Interaction of intense laser radiation with ion beams
Prof. Dr. Gerhard Paulus (Institute of Optics and Quantum Electronics, Friedrich Schiller University Jena and Helmholtz Institute Jena, Germany)

The dynamics of strong-?eld ionization plays a pivotal role for the interpretation of at- tosecond pump-probe measurements. For this reason, a number of groups are working with di?erent experimental approaches towards a direct measurement of the times at which the electrons are ionized from which sub-shell. The entire concept of a well-de?ned ionization time, however, appears to meaningful only if the ionization times are much shorter than an optical period such that the so-called Keldysh parameter is smaller than 1. Experiments that are clearly situated in the respective parameter space have been rare so far.
We show that it is advantageous to use ion beams instead of neutral atoms for such experiments. Applying intensities in excess of 1017W=cm2, Ne+ ions were ionized up to Ne6+ thus reaching uniquely low Keldysh parameters and nearly relativistic photoelectrons.
The highly charged ions are detected with a position- and time-resolving micro-channel plate. Data that are amenable to a particularly intuitive interpretation are obtained using elliptical laser polarization. There, the apparent ionization times can be determined very accurately by analogy to the hour and minute hand of a clock.