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Atom-Trap Trace-Analysis (ATTA) is a technique aimed at a measuring the relative (in comparison to the common isotope) abundance of exceedingly rare noble gas radio-isotopes (Ar-39, Kr-81 and Kr-85), for the purposes of dating water, ice and other materials that contain trapped gases. These isotopes exist with an extremely low natural abundance and require measurement capability down to parts in 1017. The noble gas dating protocol is similar to that of radio-carbon dating but, the three isotopes together, allow dating of materials in complementary ranges to that possible with the radiocarbon technique. In a facility jointly created by CSIRO, Griffith and Adelaide Universities, we use laser guidance, cooling and trapping to perform isotopic separation using the isotopic dependence of atomic energy levels. The facility is currently undergoing construction but has been shown to be able to produce metastable Ne and Ar, which can then be laser cooled and trapped. When combined with the existing gas separation facility at the CSIRO Waite facility, which efficiently extracts pure noble gases from that are dissolved in water or ice samples, we should be in a position to start measuring the age of real-world samples in the early part of 2020. In addition, to describing the current state of construction, we will also discuss two more efficient approaches for creating metastable atoms that can reduce samples size and avoid unwanted cross-contamination between samples.

Using asymmetric two-color laser fields composed of 1800 and 900nm, we have simultaneously controlled four well identified fragmentation channels in dissociative ionization of the hydrogen molecule, resulting in enhanced electron-localization sensitivities of up to 65%.

Measurements to control the morphology and characteristics of a picosecond laser produced chromium plasma plume upon double-pulse (DP) irradiation are presented and compared to their single-pulse (SP) counterpart. DP schemes are implemented by employing two geometries where the inter-pulse delay and the spatial separation are the control parameters. The ratio of plume length to plume width decreases upon increasing the inter-pulse delay and/or the energy of the second pulse in the collinear DP scheme. Interestingly, plasmas generated in the DP scheme at lower pressures resemble the expansion features of the plasma generated in the SP scheme at higher pressures. We find that DP schemes are advantageous for applications such as high harmonic generation and the production of quality thin films.

Time-resolved optical emission spectroscopic measurements of a plasma generated by irradiating a Cr target using 60 picosecond (ps) and 300 ps laser pulses is carried out to investigate the variation in the linewidth ($\delta\lambda$) of emission from neutrals and ions for increasing ambient pressures. Measurements ranging from 10$^{-6}$ Torr to 10$^2$ Torr show a distinctly different variation in the $\delta\lambda$ of neutrals (Cr I) compared to that of singly ionized Cr (Cr II), for both irradiations. $\delta\lambda$ increases monotonously with pressure for Cr II, but an oscillation is evident at intermediate pressures for Cr I. This oscillation does not depend on the laser pulse widths used. In spite of the differences in the plasma formation mechanisms, it is experimentally found that there is an optimum intermediate background pressure for which $\delta\lambda$ of neutrals drops to a minimum. Importantly, these results underline the fact that for intermediate pressures, the usual practice of calculating the plasma number density from the $\delta\lambda$ of neutrals needs to be judiciously done, to avoid reaching inaccurate conclusions.

We report on a systematic investigation of the dissociation dynamics of ${{\mathrm{H}}_{2}}^{+}$ in intense laser fields, and study how the kinetic energy spectrum of the dissociating proton can be modulated by the wavelength and intensity of the driving laser field. In the experiment, ${\mathrm{H}}_{2}$ is dissociatively ionized by an intense laser pulse with varying carrier wavelengths ranging from 800 to 1800 nm and varying peak intensities. A model based on Floquet theory and Landau-Zener theory is adopted to explain the experimental observations. The intensity effect is further explored in a few-cycle pump-probe experiment. We observed a significant intensity-dependent proton kinetic energy shift, which can also be well explained by the theoretical simulation. The wavelength- and intensity-dependent proton spectra reveal the mechanism of selective excitation of vibrational levels of ${{\mathrm{H}}_{2}}^{+}$ in intense laser fields.

