Timing the release in sequential double ionization

Abstract

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.A mong all the methods used to measure time, one of the most fundamental 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 the momentum, 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 measuring the emission angle of the electron enables us to measure time at a precision well below one optical period 2 . Thus the measurement provides attosecond timing without using an attosecond pulse.Here we use the attoclock to measure the ionization times of the two electrons in the double ionization of argon. As a result of depletion the averaged ionization time of the electrons is shifted towards the beginning of the pulse, thus requiring a multi-cycle measurement. The magnitude of the electron momenta follows the envelope of the laser pulse and gives a coarse timing for the electron release (that is 'the hour hand of the clock'). The emission angle of the electrons subsequently gives the fine timing (that is 'the minute hand of the clock').The result of the attoclock measurements addresses a fundamental question in double ionization: are there electron correlation mechanisms that are not induced by recollision? With linearly polarized fields in strong-field double ionization the dominating ionization mechanism is induced by recollision of the first emitted electron 3,4 . With close-to-circular polarization, however, we can avoid this recollision and therefore investigate a conceptually even simpler process of few-body quantum mechanics.It is impossible, at present, to simulate this process based on the time dependent Schrödinger equation (TDSE) because of exceedingly large computing time requirements 5 . Instead one describes the process usually in terms of simplifying mechanisms, which can be classified as sequential double ionization (SDI) or non-sequential double ionization ...

Understanding the role of phase in chemical bond breaking with coincidence angular streaking

“…We use the rotating electric-field vector of the laser pulse as ultrafast clockwork and measure the asymptotic emission direction f el of the tunnelionized electron. The value of f el between 0 and 2p provides the laser phase, relative to an arbitrary reference axis in the laboratory frame, at the ionization instant t i [10][11][12][13][14][15] . However, it is not f el, but rather f el mol , the phase of the laser field relative to the molecular axis at time t i , which determines the degree of anisotropy in bond breaking.…”

“…The first technique combines a single attosecond pulse with an intense near-infrared laser pulse, such that the attosecond pulse acts as a start and the phase-locked infrared pulse streaks a freed electron in energy [6][7][8][9] . The second technique, termed 'attosecond angular streaking', uses elliptically polarized light [10][11][12][13][14][15] and adopts either the major axis of the elliptical field or the carrier-envelope phase (CEP) of a CEP-stabilized pulse to provide a reference time. A released electron is then driven to a certain direction by the rotating electric field, which maps the ionization instant within one laser cycle (T p B2.6 fs at 790 nm) to the 2p interval of electron emission directions (B7.3 attosecond for 1°) in the polarization plane.…”

Attosecond tracing of correlated electron-emission in non-sequential double ionization

“…The identification of suitable approximations that capture the essential features of MED relies on the development of experiments aimed at resolving the electron dynamics on the characteristic attosecond timescale of their motion. An established technique for resolving one-electron dynamics on subfemtosecond scales involves the use of a strong few-cycle infrared laser field as a clock [1][2][3] . The well-known concept of attosecond streaking spectroscopy has permitted the observation of singleelectron dynamics with attosecond time resolution 4 .…”

Extreme-ultraviolet pump–probe studies of one-femtosecond-scale electron dynamics

“…20, seem to be less relevant to our case, in which interactions are perturbative and in the multi-photon regime. Using a sequence of two XUV pulses, and varying the delay τ between them, we are able to: (1) measure their duration by means of a second order intensity volume autocorrelation (second order IVAC) from the part of the trace in which the two pulses are overlapping and (2) induce, control and probe a fast-evolving coherence in the structured continuum, by evaluating the part of the trace where the two pulses are not overlapping.…”