It is well established that electrons can escape from atoms through tunneling under the influence of strong laser fields, but the timing of the process has been controversial and far too rapid to probe in detail. We used attosecond angular streaking to place an upper limit of 34 attoseconds and an intensity-averaged upper limit of 12 attoseconds on the tunneling delay time in strong field ionization of a helium atom. The ionization field derives from 5.5-femtosecond-long near-infrared laser pulses with peak intensities ranging from 2.3 x 10(14) to 3.5 x 10(14) watts per square centimeter (corresponding to a Keldysh parameter variation from 1.45 to 1.17, associated with the onset of efficient tunneling). The technique relies on establishing an absolute reference point in the laboratory frame by elliptical polarization of the laser pulse, from which field-induced momentum shifts of the emergent electron can be assigned to a temporal delay on the basis of the known oscillation of the field vector.

In the research area of strong-laser-field interactions and attosecond science 1 , tunnelling of an electron through the barrier formed by the electric field of the laser and the atomic potential is typically assumed to be the initial key process that triggers subsequent dynamics [1][2][3] . Here we use the attoclock technique 4 to obtain experimental information about the electron tunnelling geometry (the natural coordinates of the tunnelling current flow) and exit point. We confirm vanishing tunnelling delay time, show the importance of the inclusion of Stark shifts 5,6 and report on multi-electron effects clearly identified by comparing results in argon and helium atoms. Our combined theory and experiment allows us to single out the geometry of the inherently one-dimensional tunnelling problem, through an asymptotic separation of the full three-dimensional problem. Our findings have implications for laser tunnel ionization in all atoms and in particular in larger molecular systems with correspondingly larger dipoles and polarizabilities.One of the most striking manifestations of the rules of quantum mechanics is the possibility for a particle to move from one side of a potential barrier to the other regardless of the energy height of that barrier. This includes the classically forbidden case, referred to as tunnelling, where the potential energy of the barrier is higher than the energy of the particle (Fig. 1a). In linearly polarized laser fields, electron tunnelling is expected to eventually lead to above-threshold ionization, enhanced double ionization and coherent emission up to the X-ray regime with high-order harmonic generation [7][8][9][10] . Therefore, a detailed understanding of the tunnelling step is of paramount importance for attosecond science, including generation of attosecond pulses 11,12 and attosecond measurement techniques 4,13,14 . The attoclock 4 is an attosecond streaking technique 13 . The rotating electric field vector of a close-to-circularly polarized laser field gives the time reference, in a manner similar to the hands of a clock, and the time is measured by counting fractions of cycles with the exact angular position of the rotating electric field. In this way it is possible to obtain attosecond time resolution by employing a femtosecond pulse. The attoclock was used to set an upper limit to the tunnelling delay time during the tunnel ionization process in helium 15 , and to measure the ionization times in double ionization of argon 16,17 . For the attoclock, a very short few-femtosecond pulse is used to both ionize an atom and to provide the time reference. The pulse duration is kept sufficiently short such that the ionization event is limited to within one optical cycle around the peak of the pulse. As a result of the close-to-circular polarization, re-scattering of the liberated electron with the parent ion is mostly suppressed. Assuming classical 1 Physics Department, ETH Zurich, 8093 Zurich, Switzerland, 2 Lundbeck Foundation Theoretical Center for Quantum System Research, De...

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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 ...

Light-driven sodium pumps actively transport small cations across cellular membranes 1 .They are used by microbes to convert light into membrane potential and have become useful optogenetic tools with applications in neuroscience. While resting state structures of the prototypical sodium pump Krokinobacter eikastus rhodopsin 2 (KR2) have been solved 2,3 , it is unclear how structural alterations over time allow sodium translocation against a concentration gradient. Using the Swiss X-ray Free Electron Laser 4 , we have collected serial crystallographic data at ten pump-probe delays from femtoseconds to milliseconds. Highresolution structural snapshots throughout the KR2 photocycle show how retinal isomerization is completed on the femtosecond timescale and changes the local structure of the binding pocket in the early nanoseconds. Subsequent rearrangements and deprotonation of the retinal Schiff base open an electrostatic gate in microseconds. Structural and spectroscopic data in combination with quantum chemical calculations indicate transient binding of a sodium ion close to the retinal within one millisecond. In the last structural intermediate at 20 ms after activation, we identified a potential second sodium binding site close to the extracellular exit. These results provide direct molecular insight into the dynamics of active cation transport across biological membranes.

