Timing the release in sequential double ionization

The timing of electron release in strong-field double ionization poses great challenges both for conceptual definition and for conducting experimental measurements. Here we present coincidence momentum measurements of the doubly charged ion and of the two electrons arising from double ionization of argon using elliptically polarized laser pulses. Based on a semi-classical model, the ionization times are calculated from the measured electron momenta across a large intensity range. This paper discusses how this method provides timings on a coarse and on a fine scale, similar to the hour and the minute hand of a clock. We found that the ionization time of the first electron is in good agreement with the simulation, whereas the ionization of the second electron occurs significantly earlier than predicted. Among all themethods used tomeasure time, one of themostfundamental is to measure the angle of a rotating hand,such as is done on an analogue watch face. This principle can be employed in strong-field ionization using laser pulses with close-to-circular polarization. In the attoclock the rotating electric field vector is used to deflect photo-ionized electrons, such that the instant of ionization is mapped to the final angle of themomentum, similar to the minute hand of a clock. The attoclock technique is based on the definition of ‘time ’ by ‘counting cycles’1,2. During one period the watch hand completes one cycle, such that measurin