Multi-channel electronic and vibrational dynamics in polyatomic resonant high-order harmonic generation

Ultrafast Resonant State Formation by the Coupling of Rydberg and Dark Autoionizing States

Studying the dynamics of dark states is challenging due to their inability to undergo single-photon emission or absorption. This challenge is made even more difficult for dark autoionizing states owing to their ultrashort lifetime of a few femtoseconds. High-order harmonic spectroscopy recently appeared as a novel method to probe the ultrafast dynamics of a single atomic or molecular state. Here, we demonstrate the emergence of a new type of ultrafast resonance state as a manifestation of coupling between Rydberg and a dark autoionizing state dressed by a laser photon. Through high-order harmonic generation, this resonance results in extreme ultraviolet light emission that is more than one order of magnitude stronger than for the off-resonance case. The induced resonance can be leveraged to study the dynamics of a single dark autoionizing state and the transient changes in the dynamics of real states due to their overlap with the virtual laser-dressed states. In addition, the present results allow the generation of coherent ultrafast extreme ultraviolet light for advanced ultrafast science applications.

Wavelength dependence of the multiphoton ionization of CH3I by intense femtosecond laser pulses through Freeman resonances

Multiphoton ionization (MPI) of methyl iodide, CH₃I, has been investigated with the photoelectron imaging (PEI) technique, using high intensity femtosecond laser pulses at different central wavelengths. The use of high laser field strengths alters the way in which matter-radiation interaction takes place. This generates highly nonlinear phenomena, among which we can highlight the Stark shift effect. It can distort the potential energy surfaces of atoms and molecules, varying both the energy of electronic and rovibrational states of these systems and their ionization potentials. In this way, the Stark shift can generate resonances between intermediate states and an integer number of laser photons of a given wavelength, which would be absent in the low intensity regime. The main purpose of this work is the generation, detection and characterization of resonances produced by the Stark shift, commonly known as Freeman resonances, induced by multiphoton ionization of gas-phase CH₃I at different laser wavelengths. The results obtained reveal that a multitude of resonances are induced in the ionization of CH₃I in the range of intensities employed, involving several Rydberg states. Ionization pathways associated with different degrees of vibrational excitation in both the intermediate states and the molecular cation generated in each of the experiments are proposed.

Probing phonon dynamics with multidimensional high harmonic carrier-envelope-phase spectroscopy

High harmonic generation (HHG) has recently been established as a powerful method for probing ultrafast electron dynamics in solids. However, it remains unknown if HHG can be similarly applied for probing lattice distortions such as phonons. Specifically, it is unclear if the extreme nonlinearity of HHG can contribute to enhanced temporal resolution or sensitivity for probing lattice dynamics (compared to other, perturbative, methods). Here, we theoretically explore HHG in solids with active phonons. We present a pump-probe and multidimensional spectroscopy approach that relies on carrier-envelope-phase-sensitivity, in which HHG is highly sensitive for phonon dynamics. Strikingly, the predicted temporal resolution is ∼1 femtosecond, much below the probe pulse duration, owing to the subcycle nature of the approach.

We explore pump-probe high harmonic generation (HHG) from monolayer hexagonal-boron-nitride, where a terahertz pump excites coherent optical phonons that are subsequently probed by an intense infrared pulse that drives HHG. We find, through state-of-the-art ab initio calculations, that the structure of the emission spectrum is attenuated by the presence of coherent phonons and no longer comprises discrete harmonic orders, but rather a continuous emission in the plateau region. The HHG yield strongly oscillates as a function of the pump-probe delay, corresponding to ultrafast changes in the lattice such as specific bond compression or stretching dynamics. We further show that in the regime where the excited phonon period and the pulse duration are of the same order of magnitude, the HHG process becomes sensitive to the carrier-envelope phase (CEP) of the driving field, even though the pulse duration is so long that no such sensitivity is observed in the absence of coherent phonons. The degree of CEP sensitivity versus pump-probe delay is shown to be a highly selective measure for instantaneous structural changes in the lattice, providing an approach for ultrafast multidimensional HHG spectroscopy. Remarkably, the obtained temporal resolution for phonon dynamics is ∼1 femtosecond, which is much shorter than the probe pulse duration because of the inherent subcycle contrast mechanism. Our work paves the way toward routes of probing phonons and ultrafast material structural changes with subcycle temporal resolution and provides a mechanism for controlling the HHG spectrum.

