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[en] Interaction of atoms and molecules with strong electric fields is a fundamental process in many fields of research, particularly in the emerging field of attosecond science. Therefore, understanding the physics underpinning those interactions is of significant interest to the scientific community. One crucial step in this understanding is accurate knowledge of the few-cycle laser field driving the process. Atomic hydrogen (H), the simplest of all atomic species, plays a key role in benchmarking strong-field processes. Its wide-spread use as a testbed for theoretical calculations allows the comparison of approximate theoretical models against nearly-perfect numerical solutions of the three-dimensional time-dependent Schrödinger equation. Until recently, relatively little experimental data in atomic H was available for comparison to these models, and was due mostly due to the difficulty in the construction and use of atomic H sources. Here, we review our most recent experimental results from atomic H interaction with few-cycle laser pulses and how they have been used to calibrate important laser pulse parameters such as peak intensity and the carrier-envelope phase (CEP). Quantitative agreement between experimental data and theoretical predictions for atomic H has been obtained at the 10% uncertainty level, allowing for accurate laser calibration intensity at the 1% level. Using this calibration in atomic H, both accurate CEP data and an intensity calibration standard have been obtained Ar, Kr, and Xe; such gases are in common use for strong-field experiments. This calibration standard can be used by any laboratory using few-cycle pulses in the 10¹⁴ W cm⁻² intensity regime centered at 800 nm wavelength to accurately calibrate their peak laser intensity to within few-percent precision. (invited article)

[en] We present a technique for generating phase-locked two-colour pulses with controllable electric field waveform, whereby the polarization states of the two frequency components, as well as their intensities, relative phase and pulse delay can be completely controlled. To characterize the phase stability and the waveform of the output two-colour pulse in situ, we measured the momentum spectra of protons from dissociative ionization of the hydrogen molecule for different combinations of polarizations. (paper)

[en] We studied the dependence on the dissociative ionization of H_2 as a function of carrier-envelope phase (CEP) of few-cycle (6 fs) near-infrared (NIR) laser pulses. For low-energy channels, we present the first experimental observation of CEP dependence for combined dissociation yield (with protons emitted in both directions) and the highest degree of asymmetry reported to date (40%).

[en] We studied the dependence of dissociative ionization in H₂ on carrier-envelope phase (CEP) of few-cycle (6 fs) near-infrared laser pulses. For low-energy channels, we present the first experimental observation of the CEP dependence of combined dissociation yield (with protons emitted in both directions), as well as the highest degree of asymmetry reported to date (40%). The observed modulations in both asymmetry and combined yield could be understood in terms of interference between different n-photon dissociation pathways—n and (n + 1) photon channels for asymmetry, n and (n + 2) photon channels for yield—as suggested by the general theory of CEP effects (Roudnev and Esry 2007 Phys. Rev. Lett. 99 220406). The yield modulation is found to be π-periodic in CEP, with its phase strongly dependent on fragment kinetic energy (and reversing its sign within the studied energy range), indicating that the dissociation yield does not simply follow the CEP dependence of maximum electric field, as a naïve intuition might suggest. We also find that a positively chirped pulse can lead to a higher dissociation probability than a transform-limited pulse. (paper)

[en] Laser-induced dissociation of O₂⁺ is studied in the strong-field limit using two independent methods, namely a crossed laser-ion-beam coincidence 3D momentum imaging method and a supersonic gas jet velocity map imaging technique (790 and 395 nm, 8-40 fs, ∼10¹⁵ W/cm²). The measured kinetic energy release spectra from dissociation of O₂⁺ and dissociative ionization of O₂ reveal vibrational structure which persists over a wide range of laser intensities. The vibrational structure is similar for O₂⁺ produced incoherently in an ion source and coherently by laser pulses. By evaluation of the potential energy curves, we assign the spectral energy peaks to dissociation of the v=10-15 vibrational states of the metastable a ⁴Π_u state via the dissociation pathway |a ⁴Π_u>→|f ⁴Π_g-1ω>--a mechanism equivalent to bond softening in H₂⁺.

[en] The generation of attosecond XUV pulses has been optimized by characterizing the effects of experimental conditions. The optimized high harmonic generation is verified and the CEP-dependent spectrum indicates the signature of isolated attosecond pulse generation.

[en] As the simplest atomic system, the hydrogen atom plays a key benchmarking role in laser and quantum physics. Atomic hydrogen is a widely used atomic test system for theoretical calculations of strong-field ionization, since approximate theories can be directly compared to numerical solutions of the time-dependent Schrödinger equation. However, relatively little experimental data is available for comparison to these calculations, since atomic hydrogen sources are difficult to construct and use. We review the existing experimental results on strong-field ionization of atomic hydrogen in multi-cycle and few-cycle laser pulses. Quantitative agreement has been achieved between experiment and theoretical predictions at the 10% uncertainty level, and has been used to develop an intensity calibration method with 1% uncertainty. Such quantitative agreement can be used to certify experimental techniques as being free from systematic errors, guaranteeing the accuracy of data obtained on species other than H. We review the experimental and theoretical techniques that enable these results. (review article)

[en] We have used few-cycle IR and EUV laser pulses in pump-probe arrangement to trace out the dissociation pathways in CO when exploded by strong laser fields. We present two preliminary sets of data of different pump pulses. In these sets, different excited state of CO cations are populated using (< 10 fs) IR, and EUV pulses respectively. We followed the time evolution of these states using the time-resolved Coulomb explosion imaging technique. We compare the time evolution of IR- and EUV-induced excited states by measuring the KER of the fragment ions as a function of the time delay between the pump and the IR probe pulse.

[en] We describe an extreme ultraviolet (XUV) interferometric technique that can resolve ∼100 zeptoseconds (10⁻²¹ s) delay between high harmonic emissions from two successive sources separated spatially along the laser propagation in a single Gaussian beam focus. Several improvements on our earlier work have been implemented in the advanced interferometer. In this paper, we report on the design, characterization and optimization of the advanced Gouy phase interferometer. Temporal coherence for both atomic argon and molecular hydrogen gases has been observed for several harmonic orders. It has been shown that phase shift of XUV pulses mainly originates from the emission time delay due to the Gouy phase in the laser focus and the observed interference is independent of the generating medium. This interferometer can be a useful tool for measuring the relative phase shift between any two gas species and for studying ultrafast dynamics of their electronic and nuclear motion. (paper)