We review the first ten years of attosecond science with a selection of recent highlights and trends and give an outlook on future directions. After introducing the main spectroscopic tools, we give recent examples of representative experiments employing them. Some of the most fundamental processes in nature have been studied with some results initiating controversial discussions. Experiments on the dynamics of single-photon ionization illustrate the importance of subtle effects on such extreme timescales and lead us to question some of the well-established assumptions in this field. Attosecond transient absorption, as the first all-optical approach to resolve attosecond dynamics, has been used to study electron wave packet interferences in helium. The attoclock, a recent method providing attosecond time resolution without the explicit need for attosecond pulses, has been used to investigate electron tunneling dynamics and geometry. Pushing the frontiers in attosecond quantum mechanics with increasing temporal and spatial resolution and often limited theoretical models results in unexpected observations. At the same time, attosecond science continues to expand into more complex solid-state and molecular systems, where it starts to have impact beyond its traditional grounds.

Abstract. We present an ellipticity resolved study of momentum distribution arising from strong-field ionization of helium. The influence of the ion potential on the departing electron is considered within a semi-classical model consisting of an initial tunneling step and subsequent classical propagation. We find that the momentum distribution can be explained by including the longitudinal momentum spread of the electron at the exit from the tunnel. Our combined experimental and theoretical study provides an estimate of this momentum spread.In strong-field physics and attoscience, it is often assumed that tunnel ionization is the first step that initiates the subsequent dynamics [1]. Therefore, the understanding of the electron-parent ion interaction in the presence of a femtosecond laser pulse is of fundamental importance to draw conclusions on various types of experiments. Providing a physical insight into this interaction, semiclassical models are indispensable for guiding ultrafast experiments and inventing new ultrafast measurement techniques such as tomography of molecular orbitals [2] and the attoclock [3] for example.In the semiclassical model of strong-field ionization different steps explain the overall dynamics starting with the initial tunnel process followed by classical trajectories and potential rescattering or recombination. In a recent attoclock experiments [4] we obtained experimental information about the electron tunneling geometry (the natural coordinates of the tunneling current flow) and showed the importance of accurately accounting for the effective potential with the exact tunnel exit in semiclassical models. However, to date we did not address the momentum space distribution of the electron wavepacket at the tunnel exit. The momentum spread of the electronic wavepacket in the direction transverse to the field has been extensively studied both theoretically and experimentally [5]. Here we present experimental results, which can be explained with an initial longitudinal momentum spread of the electron at the tunnel exit.The momentum spread of the electronic wavepacket at the tunnel exit point in the longitudinal direction has recently raised substantial interest. Tunneling theories impose the longitudinal spread at the tunnel exit to be zero [6]. However, a zero initial longitudinal momentum spread has failed to explain our experimental results. We measured the ion momentum distributions arising from strongfield ionization of helium in the tunneling regime, recorded over the full range of ellipticity. By comparing the experimental momentum distributions with the classical trajectory Monte Carlo (CTMC) simulations based on the Tunnel Ionization in Parabolic coordinates with Induced dipole This is an Open Access article distributed under the terms of the Creative Commons Attribution License 2.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Electron correlation and multielectron effects are fundamental interactions that govern many physical and chemical processes in atomic, molecular and solid state systems. The process of autoionization, induced by resonant excitation of electrons into discrete states present in the spectral continuum of atomic and molecular targets, is mediated by electron correlation. Here we investigate the attosecond photoemission dynamics in argon in the 20–40 eV spectral range, in the vicinity of the 3s−1np autoionizing resonances. We present measurements of the differential photoionization cross section and extract energy and angle-dependent atomic time delays with an attosecond interferometric method. With the support of a theoretical model, we are able to attribute a large part of the measured time delay anisotropy to the presence of autoionizing resonances, which not only distort the phase of the emitted photoelectron wave packet but also introduce an angular dependence.