Ab Initio Cluster Approach for High Harmonic Generation in Liquids

High harmonic generation (HHG) takes place in all phases of matter. In gaseous atomic and molecular media, it has been extensively studied and is very well understood. In solids, research is ongoing, but a consensus is forming for the dominant microscopic HHG mechanisms. In liquids, on the other hand, no established theory yet exists, and approaches developed for gases and solids are generally inapplicable, hindering our current understanding. We develop here a powerful and reliable ab initio cluster-based approach for describing the nonlinear interactions between isotropic bulk liquids and intense laser pulses. The scheme is based on time-dependent density functional theory and utilizes several approximations that make it feasible yet accurate in realistic systems. We demonstrate our approach with HHG calculations in water, ammonia, and methane liquids and compare the characteristic response of polar and nonpolar liquids. We identify unique features in the HHG spectra of liquid methane that could be utilized for ultrafast spectroscopy of its chemical and physical properties, including a structural minimum at 15-17 eV that is associated solely with the liquid phase. Our results pave the way to accessible calculations of HHG in liquids and illustrate the unique nonlinear nature of liquid systems.

Channel Coupling Dynamics of Deep-Lying Orbitals in Molecular High-Harmonic Generation

Investigation on structures in the high-harmonic spectrum has provided profuse information of molecular structure and dynamics in intense laser fields, based on which techniques of molecular ultrafast dynamics imaging have been developed. Combining ab initio calculations and experimental measurements on the high-harmonic spectrum of the CO_{2} molecule, we find a novel dip structure in the low-energy region of the harmonic spectrum which is identified as fingerprints of participation of deeper-lying molecular orbitals in the process and decodes the underlying attosecond multichannel coupling dynamics. Our work sheds new light on the ultrafast dynamics of molecules in intense laser fields.

A systematic construction of Gaussian basis sets for the description of laser field ionization and high-harmonic generation

A precise understanding of mechanisms governing the dynamics of electrons in atoms and molecules subjected to intense laser fields has a key importance for the description of attosecond processes such as the high-harmonic generation and ionization. From the theoretical point of view, this is still a challenging task, as new approaches to solve the time-dependent Schrödinger equation with both good accuracy and efficiency are still emerging. Until recently, the purely numerical methods of real-time propagation of the wavefunction using finite grids have been frequently and successfully used to capture the electron dynamics in small one- or two-electron systems. However, as the main focus of attoscience shifts toward many-electron systems, such techniques are no longer effective and need to be replaced by more approximate but computationally efficient ones. In this paper, we explore the increasingly popular method of expanding the wavefunction of the examined system into a linear combination of atomic orbitals and present a novel systematic scheme for constructing an optimal Gaussian basis set suitable for the description of excited and continuum atomic or molecular states. We analyze the performance of the proposed basis sets by carrying out a series of time-dependent configuration interaction calculations for the hydrogen atom in fields of intensity varying from 5 × 10¹³ W/cm² to 5 × 10¹⁴ W/cm². We also compare the results with the data obtained using Gaussian basis sets proposed previously by other authors.

Aurore: A platform for ultrafast sciences

We present the Aurore platform for ultrafast sciences. This platform is based on a unique 20 W, 1 kHz, 26 fs Ti:sapphire laser system designed for reliable operation and high intensity temporal contrast. The specific design ensures the high stability in terms of pulse duration, energy, and beam pointing necessary for extended experimental campaigns. The laser supplies 5 different beamlines, all dedicated to a specific field: attosecond science (Aurore 1), ultrafast phase transitions in solids (Aurore 2 and 3), ultrafast luminescence in solids (Aurore 4), and femtochemistry (Aurore 5). The technical specifications of these five beamlines are described in detail, and examples of the recent results are given.