This work describes the first observations of the ionisation of neon in a metastable atomic state utilising a strong-field, few-cycle light pulse. We compare the observations to theoretical predictions based on the Ammosov-Delone-Krainov (ADK) theory and a solution to the time-dependent Schrödinger equation (TDSE). The TDSE provides better agreement with the experimental data than the ADK theory. We optically pump the target atomic species and measure the ionisation rate as the a function of different steady-state populations in the fine structure of the target state which shows significant ionisation rate dependence on populations of spin-polarised states. The physical mechanism for this effect is unknown.

Ionization of atoms and molecules in strong laser fields is a fundamental process in many fields of research, especially in the emerging field of attosecond science. So far, demonstrably accurate data have only been acquired for atomic hydrogen (H), a species that is accessible to few investigators. Here we present measurements of the ionization yield for argon, krypton, and xenon with percentlevel accuracy, calibrated using H, in a laser regime widely used in attosecond science. We derive a transferrable calibration standard for laser peak intensity, accurate to 1.3%, that is based on a simple reference curve. In addition, our measurements provide a much-needed benchmark for testing models of ionisation in noble-gas atoms, such as the widely employed single-active electron approximation., Article: 5 pages, 2 figures, submitted to PRL (manuscript number LZ14457). Supplementary information: 7 pages, 6 figures, appended to end of main Article

We demonstrate that the coherent control of unimolecular reactions by using a waveform-controlled laser fields can lead to a strong modulation on the yield of the reaction. By using a synthesized \ensuremath{\omega} (1800-nm) and 3\ensuremath{\omega} (600-nm) two-color laser field, the probability of photodissociation of ${\mathrm{H}}_{2}{}^{+}$ can be strongly modulated by varying the relative phase between the two colors. The dissociation probability maximizes at different relative phases for protons with different kinetic energy, and such energy dependence can also be qualitatively reproduced by our simulation. We attribute the observed dissociation probability modulation to the interference between two different dissociation pathways which start from the same electronic states and end with the same kinetic energy.

International audience; Steering the electrons during an ultrafast photo-induced process in a molecule influences the chemical behavior of the system, opening the door to the control of photochemical reactions and photobiological processes. Electrons can be efficiently localized using a strong laser field with a well-designed temporal shape of the electric component. Consequently, many experiments have been performed with laser sources in the near-infrared region (800 nm) in the interest of studying and enhancing the electron localization. However, due to its limited accessibility, the mid-infrared (MIR) range has barely been investigated, although it allows to efficiently control small molecules and even more complex systems. To push further the manipulation of basic chemical mechanisms, we used a MIR two-color (1800 and 900 nm) laser field to ionize H$_2$ and D$_2$ molecules and to steer the remaining electron during the photo-induced dissociation. The study of this prototype reaction led to the simultaneous control of four fragmentation channels. The results are well reproduced by a theoretical model solving the time-dependent Schrödinger equation for the molecular ion, identifying the involved dissociation mechanisms. By varying the relative phase between the two colors, asymmetries (i.e., electron localization selectivity) of up to 65% were obtained, corresponding to enhanced or equivalent levels of control compared to previous experiments. Experimentally easier to implement, the use of a two-color laser field leads to a better electron localization than carrier-envelope phase stabilized pulses and applying the technique in the MIR range reveals more dissociation channels than at 800 nm.

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 present accurate measurements of carrier-envelope phase effects on ionisation of the noble gases with few-cycle laser pulses. The experimental apparatus is calibrated by using atomic hydrogen data to remove any systematic offsets and thereby obtain accurate CEP data on other generally used noble gases such as Ar, Kr and Xe. Experimental results for H are well supported by exact TDSE theoretical simulations however significant differences are observed in case of noble gases., Comment: 4 pages, 5 figures, submitted to PRL (manuscript number LH15031)

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.

