Abstract
We investigate theoretically electron dynamics under a vacuum ultraviolet (VUV) attosecond pulse train, which has a controlled phase delay with respect to an additional strong infrared laser field. Using the strong field approximation and the fact that the attosecond pulse is short compared to the excited electron dynamics, we arrive at a minimal analytical model for the kinetic energy distribution of the electron as well as the photon absorption probability as a function of the phase delay between the fields. We analyze the dynamics in terms of electron wave packet replicas created by the attosecond pulses. The absorption probability shows strong modulations as a function of the phase delay for VUV photons of energy comparable to the binding energy of the electron, while for higher photon energies the absorption probability does not depend on the delay, in line with the experimental observations for helium and argon, respectively.
GENERAL SCIENTIFIC SUMMARY Introduction and background. Attosecond science is a rapidly developing field with great promise and great surprises. One of them is the complicated ionization response of an atom exposed to an attosecond pulse train in combination with an infrared laser pulse with well controlled phase delay as measured recently. The theoretical problem is single electron dynamics for which the Schroedinger equation can be solved numerically with high accuracy and in quantitative agreement with experiments. Yet, the numerical solution confirmed the complicated electron response without being able to explain it.
Main results. The present work offers a minimal analytical approach to elucidate the electron dynamics under attosecond pulse trains on top of infrared pulses. The picture which emerges is a multi-component electron wavepacket which has two factors: a comb like periodic factor in the electron momentum induced by the pulse train, independent of the phase delay, and a factor smoothly dependent on the phase delay with an imprint of the infrared laser amplitude. While only approximate, this analytical formulation provides insight into how the complicated electron dynamics emerges from the interplay of these two factors.
Wider implications. A simple yet detailed understanding of how matter reacts to attosecond pulses is essential to develop interesting applications of attosecond pulses. The present work aims to contribute to this understanding.