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Journal of physics / B 55(14), 144001 - (2021). doi:10.1088/1361-6455/ac6b6b, We demonstrate the application of event-driven Timepix3-based detectors in combination with a double-sided velocity-map-imaging spectrometer to record the full 3D momentum of charged particles at the free-electron-laser facility FLASH. We measured the XUV induced fragmentation of $\text{N}_2$ using 250 kHz FLASH bursts with sub-pixel spatial resolution and up to 1.7 ns temporal resolution for photoelectrons. To further demonstrate the capabilities of this camera at even higher repetition rates we measured single-shot images of He($1s$) photoelectrons for bursts with a repetition rate of 1 MHz. Overall, with the Timepix3 camera we overcome limitations of standard-camera technology for advanced-imaging experiments with requirements on high event-rates and high spatio-temporal resolution., Published by IOP Publ., Bristol

The tunnelling of a particle through a potential barrier is a key feature of quantum mechanics that goes to the core of wave–particle duality. The phenomenon has no counterpart in classical physics, and there are no well constructed dynamical observables that could be used to determine ‘tunnelling times’. The resulting debate1–5 about whether a tunnelling quantum particle spends a finite and measurable time under a potential barrier was reignited in recent years by the advent of ultrafast lasers and attosecond metrology6. Particularly important is the attosecond angular streaking (‘attoclock’) technique7, which can time the release of electrons in strong-field ionization with a precision of a few attoseconds. Initial measurements7–10 confirmed the prevailing view that tunnelling is instantaneous, but later studies11,12 involving multi-electron atoms—which cannot be accurately modelled, complicating interpretation of the ionization dynamics—claimed evidence for finite tunnelling times. By contrast, the simplicity of the hydrogen atom enables precise experimental measurements and calculations13–15 and makes it a convenient benchmark. Here we report attoclock and momentum-space imaging16 experiments on atomic hydrogen and compare these results with accurate simulations based on the three-dimensional time-dependent Schrodinger equation and our experimental laser pulse parameters. We find excellent agreement between measured and simulated data, confirming the conclusions of an earlier theoretical study17 of the attoclock technique in atomic hydrogen that presented a compelling argument for instantaneous tunnelling. In addition, we identify the Coulomb potential as the sole cause of the measured angle between the directions of electron emission and peak electric field: this angle had been attributed11,12 to finite tunnelling times. We put an upper limit of 1.8 attoseconds on any tunnelling delay, in agreement with recent theoretical findings18 and ruling out the interpretation of all commonly used ‘tunnelling times’19 as ‘time spent by an electron under the potential barrier’20. Simulation and measurement of the photoionization of atomic hydrogen at attosecond resolution confirm that the tunnelling of the ejected electron is instantaneous.

We present a detailed experimental and theoretical study on the relativistic non-dipole effects in strong-field atomic ionisation by near-infrared linearly-polarised few-cycle laser pulses in the intensity range 1014 -1015 W/cm2. We record high-resolution photoelectron momentum distributions of argon using a reaction microscope and compare our measurements with a truly ab-initio fully relativistic 3D model based on the time-dependent Dirac equation. We observe counter-intuitive peak shifts of the transverse electron momentum distribution in the direction opposite to that of laser propagation as a function of laser intensity and demonstrate an excellent agreement between experimental results and theoretical predictions., 6 pages, 6 figures, Manuscript prepared for submission to Physical Review Letters

Synopsis Quantum mechanical description of the relativistic non-dipole effects is of fundamental interest in strong-field physics community, as they may have direct implications on the processes relying on re-collision dynamics. Here, we present a detailed experimental and theoretical study on the relativistic non-dipole effects in strong-field ionisation at moderate intensities. We compare our precise measurements with a truly ab-initio fully relativistic 3D model based on the time-dependent Dirac equation and demonstrate an excellent agreement between experimental results and theoretical predictions.

The dynamics of molecules and molecular reactions that generate chemistry such as bond breaking, bond formation or bond rearrangement occur on the femtosecond (10􀀀15 s) time scale. Real-time observations and investigations of these reactions are fundamental to understanding these dynamics. Ultra-fast laser pulses with intensities around 1014 W=cm2, impact on the electronic and nuclear motion on femtosecond timescale. In order to understand the region of transition states between the initial and nal state of a reaction, that determines the fate of the products, detailed experimental investigations are necessary. This work focuses on three separate investigations, one addressing the strong eld control of the electronic dynamics in the hydrogen molecule and two addressing the strong eld ionization dynamics in small but complex hydrocarbon molecule acetylene (C2H2). The pump-probe approach is applied to observe the real-time femtochemistry. The rst part of this thesis deals with the strong eld dissociation of H+ 2 . The goal is to control the dissociating dynamics by utilizing the control capability of the carrierenvelope phase (CEP) in a pump-probe approach. The work shows the response of the bound electron during the dissociation process. The timing of the electron localization and time-dependent distribution of the inter-nuclear separation is retrieved. Second and third part of this thesis deals with the investigations of the fragmentation dynamics in acetylene in the strong laser eld. Time-resolved vibrational dynamics and enhanced ionization (EI) are studied by using the pump-probe approach with reaction microscope in the second part. A wave-packet is launched onto the acetylene cation by the pump pulse and vibrational dynamics are traced by the probe pulse in di-cation fragmentation of the acetylene. Whereas laser parameters to control particular fragmentation and bond breaking are investigated in the third section of the thesis. This is illustrated by taking the experimental measurements at di erent laser intensities and ellipticities and parameters that govern the ultra-fast proton migration in acetylene dication are investigated. It is found that C-C bond breaking after proton migration in acetylene dication is favored in circularly polarized laser elds over C-C bond breaking without proton migration.