When a diatomic molecule is ionized by an intense laser field, the ionization rate depends very strongly on the inter-nuclear separation. That dependence exhibits a pronounced maximum at the inter-nuclear separation known as the “critical distance”. This phenomenon was first demonstrated theoretically in H2+ and became known as “charge-resonance enhanced ionization” (CREI, in reference to a proposed physical mechanism) or simply “enhanced ionization”(EI). All theoretical models of this phenomenon predict a double-peak structure in the R-dependent ionization rate of H2+. However, such double-peak structure has never been observed experimentally. It was even suggested that it is impossible to observe due to fast motion of the nuclear wavepackets. Here we report a few-cycle pump-probe experiment which clearly resolves that elusive double-peak structure. In the experiment, an expanding H2+ ion produced by an intense pump pulse is probed by a much weaker probe pulse. The predicted double-peak structure is clearly seen in delay-dependent kinetic energy spectra of protons when pump and probe pulses are polarized parallel to each other. No structure is seen when the probe is polarized perpendicular to the pump.

We study transverse electron momentum distribution (TEMD) in strong field atomic ionization driven by laser pulses with varying ellipticity. We show, both experimentally and theoretically, that the TEMD in the tunneling and over the barrier ionization regimes evolves in a qualitatively different way when the ellipticity parameter describing polarization state of the driving laser pulse increases., Comment: 5 pages, 6 figures

We present momentum images of the ionic products from the ionization of D2 and C2H2 by short laser pulses. For D2, we use a pump–probe approach to investigate the dependence of the enhanced ionization on the internuclear distance. Evidence for two (not well separated) regions of enhancement is found near internuclear distances of 6 and 10 au. In the case of acetylene, we report clear evidence for the production of both acetylene and vinylidene dications with kinetic energy releases similar to those reported earlier by core electron removal. We also find very different angular distributions for the fragments in the two channels, consistent with a finite time for the isomerization.

We have measured momentum-space images of low-energy electrons generated by the interaction of short intense laser pulses with argon atoms at high intensities. We have done this over a wavelength range from 400 to 800 nm. The spectra show considerable structure in both the energy and angular distributions of the electrons. Some, but not all, energy features can be identified as multi-photon resonances. The angular structure shows a regularity which transcends the resonant structure and may be due instead to diffraction. The complexity of the results defies easy model-dependent interpretations and invites full solutions to Schrodinger's equation for these systems.

Field-free alignment of gas-phase molecules can be achieved by creating rotational wavepackets with a short laser pulse. We demonstrate that the degree of alignment can be improved by illuminating the molecules with a second laser pulse at a specific time during the evolution of the original rotational wavepacket.

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 Schrodinger 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.

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 explore the effect of nuclear mass on the laser-driven electron localization process. We dissociate a mixed H2 and D2 target with intense, carrier-envelope-phase (CEP) stable 6 fs laser pulses and detect the products in a reaction microscope. We observe a very strong CEP-dependent asymmetry in proton and deuteron emission for low dissociation energy channels. This asymmetry is stronger for H2 than for D2. We also observe a large CEP offset between the asymmetry spectra for H2 and D2. Our theoretical simulations, based on a one-dimensional two-channel model, agree very well with the asymmetry spectra, but fail to account properly for the phase difference between the two isotopes.

The transition between two distinct mechanisms for the laser-induced field-free orientation of CO molecules is observed via measurements of orientation revival times and subsequent comparison to theoretical calculations. In the first mechanism, which we find responsible for the orientation of CO up to peak intensities of 8 x 10^13 W/cm^2, the molecules are impulsively oriented through the hyperpolarizability interaction. At higher intensities, asymmetric depletion through orientation-selective ionization is the dominant orienting mechanism. In addition to the clear identification of the two regimes of orientation, we propose that careful measurements of the onset of the orientation depletion mechanism as a function of the laser intensity will provide a relatively simple route to calibrate absolute rates of non-perturbative strong-field molecular ionization., Comment: 5 pages, 2 figures

Accurate knowledge of the intensity of focused ultrashort laser pulses is crucial to the correct interpretation of experimental results in strong-field physics. We have developed a technique to measure laser intensities approaching ${10}^{15}\phantom{\rule{0.28em}{0ex}}\text{W}/{\text{cm}}^{2}$ with an accuracy of 1$%$. This accuracy is achieved by comparing experimental photoelectron yields from atomic hydrogen with predictions from exact numerical solutions of the three-dimensional time-dependent Schr\"odinger equation. Our method can be extended to relativistic intensities and to the use of other atomic species.

