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We investigate the role of nuclear motion and strong-field-induced electronic couplings during the double ionization of deuterated water using momentum-resolved coincidence spectroscopy. By examining the three-body dicationic dissociation channel, D$^{+}$/D$^{+}$/O, for both few- and multi-cycle laser pulses, strong evidence for intra-pulse dynamics is observed. The extracted angle- and energy-resolved double ionization yields are compared to classical trajectory simulations of the dissociation dynamics occurring from different electronic states of the dication. In contrast with measurements of single photon double ionization, pronounced departure from the expectations for vertical ionization is observed, even for pulses as short as 10~fs in duration. We outline numerous mechanisms by which the strong laser field can modify the nuclear wavefunction en-route to final states of the dication where molecular fragmentation occurs. Specifically, we consider the possibility of a coordinate-dependence to the strong-field ionization rate, intermediate nuclear motion in monocation states prior to double ionization, and near-resonant laser-induced dipole couplings in the ion. These results highlight the fact that, for small and light molecules such as D$_2$O, a vertical-transition treatment of the ionization dynamics is not sufficient to reproduce the features seen experimentally in the strong field coincidence double-ionization data., Comment: 14 pages, 13 figures

Polyatomic molecules in strong laser fields can undergo substantial nuclear motion within tens of femtoseconds. Ion imaging methods based on dissociation or Coulomb explosion therefore have difficulty faithfully recording the geometry dependence of the field ionization that initiates the dissociation process. Here we compare the strong-field double ionization and subsequent dissociation of water (both H$_2$O and D$_2$O) in 10-fs and 40-fs 800-nm laser pulses. We find that 10-fs pulses turn off before substantial internuclear motion occurs, whereas rapid internuclear motion can take place during the double ionization process for 40-fs pulses. The short-pulse measurements are consistent with a simple tunnel ionization picture, whose predictions help interpret the motion observed in the long-pulse measurements., 9 pages, 5 figures

We simulate the pump-probe experiments of lasing in molecular nitrogen ions with particular interest in the effects of rotational wave-packet dynamics. Our computations demonstrate that the coherent preparation of rotational wave packets in ${\mathrm{N}}_{2}{}^{+}$ by an intense short nonresonant pulse results in a modulation of the subsequent emission from ${B}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{\ensuremath{\Sigma}}}_{u}{}^{+}\ensuremath{\rightarrow}{X}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{\ensuremath{\Sigma}}}_{g}{}^{+}$ transitions induced by a resonant seed pulse. We model the dynamics of such pumping and emission using density-matrix theory to describe the ${\mathrm{N}}_{2}{}^{+}$ dynamics and the Maxwell wave equation to model the seed pulse propagation. We show that the gain and absorption of a delayed seed pulse is dependent on the pump-seed delay, that is, the rotational coherences excited by the pump pulse can modulate the gain and absorption of the delayed seed pulse. Further, we demonstrate that the coherent rotational dynamics of the nitrogen ions can cause lasing without electronic inversion.

Air lasing in the nitrogen molecular ion is not well understood because the complex physics responsible for gain is interwoven with pulse propagation in an extreme environment. Here we use a short gas jet to limit the interaction length, thereby removing the propagation effects. We report on several mechanisms that contribute to the decay of gain in different conditions, and experimentally isolate two decay timescales: the decay of long-term gain due to collisional state mixing, and short-term gain that cannot be explained by population inversion. To test the former, we control the inelastic electron scattering rate by varying the gas concentration while keeping the propagation length fixed, and predict the change of the decay using a model of collisional state mixing. We show that the same mechanism causes the decay of rotational wave packets in the states of the ion. Finally, we simulate the complex modulations of gain due to rotational wave packets and the propagation of the probe pulse through the evolving rotationally excited and inverted medium.

We demonstrate an efficient approach for enhancing the spectral broadening of long laser pulses and for efficient frequency redshifting by exploiting the intrinsic temporal properties of molecular alignment inside a gas-filled hollow-core fiber (HCF). We find that laser-induced alignment with durations comparable to the characteristic rotational time scale T R o t A l i g n enhances the efficiency of redshifted spectral broadening compared to noble gases. The applicability of this approach to Yb lasers with (few hundred femtoseconds) long pulse duration is illustrated, for which efficient broadening based on conventional Kerr nonlinearity is challenging to achieve. Furthermore, this approach proposes a practical solution for high energy broadband long-wavelength light sources, and it is attractive for many strong field applications.