The tunnelling of a particle through a potential barrier is a key feature of quantum mechanics that goes to the core of wave-particle duality. The phenomenon has no counterpart in classical physics, and there are no well constructed dynamical observables that could be used to determine 'tunnelling times'. The resulting debate

In this Letter, the statement 'I.I. and A.B. performed computations at the NCI Australia' was missing from the Acknowledgements section. This has been corrected online.

We have investigated nuclear dynamics in bound and dissociating acetylene molecular ions in a time-resolved reaction microscopy experiment with a pair of few-cycle pulses. Different ionization processes were observed for acetylene. These time-dependent ionization processes are simulated using semi-classical on-the-fly dynamics revealing the underling mechanisms.

Dissociation of diatomic molecules with odd number of electrons always causes the unpaired electron to localize on one of the two resulting atomic fragments. In the simplest diatomic molecule H2+ dissociation yields a hydrogen atom and a proton with the sole electron ending up on one of the two nuclei. That is equivalent to breaking of a chemical bond—the most fundamental chemical process. Here we observe such electron localization in real time by performing a pump–probe experiment. We demonstrate that in H2+ electron localization is complete in just 15 fs when the molecule’s internuclear distance reaches 8 atomic units. The measurement is supported by a theoretical simulation based on numerical solution of the time-dependent Schrödinger equation. This observation advances our understanding of detailed dynamics of molecular dissociation., Time resolved measurements provide insights to the intriguing process of ultrafast molecular fragmentation. Here the authors use CEP-locked laser pulses in pump–probe scheme to explore the H2 + dissociation and find out that the electron localization to one of the nuclei occurs in about 15 fs.

We studied the dynamics of dissociative double ionization of acetylene using pump-probe technique with few-cycle laser pulses and Reaction Microscope detection system.

We have investigated nuclear dynamics in bound and dissociating acetylene molecular ions in a time-resolved reaction microscopy experiment with a pair of few-cycle pulses. Vibrating bound acetylene cations or dissociating dications are produced by the first pulse. The second pulse probes the nuclear dynamics by ionization to higher charge states and Coulomb explosion of the molecule. For the bound cations, we observed vibrations in acetylene (HCCH) and its isomer vinylidene (CCHH) along the CC-bond with a periodicity of around 26 fs. For dissociating dication molecules, a clear indication of enhanced ionization is found to occur along the CH- and CC-bonds after 10 fs to 40 fs. The time-dependent ionization processes are simulated using semi-classical on-the-fly dynamics revealing the underling mechanisms.

It has been recently predicted theoretically that due to nuclear motion light and heavy hydrogen molecules exposed to strong electric field should exhibit substantially different tunneling ionization rates (O.I. Tolstikhin, H.J. Worner and T. Morishita, Phys. Rev. A 87, 041401(R) (2013) [1]). We studied that isotope effect experimentally by measuring relative ionization yields for each species in a mixed H2/D2 gas jet interacting with intense femtosecond laser pulses. In a reaction microscope apparatus we detected ionic fragments from all contributing channels (single ionization, dissociation, and sequential double ionization) and determined the ratio of total single ionization yields for H2 and D2. The measured ratio agrees quantitatively with the prediction of the generalized weak-field asymptotic theory in an apparent failure of the frozen-nuclei approximation., Comment: 11 pages, 4 figures

Synopsis We present an intensity dependent study of transverse electron momentum distribution obtained from strong field ionization of argon using linearly polarized few-cycle (6 fs) laser pulses in the intensity range 1015W/cm2-1016W/cm2.

We present the results of attosecond angular streaking of atomic Hydrogen using elliptically polarized, 5.5 fs pulses that are centered around 770 nm, within the intensity range of 1.65 to 4 × 1014 W/cm2. We measure angular offsets in the photo-electron momentum distribution relative to the peak of the electric field of light in the polarization plane. We find a strong agreement in these results with the solutions of complete 3D-TDSE simulations. Further, we compute the contribution of coulomb effects (between the ionized electron-parent ion) to the measured angular offsets using Yukawa potential, subtracting which yields the real 'tunneling delays'.