Laser produced plasmas are transient in nature, and their properties, which depend on the laser parameters as well as the material properties and the irradiation conditions, can be tailored for different applications. Highly ionized Al plasmas generated using 7 ns and negatively chirped 60 ps pulses are optimized for the purpose of generating Al IV and Al III, respectively. The plasma is optimized spatio-temporally for Al IV or Al III with irradiation energy as the control parameter using time-resolved optical emission spectroscopy. Plasmas attuned for higher charged states could be utilized as a good alternative source for the generation of high order harmonics.

Our principal goal was the experimental demonstration of Laser-Induced Electron Diffraction (LIED). Key steps along the development of this experimental technique have been accomplished and reported in the publications listed in this brief report. We started with measuring 3D electron momenta spectra in aligned nitrogen and oxygen molecules. Chakra Maharjan (Ph.D. student of Lew Cocke) was a lead researcher on this project. Although Chakra succeeded in obtaining those spectra, we were scooped by the publication of identical results in Science by the NRC Ottawa group. Our results were never published as a refereed article, but became a part of Chakra's Ph.D. dissertation. That Science paper was the first experimental demonstration of Laser-Induced Electron Diffraction (LIED). Chakra also worked on wavelength dependence of 3D ATI spectra of atoms and molecules using tunable OPA pulses. Another Ph.D. student, Maia Magrakvelidze (her GRA was funded by the grant), started working on COLTRIMS experiments using OPA pulses (1800 nm wavelength). After some initial experiments it became apparent that COLTRIMS did not yield sufficient count rates of electrons in the high-energy part of the spectrum to see diffraction signatures with acceptable statistics (unfavorable scaling of the electron yield with laser wavelength was partly to blame). Nevertheless, Maia managed to use COLTRIMS and OPA to measure the angular dependence of the tunneling ionization rate in D{sub 2} molecules. Following the initial trial experiments, the decision was made to switch from COLTRIMS to VMI in order to increase the count rates by a factor of {approx}100, which may have given us a chance to see LIED. Research Associate Dr. Sankar De (his salary was funded by the grant), in collaboration with Matthias Kling's group (then at MPQ Garching), proceeded to design a special multi-electrode VMI spectrometer for capturing high-energy ATI electrons and to install it in place of COLTRIMS inside our experimental chamber. That apparatus was later used for the first demonstration of field-free orientation in CO using two-color laser pulses as well as for a series of other experiments, such as pump-probe studies of molecular dynamics with few-cycle laser pulses, control of electron localization in dissociating hydrogen molecules using two-color laser pulses, and ATI spectra of Xe ionized by two-color laser pulses. In parallel, Dipanwita Ray (Ph.D. student of Lew Cocke) worked on measuring angle-resolved ATI spectra of noble gases using a stereo-ATI phasemeter as a TOF electron spectrometer. She observed the angular diffraction structures in 3D ATI spectra of Ar, Kr and Xe, which were interpreted in terms of the Quantitative Rescattering theory newly developed by C.D. Lin. We also attempted to use a much more powerful OPA (five times more energy per pulse than the one we had at JRML) available at the Advanced Laser Light Source (ALLS) in Montreal to observe LIED. Two visits to ALLS by the PI, Igor Litvinyuk, and one visit by the PI's Ph.D. student (Irina Bocharova) were funded by the grant. Though we failed to observe LIED (the repetition rate of the ALLS OPA was too low at only 100 Hz), this international collaboration resulted in several publications on other related subjects, such as the wavelength dependence of laser Coulomb explosion of hydrogen, the wavelength dependence of non-sequential double ionization of neon and argon, the demonstration of charge-resonance enhanced ionization in CO{sub 2}, and the study of non-elastic scattering processes in H{sub 2}. Theoretical efforts to account for the hydrogen Coulomb explosion experiment resulted in another paper by Maia Magrakvelidze as lead author. Although for various reasons we failed to achieve our main goal of observing LIED, we salute the recent success in this endeavor by Lou DiMauro's group (with theoretical support from our KSU colleague C.D. Lin) published in Nature, which validates our approach.