We show that lasing without inversion arises naturally during propagation of intense femtosecond laser pulses in air. It is triggered by the combination of molecular ionization and molecular alignment, both unavoidable in intense light fields., High Intensity Lasers and High Field Phenomena 2020, High-field Re-scattering Physics, Relativistic Nonlinear Phenomena, Intense Pulse Propagation and Filamentation (HM1B) Session, November 16–20, 2020, Washington, DC, United States, Series: OSA Technical Digest

The strong coupling between intense laser fields and valence electrons in molecules causes distortions of the potential energy hypersurfaces which determine the motion of the nuclei and influence possible reaction pathways. The coupling strength varies with the angle between the light electric field and valence orbital, and thereby adds another dimension to the effective molecular potential energy surface, leading to the emergence of light-induced conical intersections. Here, we demonstrate that multiphoton couplings can give rise to complex light-induced potential energy surfaces that govern molecular behavior. In the laser-induced dissociation of H2+, the simplest of molecules, we measure a strongly modulated angular distribution of protons which has escaped prior observation. Using two-color Floquet theory, we show that the modulations result from ultrafast dynamics on light-induced molecular potentials. These potentials are shaped by the amplitude, duration and phase of the dressing fields, allowing for manipulating the dissociation dynamics of small molecules., Conical intersections, a hallmark of polyatomic molecules, can be induced with light, leading to new reaction pathways. Here, the authors show that intense fields can create complex, beyond-conical intersections even in diatomics, resulting in an unexpected angular distribution of fragment ions.

The XUV field emitted by impulsively aligned ethylene molecules during high-order harmonic generation is retrieved as a function of molecular orientation. The results can be ascribed to multielectron contributions to the harmonic emission.

Synopsis Using theory and Cold Target Recoil Ion Momentum Spectroscopy we find signatures of light-induced molecular potential energy surfaces in the 3-dimensional proton momentum distributions of dissociating H 2 + .

We have trained a deep (convolutional) neural network to predict the ground-state energy of an electron in four classes of confining two-dimensional electrostatic potentials. On randomly generated potentials, for which there is no analytic form for either the potential or the ground-state energy, the neural network model was able to predict the ground-state energy to within chemical accuracy, with a median absolute error of 1.49 mHa. We also investigate the performance of the model in predicting other quantities such as the kinetic energy and the first excited-state energy of random potentials.

We discuss the seminal three-step model and the recollision picture in the context of high harmonic generation in molecules. In particular, we stress the importance of multi-electron correlation during the first and the third of the three steps of the process: (1) the strong field ionization and (3) the recombination. We point out how accurate account for multi-electron correlations during the third, recombination, step allows one to gauge the importance of pre-collision: the term coined by J. H. Eberly to describe unusual pathways during the first, ionization, step., Comment: 28 pages, 11 figures

We measure the ellipticity dependence of high-harmonic generation (HHG) from unaligned gas-phase ensembles of the five-membered ring molecules 2,3-dihydrofuran (${\mathrm{C}}_{4}{\mathrm{H}}_{6}\mathrm{O}$), furan (${\mathrm{C}}_{4}{\mathrm{H}}_{4}\mathrm{O}$), and thiophene (${\mathrm{C}}_{4}{\mathrm{H}}_{4}\mathrm{S}$). As is normally the case, the HHG emission is suppressed for increased ellipticity since the recollision wave packet leading to HHG is steered away from the parent ion for large ellipticity. However, through comparison with computations of the first step in HHG, namely, strong-field ionization (SFI), we show that the observed differences in the ellipticity dependence for the three molecular species reflect the lateral momentum distributions of the continuum electron responsible for HHG, which in turn provides information about the particular orientation between the molecular axis and the laser field that maximizes the SFI probability. Strikingly, and contrary to the conventional wisdom in the strong-field community, we find that for furan and thiophene the most probable orientation for SFI occurs when the electric field of the laser is aligned near a nodal plane of the corresponding ionizing orbital. 2,3-dihydrofuran, on the other hand, follows the standard expectation that the most probable orientation for SFI occurs when the electric field is aligned away from any nodal plane.

