Photoelectron sidebands induced by a chirped laser field for shot-by-shot temporal characterization of FEL pulses

We theoretically investigate the laser-assisted photoionization of He by an extreme ultra violet (XUV) pulse in the presence of a linearly chirped intense laser pulse by solving the time-dependent Schrodinger equation within the single-active-electron approximation. Analysis based on the time-dependent perturbation theory is also carried out to provide more physical insights. A new scheme is shown to be capable of extracting the arrival time of an XUV free-electron laser (FEL) pulse relative to an external laser pulse as well as the XUV pulse duration from the photoelectron sidebands resulting from XUV ionization in the presence of a chirped laser pulse. This scheme is independent of the energy fluctuation and the timing jittering of the FEL pulse. Therefore it can be implemented in a non-invasive way to characterize FEL pulses on a shot-by-shot basis in time-resolved spectroscopy.

Resolving the enhanced nonlinear double excitation of helium in intense ultra-short extreme-ultraviolet pulses

We present a theoretical study for multiphoton double excitation of He atoms exposed to an intense ul-trashort extreme-ultraviolet (EUV) pulse, where the photon energy coincides with the transition energy from the ground state to a Rydberg state. We identify the mechanism of the enhanced three-photon absorption probability as observed in the experiment.

Theoretical study of multi-photon double electron excitation of He by intense ultrashort EUV pulses

We use the time-dependent hypersperhical close-coupling method to study the double excitation of helium atoms by intense ultrashort XUV pulses. In addition to showing the photoelectron yield spectra, we demonstrate that free electron lasers can be used to efficiently create doubly excited electron wave packets, opening up the possibility of visualizing the correlated motion of electrons in atoms.

Probing the molecule-like modes of doubly excited states of helium atoms

We use the time-dependent hypersperhical close-coupling method to study the double excitation of helium atoms by attosecond XUV pulses in the presence of few-cycle infrared lasers, aiming to probe the time-resolved correlated electron motion and to reveal the underlying ro-vibration modes directly in time domain.

Numerical study of contrast degradation of BEC interferometers and countermeasures

Despite promising features, the BEC-based atomic interferometer needs to overcome momentum recoil caused by the internal atom-atom interaction so as to serve for high precision measurements. We visualize the momentum recoil using the Husimi distribution function and study the BECs dynamics in phase space. In so doing, we discuss possible countermeasures for contrast degradation in the manner of ref.[1] as well as hitherto unexpected interesting dynamical features.