We present a new interferometer technique whereby multiple extreme ultraviolet light pulses are generated at different positions within a single laser focus (i.e., from successive sources) with a highly controllable time delay. The interferometer technique is tested with two generating media to create two extreme ultraviolet light pulses with a time delay between them. The delay is found to be a consequence of the Gouy phase shift. Ultimately the apparatus is capable of accessing unprecedented time scales by allowing stable and repeatable delays as small as 100 zs. © 2012 American Physical Society.

We present the first experimental data on strong-field ionization of atomic hydrogen by few-cycle laser pulses. We obtain quantitative agreement at the 10% level between the data and an {\it ab initio} simulation over a wide range of laser intensities and electron energies.

We used Coulomb explosion imaging (CEI) to observe enhanced ionization (EI) in nitrogen and carbon dioxide molecules. In N 2 we observed electron localization with time-resolved CEI using few-cycle laser pulses. In CO 2 we reconstructed molecular structure from triple coincidence measurements while varying pulse duration from 7 fs to 200 fs.

The process by which a molecule in an intense laser field ionizes more efficiently as its bond length increases towards a critical distance ${R}_{c}$ is known as charge resonance enhanced ionization (CREI). We make a series of measurements of this process for ${\mathrm{CO}}_{2}$, by varying pulse duration from 7 to 200 fs, in order to identify the charge states and time scales involved. We find that for the $4+$ and higher charge states, 100 fs is the time scale required to reach the critical geometry $⟨{R}_{\mathrm{CO}}⟩\ensuremath{\approx}2.1\text{ }\text{ }\AA{}$ and $⟨{\ensuremath{\theta}}_{\mathrm{OCO}}⟩\ensuremath{\approx}163\ifmmode^\circ\else\textdegree\fi{}$ (equilibrium ${\mathrm{CO}}_{2}$ geometry is $⟨{R}_{\mathrm{CO}}⟩\ensuremath{\approx}1.16\text{ }\text{ }\AA{}$ and $⟨{\ensuremath{\theta}}_{\mathrm{OCO}}⟩\ensuremath{\approx}172\ifmmode^\circ\else\textdegree\fi{}$). The $\mathrm{CO}_{2}{}^{3+}$ molecule, however, appears always to begin dissociation from closer than 1.7 \AA{} indicating that dynamics on charge states lower than $3+$ is not sufficient to initiate CREI. Finally, we make quantum ab initio calculations of ionization rates for ${\mathrm{CO}}_{2}$ and identify the electronic states responsible for CREI.

We have used velocity-map imaging to measure the three-dimensional momenta of electrons rescattered from Xe and Ar following the liberation of the electrons from these atoms by 45 fs, 800 nm intense laser pulses. Strong structure in the rescattering region is observed in both angle and energy, and is interpreted in terms of quantitative rescattering (QRS) theory. Momentum images have also been taken with two-color (800 nm $+$ 400 nm) pulses on Xe targets. A strong dependence of the spectra on the relative phase of the two colors is observed in the rescattering region. Interpretation of the phase dependence using both QRS theory and a full solution to the time-dependent Schr\"odinger equation shows that the rescattered electrons provide a much more robust method for determining the relative phase of the two colors than do the direct electrons.

We have performed the first experiment on the interaction of intense few-cycle laser pulses with atomic hydrogen. Experimental data is compared with an advanced ab initio simulation and agrees quantitatively with simulations at the 10% level.

We report the first experimental observation of nonadiabatic field-free orientation of a heteronuclear diatomic molecule (CO) induced by an intense two-color (800 and 400 nm) femtosecond laser field. We monitor orientation by measuring fragment ion angular distributions after Coulomb explosion with an 800 nm pulse. The orientation of the molecules is controlled by the relative phase of the two-color field. The results are compared to quantum mechanical rigid rotor calculations. The demonstrated method can be applied to study molecular frame dynamics under field-free conditions in conjunction with a variety of spectroscopy methods, such as high-harmonic generation, electron diffraction, and molecular frame photoelectron emission.

Two-color (800 and 400 nm) short (45 fs) linearly polarized pulses are used to ionize and dissociate ${\mathrm{D}}_{2}$ into a neutral deuterium atom and a deuteron. The yields and energies of the ions are measured left and right along the polarization vector. As the relative phase of the two colors is varied, strong yield asymmetries are found in the ion-energy regions traditionally identified as bond softening, above-threshold dissociation and rescattering. The asymmetries in these regions are quite different. A model based on the dynamic coupling by the laser field of the gerade and ungerade states in the molecular ion accounts for many of the observed features.