The encoding of quantum information in photonic time-bin qubits is apt for long-distance quantum communication schemes. In practice, due to technical constraints such as detector response time, or the speed with which copolarized time-bins can be switched, other encodings, e.g., polarization, are often preferred for operations like state detection. Here, we present the conversion of qubits between polarization and time-bin encodings by using a method that is based on an ultrafast optical Kerr shutter and attain efficiencies of 97% and an average fidelity of $0.827\ifmmode\pm\else\textpm\fi{}0.003$ with shutter speeds near 1 ps. Our demonstration delineates an essential requirement for the development of hybrid and high-rate optical quantum networks.

Ionization of an atom or molecule by a strong laser field produces sub-optical cycle wave packets whose control has given rise to attosecond science. The final states of the wave packets depend on ionization and deflection by the laser field, which are convoluted in conventional experiments. Here, we demonstrate a technique enabling efficient electron deflection, separate from the field driving strong-field ionization. Using a mid-infrared deflection field permits one to distinguish electron wave packets generated at different field maxima of an intense few-cycle visible laser pulse. We utilize this capability to trace the scattering of low-energy electrons driven by the mid-infrared field. Our approach represents a general technique for studying and controlling strong-field ionization dynamics on the attosecond time scale., Comment: 5 Pages, 4 figures

Laser-driven electron recollision is at the heart of the rapidly growing field of attosecond science. The recollision wavepacket is qualitatively described within the strong-field approximation, which commonly assumes tunnelling ionization and plane-wave propagation of the liberated electron in the continuum. However, with increasing experimental sophistication, refinements to this simple model have become necessary. Through careful modelling and measurements of laser-induced recollision holography using aligned N 2 molecules, we demonstrate that the continuum electron wavepacket already carries a non-trivial spatial phase structure immediately following ionization. This effect is of rather general character: any molecule and any non-isotropic system that is ionized by a strong laser field will exhibit an offset in the phase of the continuum electron wavepacket. Specifically, this has important implications for any coherent scattering process in molecules, such as high-harmonic generation or laser-induced electron holography.

We study the near-threshold photodissociation dynamics of NO2 by a kinematically complete femtosecond pump-probe scheme using a cold target recoil ion momentum spectrometer. We excite NO2 to the optically bright Ã2B2 state with a 400 nm pulse and probe the ensuing dynamics via strong field single and double ionization with a 25 fs, 800 nm pulse. The pump spectrum spans the NO(X2Π) + O(3P) dissociation channel threshold, and therefore, following internal conversion, excited NO2 is energetically prepared both “above threshold” (dissociating) and “below threshold” (nondissociating). Experimentally, we can clearly discriminate a weak two-photon pump channel from the dominant single-photon data. In the single ionization channel, we observe NO+ fragments with nonzero momentum at 200 fs delay and an increasing yield of NO+ fragments with near-zero momentum at 3.0 ps delay. For double ionization events, we observe a time-varying Coulombic kinetic energy release between the NO+ and O+ fragments impulsively created from the evolving “hot” neutral ground state. Supported by classical trajectory calculations, we assign the decreasing Coulombic kinetic energy release at longer time delays to the increasing average NO–O distances in the ground electronic state during its large amplitude phase space evolution toward free products. The time-resolved kinetic energy release in the double ionization channel probes the large amplitude ground state evolution from a strongly coupled “inner region” to a loosely coupled “outer region” where one O atom is on average much further away from the NO. Both the time evolution of the kinetic energy release and the NO+ angular distributions support our assignments.

Strong field ionization (SFI) and high harmonic generation (HHG) are the central processes that enable attosecond physics. To date, most theoretical investigations of SFI and HHG rely on semiclassical methods, most notably the strong field approximation (SFA), which treats the continuum states of the liberated electron as plane waves. SFA is a very successful conceptual model for atoms and small molecules, and leads to an intuitively satisfying picture of HHG (i.e. the three-step model). However, as SFI and HHG experiments enter the regime of polyatomic molecules, many approximations made in SFA-type models can no longer be ignored even for a qualitatively correct understanding. In this contribution, we present a selection of results from our efforts toward ab initio modeling of strong field processes in molecular systems. The method is applicable to single-electron ionization and HHG of gas-phase polyatomic molecular targets. © 2012 Elsevier B.V. All rights reserved.

