Atomic excitation during recollision-free ultrafast multi-electron tunnel ionization

TL;DR

This study demonstrates that during ultrafast multi-electron tunnel ionization, simultaneous electron excitation occurs via shake-up, challenging the traditional view of isolated ionization events and impacting future attosecond pulse applications.

Contribution

It provides the first experimental evidence that shake-up excitation accompanies tunnel ionization in atoms under intense ultrafast laser pulses, independent of focal geometry and recollision effects.

Findings

Tunnel ionization induces simultaneous electron excitation.

Shake-up process occurs during ultrafast ionization.

Results impact development of attosecond XUV sources.

Abstract

Modern intense ultrafast pulsed lasers generate an electric field of sufficient strength to permit tunnel ionization of the valence electrons in atoms. This process is usually treated as a rapid succession of isolated events, in which the states of the remaining electrons are neglected. Such electronic interactions are predicted to be weak, the exception being recollision excitation and ionization caused by linearly-polarized radiation. In contrast, it has recently been suggested that intense field ionization may be accompanied by a two-stage `shake-up' reaction. Here we report a unique combination of experimental techniques that enables us to accurately measure the tunnel ionization probability for argon exposed to 50 femtosecond laser pulses. Most significantly for the current study, this measurement is independent of the optical focal geometry, equivalent to a homogenous electric field. Furthermore, circularly-polarized radiation negates recollision. The present measurements indicate that tunnel ionization results in simultaneous excitation of one or more remaining electrons through shake-up. From an atomic physics standpoint, it may be possible to induce ionization from specific states, and will influence the development of coherent attosecond XUV radiation sources. Such pulses have vital scientific and economic potential in areas such as high-resolution imaging of in-vivo cells and nanoscale XUV lithography.