Based on a quantum-mechanical model, we calculate the time evolution of an initial nuclear vibrational wave packet in $\text{D}_{2}{}^{+}$ generated by the rapid ionization of ${\text{D}}_{2}$ in an ultrashort pump-laser pulse. By Fourier transformation of the nuclear probability density with respect to the time delay between the pump pulse and the instant destructive Coulomb-explosion imaging of the wave packet at the high-intensity spike of an intense probe-laser pulse, we provide two-dimensional internuclear-distance-dependent power spectra that serve as a tool for visualizing and analyzing the nuclear dynamics in $\text{D}_{2}{}^{+}$ in an intermittent external laser field. The external field models the pedestal to the central ultrashort spike of a realistic probe pulse. Variation in the intensity, wavelength, and duration of this probe-pulse pedestal (i) allows us to identify the optimal laser parameters for the observation of field-induced bond softening and bond hardening in $\text{D}_{2}{}^{+}$ and (ii) suggests a scheme for quantitatively testing the validity of the ``Floquet picture'' that is commonly used for the interpretation of short-pulse laser-molecule interactions, despite its implicit ``continuum wave'' (infinite pulse length) assumption.

Series: Springer Series in Optical Sciences

We have measured full momentum images of electrons rescattered from Xe, Kr, and Ar following the liberation of the electrons from these atoms by short, intense laser pulses. At high momenta the spectra show angular structure (diffraction) which is very target dependent and in good agreement with calculated differential cross sections for the scattering of free electrons from the corresponding ionic cores.

We present the results of an investigation of the measurement of the transverse electron momentum (TEMD) distribution of atoms ionized though the interaction of strong-field light pulses of varying ellipticity. We show though experiment and theory that the TEMD for the tunneling regime and the over-the-barrier regime (OB) evolves in a qualitatively different way when polarization state of the ionizing laser changes ellipticity.

We have measured angular distributions of ion fragments produced in dissociative double ionization of CO, CO{sub 2}, and C{sub 2}H{sub 2} by intense ultrashort laser pulses. This report extends similar recent studies of O{sub 2} and N{sub 2} to a wider set of molecules. We found that for sub-10-fs pulses of sufficiently low intensity the fragments' angular distributions for all studied molecules are determined by angular dependence of the first ionization step. Those experimental angular distributions were in good agreement with angular dependent ionization probabilities calculated with the molecular tunneling ionization theory. The measured angular distributions directly reflect the symmetry of the corresponding molecular orbitals. For higher laser intensities and longer pulse durations, dynamic alignment and postionization alignment start to affect the angular distributions and ion fragments are preferentially ejected along the laser-polarization direction.

Using the molecular clock, we time-resolved the double ionization of D2. When 8 fs pulse ∼1015 W/cm2 are used, there is a 4 fs delay between the first and the second ionization. We measure molecular structure of D2O and SO2 with ∼0.3-Angstrom resolution using laser Coulomb explosion imaging.

We solve the time-dependent Schrodinger equation in three dimensions for H-2(+) in a one-cycle laser pulse of moderate intensity. We consider fixed nuclear positions and Coulomb electron-nuclear interaction potentials. We analyze the field-induced electron interference and diffraction patterns. To extract the ionization dynamics we subtract the excitations to low-lying bound states explicitly. We follow the time evolution of a well-defined wave packet that is formed near the first peak of the laser field. We observe the fragmentation of the wave packet due to molecular focusing. We show how to retrieve a diffraction molecular image by taking the ratio of the momentum distributions in the two lateral directions. The positions of the diffraction peaks are well described by the classical double slit diffraction rule.

The yield of high harmonic generation with N2 and O2 molecules is enhanced when the molecules are aligned parallel to the laser field that produces the harmonics and suppressed for the perpendicular case, compared to randomly oriented molecules. The alignment is produced via creation of a rotational molecular wave packet with a non-ionizing 800-nm, 60-fs Ti:Sa pulse and probed at its revival via high harmonic generation with a delayed intense 800-nm, 30-fs pulse.