The influence of cyclic electron delocalization associated with aromaticity on the high-order-harmonic generation (HHG) process is investigated in organic molecules. We show that the aromatic molecules benzene $({\mathrm{C}}_{6}{\mathrm{H}}_{6})$ and furan $({\mathrm{C}}_{4}{\mathrm{H}}_{4}\mathrm{O})$ produce high-order harmonics more efficiently than nonaromatic systems having the same ring structure. We also demonstrate that the relative strength of plateau harmonics is sensitive to the aromaticity in five-membered-ring molecules using furan, pyrrole $({\mathrm{C}}_{4}{\mathrm{H}}_{4}\mathrm{NH})$, and thiophene $({\mathrm{C}}_{4}{\mathrm{H}}_{4}\mathrm{S})$. Numerical time-dependent Schr\"odinger equation simulations of total orientation-averaged strong-field ionization yields show that the HHG from aromatic molecules comes predominantly from the two highest $\ensuremath{\pi}$ molecular orbitals, which contribute to the aromatic character of the systems.

A number of techniques have been developed to characterize the attosecond emission from high-order-harmonic sources. These techniques are broadly classified as ex situ, where the attosecond pulse train photoionizes a target gas in the presence of an infrared field, and in situ, where the measurement takes place in the medium in which the attosecond pulses are generated. It is accepted that ex situ techniques measure the characteristics of the electromagnetic field, including the phase of the recombination transition moment of the emitting atom or molecule, when the phase of the second medium is known. However, there is debate about whether in situ techniques measure the electromagnetic field, or only the characteristics of the recolliding electron before recombination occurs. We show numerically that in situ measurements are not sensitive to the recombination phase, when implemented in the perturbative regime as originally envisioned, and that they do not measure the electromagnetic phase of the emission.

We propose and experimentally demonstrate a scheme to generate optically controlled delays based on off-resonant Raman absorption. Dispersion in a transparency window between two neighboring, optically activated Raman absorption lines is used to reduce the group velocity of broadband 765 nm pulses. We implement this approach in a potassium titanyl phosphate (KTP) waveguide at room temperature, and demonstrate Raman-induced delays of up to 140 fs for a 650-fs duration, 1.8-THz bandwidth, pulse. Our approach should be applicable to single-photon signals, offers wavelength tunability, and is a step toward processing ultrafast photons.

We use differential holography to overcome the forward scattering problem in strong-field photoelectron holography. Our differential holograms of ${\mathrm{H}}_{2}$ and ${\mathrm{D}}_{2}$ molecules exhibit a fishbonelike structure, which arises from the backscattered part of the recolliding photoelectron wave packet. We demonstrate that the backscattering hologram can resolve the different nuclear dynamics between ${\mathrm{H}}_{2}$ and ${\mathrm{D}}_{2}$ with subangstrom spatial and subcycle temporal resolution. In addition, we show that attosecond electron dynamics can be resolved. These results open a new avenue for ultrafast studies of molecular dynamics in small molecules.

In the initial article [Phys. Rev. Lett. 110, 044301 (2013), arXiv:1208.4611] it was claimed that human hearing can beat the Fourier uncertainty principle. In this Comment, we demonstrate that the experiment designed and implemented in the original article was ill-chosen to test Fourier uncertainty in human hearing., Comment: 2 pages, 1 figure, accepted to Phys. Rev. Lett

We consider the quantum dynamics of the kicked rotor model in the regime where the last stable resonance islands in the classical phase space disappear. It is found that phase space regions with lo...

We analyse S-matrix-type approaches to strong-field ionization by an intense few-cycle laser pulse. We derive simple modifications of approximate S-matrix-type approaches that either neglect the effect of the ionic potential after ionization, or treat this potential perturbatively. We show how these modifications eliminate some crucial problems and artefacts, such as the 'curse of the displaced atom', associated with the choice of gauge in the strong field approximation. We then discuss how these approximate S-matrix-type approaches should be applied to dissociating molecules using the example of H [image omitted]. We show that suppression of electron tunnelling between the two fragments of the molecular ion introduces dramatic changes in the spectrum of high harmonic emission generated by an intense laser pulse that probes the molecular dissociation. For large internuclear distances and even when the active electron is fully delocalized between the two fragments, the high harmonic spectrum loses all traces of the conventional interference pattern associated with a two-centre molecular structure.

We discuss the sub-cycle dynamics of strong-field ionization by an intense few-cycle laser pulse. Even at high intensities when all the excited states are embedded into the continuum, the dynamics are found to be strongly dependent both on the presence or absence of excited states in the field-free potential and on the structure of the continuum. Both Coulomb effects in the continuum and polarization of the initial state are important even for qualitative description of sub-cycle ionization. We show how to consistently include effects of the binding (Coulomb) potential on the continuum part of the electron wavefunction while retaining the effects of the laser field on the bound part. We demonstrate quantitative accuracy of our approach in the barrier-suppression region of laser intensities.

Strong field ionization is a starting point for a rich set of physical phenomena associated with attosecond science. This paper provides an introductory overview of the basic theory of strong field ionization and focuses on (i) the physics and the dynamics of the electron transition to the continuum, and (ii) the shape of the electron wavepacket as it appears in the continuum.

We present a theoretical analysis of how intense, few-cycle infrared laser pulses can be used to image the structure of small molecules with nearly 1 fs temporal and sub-Å spatial resolution. We identify and analyse several physical mechanisms responsible for the distortions of the diffraction image and describe a recipe for recovering an un-distorted image from angle and energy-resolved electron spectra. We also identify holographic patterns in the photoelectron spectra and discuss the requirements to enhancing the hologram resolution for imaging the scattering potential.

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.

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

Modern femtosecond technology can be used to create laser pulses that induce controlled spinning of anisotropic molecules to very high angular momentum states ("optical centrifuge"). In this paper we extend our previous study [M. Spanner and M. Ivanov, J. Chem. Phys. 114, 3456 (2001)] and focus on the stability of angular trapping and forced rotational acceleration of a diatomic molecule in an optical centrifuge. The effects of laser intensity modulations and rovibrational coupling are analyzed in detail, classically and quantum mechanically. The numerical simulations show excellent qualitative agreement between the quantum and classical systems. Forced rotations of the classical system can exhibit chaotic behavior, which becomes rather unique when the accelerating rotation of the angular trapping potential combines with efficient rovibrational coupling. In this regime the Lyapunov exponent becomes time-dependent and the trajectories separate as exp(lambdaF(t)).

We produce oriented rotational wave packets in CO and measure their characteristics via high harmonic generation. The wave packet is created using an intense, femtosecond laser pulse and its second harmonic. A delayed 800 nm pulse probes the wave packet, generating even-order high harmonics that arise from the broken symmetry induced by the orientation dynamics. The even-order harmonic radiation that we measure appears on a zero background, enabling us to accurately follow the temporal evolution of the wave packet. Our measurements reveal that, for the conditions optimum for harmonic generation, the orientation is produced by preferential ionization which depletes the sample of molecules of one orientation.

The multiphoton ionization rate of molecules depends on the alignment of the molecular axis with respect to the ionizing laser polarization. By studying molecular frame photo-electron angular distributions from N$_2$, O$_2$ and benzene, we illustrate how the angle-dependent ionization rate affects the photo-electron cutoff energy. We find alignment can enhance the high energy cutoff of the photo-electron spectrum when probing along a nodal plane or when ionization is otherwise suppressed. This is supported by calculations using a tunneling model with a single ion state., Comment: 4 pages, 4 figures

In a velocity map imaging spectrometer, we measured the electron momentum distributions from the ionization of O molecules with 800 nm wavelength, 40 fs laser pulses at a peak intensity of W cm. The molecules were aligned at 0, 45 and 90 relative to the laser polarization prior to ionization. We show that for all alignments the low momentum region – populated by direct electrons which do not recollide with the parent ion – is consistent with the ionized orbital being filtered and projected onto the continuum electron wave packet. In the high momentum region – populated by rescattered electrons – we observe that the pattern created by diffraction of the recolliding wave packet by the ion core disappears as the alignment gets closer to the laser field axis. We find that a two-slit diffraction model agrees well with the results for molecules aligned at 90, but only partially predicts the decrease in the diffraction signature for smaller alignment angles.

In strong-field ionization (SFI) of polyatomic molecules, the participation of multiple electronic ionization channels is emerging as a key aspect. In the molecular frame, each channel is expected to show a characteristic dependence of the SFI yield on the polarization direction of the ionizing field. We apply a new angle- and channel-resolved SFI technique to the polyatomic molecule 1,3-butadiene and compare these molecular-frame measurements with two leading theoretical models. © 2013 American Physical Society.