Ionization of D2 launches a vibrational wave packet on the ground state of D+2. Removal of the second electron places a pair of D+ ions onto a Coulombic potential. Measuring the D+ kinetic energy determines the time delay between the first and the second ionization. Caught between a falling ionization and a rapidly rising intensity, the typical lifetime of the D+2 intermediate is less than 5 fs when an intense 8.6 fs laser pulse is used. We simulate Coulomb explosion imaging of the ground state wave function of D2 by a 4 fs optical pulse and compare with our experimental observations.

We demonstrate a method to measure strong field laser ionization of aligned molecules. The method employs a macroscopic field-free dynamic alignment, which occurs during revivals of rotational wave packets produced by a femtosecond laser pulse. We investigate the dependence of strong field ionization of ${\mathrm{N}}_{\mathrm{2}}$ on molecular orientation. We determine that ${\mathrm{N}}_{\mathrm{2}}$ molecules are four times more likely to ionize when aligned parallel to the field than when aligned perpendicular to it.

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 Schrodinger 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.

A novel design for a velocity-map imaging (VMI) spectrometer with high resolution over a wide energy range surpassing a standard VMI design is reported. The main difference to a standard three-electrode VMI is the spatial extension of the applied field using 11 electrodes forming a thick-lens. This permits measurements of charged particles with higher energies while achieving excellent resolving power over a wide range of energies. Using SIMION simulations, the thick-lens VMI is compared to a standard design for up to 360 eV electrons. The simulations also show that the new spectrometer design is suited for charged-particle detection with up to 1 keV using a repeller-electrode voltage of -30 kV. The experimental performance is tested by laser-induced ionization of rare gases producing electrons up to about 70 eV. The thick-lens VMI is useful for a wide variety of studies on atoms, molecules and nanoparticles in intense laser fields and high-photon-energy fields from high-harmonic-generation or free-electron lasers.

We present a new interferometer technique based on the interference of high-order harmonic generation radiation from translatable successive gas jets. The phase shifts in the apparatus are shown to originate from the Gouy phase shift of the driving laser. The technique can be used to deliver time delays between light pulses and we demonstrate the unprecedented capability of delivering pulses of extreme ultraviolet light delayed in time by as small as 100 zeptoseconds.

We studied the dependence on the dissociative ionization of H2 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%).

Recent experiments in ultrafast physics have established the importance of above-threshold ionization (ATI) experiments in measuring and controlling the carrier-envelope phase (CEP) of few-cycle laser pulses. We have performed an investigation of atomic hydrogen subjected to intense CEP- stable few-cycle laser pulses. The experimental ATI spectra have been compared to predictions from an ab initio numerical solution of the time-dependent Schr¨ odinger equation in three dimensions. Good agreement between experiment

The interaction of intense few-cycle light pulses and matter has given rise to a number of high-field physical processes that are of great interest. The high non-linearity of such interactions necessitates complex numerical simulations in order to retrieve useful physical measurements from the experimental data. Here we demonstrate quantitative agreement at the 10% level between experimentally obtained integrated photoelectron spectra and numerical simulations for above-threshold ionization in the intense few-cycle regime. The use of atomic hydrogen provides a key experimental innovation as it is the only electronic system for which ab initio simulations in this regime are available.

Attosecond science has opened up the possibility of manipulating electrons on their fundamental timescales. Here, we use both theory and experi- ment to investigate ionization dynamics in helium on the attosecond timescale by simultaneously irradiating the atom with a soft x-ray attosecond pulse train (APT) and an ultrafast laser pulse. Because the APT has resolution in both energy and time, we observe processes that could not be observed without resolu- tion in both domains simultaneously. We show that resonant absorption is impor- tant in the excitation of helium and that small changes in energies of harmonics that comprise the APT can result in large changes in the ionization process. With the help of theory, ionization pathways for the infrared-assisted excitation and ionization of helium by extreme ultraviolet (XUV) attosecond pulses have been identified and simple model interpretations have been developed that should be of general applicability to more complex systems (Zewail A 2000 J. Phys. Chem. A 104 5660-94).