We demonstrate that high-order-harmonic generation (HHG) spectroscopy can be used to probe stereoisomers of randomly oriented 1,2-dichloroethylene (C${}_{2}$H${}_{2}$Cl${}_{2}$) and 2-butene (C${}_{4}$H${}_{8}$). The high-harmonic spectra of these isomers are distinguishable over a range of laser intensities and wavelengths. Time-dependent numerical calculations of angle-dependent ionization yields for 1,2-dichloroethylene suggest that the harmonic spectra of molecular isomers reflect differences in their strong-field ionization. The subcycle ionization yields for the cis isomer are an order of magnitude higher than those for the trans isomer. The sensitivity in discrimination of the harmonic spectra of cis- and trans- isomers is greater than 8 and 5 for 1,2-dichloroethylene and 2-butene, respectively. We show that HHG spectroscopy cannot differentiate the harmonic spectra of the two enantiomers of the chiral molecule propylene oxide (C${}_{3}$H${}_{6}$O).

We study correlations between neutral and ionic states in strong-field molecular ionization. We compare predictions based on Dyson orbital norms and quasistatic semiclassical tunneling theories (Keldysh and molecular orbital Ammosov-Delone-Krainov) with more detailed calculations of strong-field ionization which take into account (i)the Coulomb interaction between the outgoing continuum electron wave packet and the remaining bound electrons and (ii)electron-core interactions that cause distortions of the electronic continuum states during the ionization event. Our results highlight the prominence of electronic rearrangement effects in strong-field ionization with intense ultrafast laser pulses, where the outgoing continuum electron can cause electronic transitions in the parent ion. Calculations and measurements for excited uracil molecules reveal the breakdown of Keldysh-weighted Dyson norm predictions for ionization to different states of the molecular cation in the strong-field regime. © 2012 American Physical Society.

Two mechanisms of two-color (\omega + 2\omega) laser-induced field-free molecular orientation, based on the hyperpolarizability and ionization depletion, are explored and compared. The CO molecule is used as a computational example. While the hyperpolarizability mechanism generates small amounts of orientation at intensities below the ionization threshold, ionization depletion quickly becomes the dominant mechanism as soon as ionizing intensities are reached. Only the ionization mechanism leads to substantial orientation (e.g. on the order of || > 0.1). For intensities typical of laser-induced molecular alignment and orientation experiments, the two mechanism lead to robust, characteristic timings of the field-free orientation wave-packet revivals relative to the the alignment revivals and the revival time. The revival timings can be used to detect the active orientation mechanism experimentally.

We bring the methodology of orienting polar molecules together with the phase sensitivity of high harmonic spectroscopy to experimentally compare the phase difference of attosecond bursts of radiation emitted upon electron recollision from different ends of a polar molecule. This phase difference has an impact on harmonics from aligned polar molecules, suppressing emission from the molecules parallel to the driving laser field while favoring the perpendicular ones. For oriented molecules, we measure the amplitude ratio of even to odd harmonics produced when intense light irradiates CO molecules and determine the degree of orientation and the phase difference of attosecond bursts using molecular frame ionization and recombination amplitudes. The sensitivity of the high harmonic spectrum to subtle phase differences in the emitted radiation makes it a detailed probe of polar molecules and will drive major advances in the theory of high harmonic generation.

The analogy between Young’s double-slit experiment with matter and laser driven coherent control schemes is investigated, and shown to be limited. To do so, a general decomposition of observables in the Heisenberg picture into direct terms and interference contributions is introduced, and formal quantum- classical correspondence arguments in the Heisenberg picture are employed to define classical analogs of quantum interference terms. While the classical interference contributions in the double-slit experiment are shown to be zero, they can be nonzero in laser driven coherent control schemes and lead to laser control in the classical limit. This classical limit is interpreted in terms of nonlinear response theory arguments.

We present state-of-the-art ab initio--type computations of strong-field ionization (SFI) and the single-molecule response contribution to high-order-harmonic generation (HHG) in the polyatomic molecule N${}_{2}$O${}_{4}$. The numerical method uses a mixed orbital- and grid-based approach to model the multielectron bound states and single-electron continuum. The effects of ionic-core attraction and Coulomb-mediated interchannel coupling are rigorously included. We show that full-dimensionality time-dependent multielectron computations of SFI and HHG in polyatomic molecules are now feasible. The computational results indicate that (a) SFI yields in N${}_{2}$O${}_{4}$ are dominated by a single ionic state (the ${A}_{g}$ state), and are strongly modulated by the N-N stretch coordinate; and (b) the HHG radiating dipole as a function of the N${}_{2}$O${}_{4}$ N-N stretch is dominated by the same, single state. The molecular-beam coincidence measurements presented here support the computational results. Due to the differences in the estimated vibrational amplitude, however, the computations are not in full agreement with previous HHG data.

We report the measurement of high harmonics from oriented molecules in the gas phase. We show that attosecond and re-collision science provides a detailed and sensitive probe of electronic asymmetry in polar molecules., 2012 Conference on Lasers and Electro-Optics, CLEO 2012, May 6-11, 2012, San Jose, CA, USA

The extreme-ultraviolet (XUV) and strong-field valence ionization of cytosine and uracil is considered. We examine some simple estimates of the relative yields of the cation states populated following ionization and compare these to the results of a recently developed ab initio-type numerical model designed to compute strong-field ionization of molecules, the so-called time-dependent resolution in ionic states (TD-RIS) method. In analogy with one-photon XUV ionization, where the photoionization matrix elements can be related to the Dyson orbitals, we construct estimates for the yield of strong-field ionization (SFI) to different cation states based on the Dyson orbital norms and the Keldysh tunneling ionization rate. In the case of XUV ionization, the Dyson norms are shown to be good predictors of the relative cation yields when compared with the TD-RIS yields. The Dyson- and Keldysh-based models underestimate the yield to excited cation states in the case of SFI. The increased yield to the excited cation states in the TD-RIS results is attributed to the inclusion of multielectron effects and continuum structure not present in the simple models. The molecular Ammosov-Delone-Krainov (MO-ADK) method of calculating SFI is also considered. This later method is seen to agree more closely with the Dyson- and Keldysh-based estimates as it also fails to capture the multielectron effects and continuum structure included in the TD-RIS approach. ©2012 American Physical Society.

Channel-resolved Above-Threshold Ionization (CRATI) reveals the multiple electronic continua underlying many emerging attosecond strong field molecular spectroscopies. CRATI measurements in the molecular frame probe the angular ionization probability of the underlying Dyson orbitals.

We explore the effects of multiple participating final ion states during strong field ionization (SFI) and high-harmonic generation (HHG) using a two-electron two-center reduced-dimensionality model. In particular, we propose to use above threshold ionization (ATI) photoelectron spectra to identify ionic states that are active in SFI, and demonstrate the feasibility of our proposal to track multiple ionic states in a dissociation scenario. In addition to offering clear signatures of multiple participating ionic states, our method is shown to be sensitive to sub-cycle nonadiabatic laser-driven couplings between the ionic states. Finally, we calculate the high-harmonic emission and show how the multiple ionic states identified in the ATI spectra can manifest themselves in the high-harmonic spectra.

Coupled time-dependent single-particle Schroedinger equations describing the neutral amplitude and continuum electron are constructed. Results of strong field ionization on small (e.g. N2) to medium size (e.g. n-butane) molecules will be considered.

The quantum-to-classical transition of a symmetry-breaking coherent control scenario is computationally demonstrated in an optical lattice arrangement. Control is shown to survive in the classical limit and, for small effective $\ensuremath{\hbar}$, to be comparable in magnitude to quantum control. Moderate decoherence is seen to eliminate structure from the momentum space distribution, but not to cause loss of control. The proposed scenario is designed so as to be demonstrable experimentally in a moving or shaken one-dimensional optical lattice.

We analyze atomic photoelectron momentum distributions induced by bichromatic and monochromatic laser fields within the strong field approximation (SFA), separable Coulomb-Volkov approximation (SCVA), and ab initio treatment. We focus on the high frequency regime -- the smallest frequency used is larger than the ionization potential of the atom. We observe a remarkable agreement between the ab initio and velocity gauge SFA results while the velocity gauge SCVA fails to agree. Reasons of such a failure are discussed., Comment: Completely rewritten paper. Ionization by a two-color field is added