Ultrafast dissociative ionization and large-amplitude vibrational wave packet dynamics of strong-field-ionized di-iodomethane

We employ few-cycle pulses to strong-field-ionize di-iodomethane (CH₂I₂) and femtosecond extreme ultraviolet (XUV) transient absorption spectroscopy to investigate the subsequent ultrafast dissociative ionization and vibrational wave packet dynamics. Probing in the spectral region of the I 4d core-level transitions, the time-resolved XUV differential absorption spectra reveal the population of several electronic states of CH₂I₂ ⁺ by strong-field ionization. Global analysis reveals three distinct time scales for the observed dynamics: 20 ± 2 fs, 49 ± 6 fs, and 157 ± 9 fs, ascribed to relaxation of the CH₂I₂ ⁺ parent ion from the Franck-Condon region, dissociation of high-lying excited states of CH₂I₂ ⁺ to I⁺ (³P₂), CH₂I, and I₂ ⁺ (²Π_3/2,g), and dissociation of CH₂I₂ ⁺ to I (²P_3/2) and CH₂I⁺, respectively. Oscillatory features in the time-resolved XUV differential absorption spectra point to the generation of vibrational wave packets in both the residual CH₂I₂ and the CH₂I₂ ⁺ parent ion. Analysis of the oscillation frequencies and phases reveals, in the case of neutral CH₂I₂, C-I symmetric stretching induced by bond softening and I-C-I bending driven by a combination of bond softening and R-selective depletion. In the case of CH₂I₂ ⁺, both the fundamental and first overtone frequencies of the I-C-I bending mode are observed, indicating large-amplitude I-C-I bending motion, in good agreement with results obtained from ab initio simulations of the XUV transition energy along the I-C-I bend coordinate. These results show that femtosecond XUV absorption spectroscopy is well-suited for studying ultrafast photodissociation and vibrational wave packet dynamics.

High relative-phase precision beam duplicator for mid-infrared femtosecond pulses

In this Letter, we use a 0-π square-wave phase grating to shape 1350 nm and 1450 nm femtosecond pulses and create two intense lobes at the focus of a lens. We show that the relative phase between these two lobes (the 1st and -1st orders of diffraction of the grating) is controlled very simply and precisely by shifting the position of the grating in its plane. We generate high harmonic orders from the two bright lobes and record the beating between the two emissions for each harmonic order up to the 53rd harmonic order.

Symphony on strong field approximation

This paper has been prepared by the Symphony collaboration (University of Warsaw, Uniwersytet Jagielloński, DESY/CNR and ICFO) on the occasion of the 25th anniversary of the 'simple man's models' which underlie most of the phenomena that occur when intense ultrashort laser pulses interact with matter. The phenomena in question include high-harmonic generation (HHG), above-threshold ionization (ATI), and non-sequential multielectron ionization (NSMI). 'Simple man's models' provide both an intuitive basis for understanding the numerical solutions of the time-dependent Schrödinger equation and the motivation for the powerful analytic approximations generally known as the strong field approximation (SFA). In this paper we first review the SFA in the form developed by us in the last 25 years. In this approach the SFA is a method to solve the TDSE, in which the non-perturbative interactions are described by including continuum-continuum interactions in a systematic perturbation-like theory. In this review we focus on recent applications of the SFA to HHG, ATI and NSMI from multi-electron atoms and from multi-atom molecules. The main novel part of the presented theory concerns generalizations of the SFA to: (i) time-dependent treatment of two-electron atoms, allowing for studies of an interplay between electron impact ionization and resonant excitation with subsequent ionization; (ii) time-dependent treatment in the single active electron approximation of 'large' molecules and targets which are themselves undergoing dynamics during the HHG or ATI processes. In particular, we formulate the general expressions for the case of arbitrary molecules, combining input from quantum chemistry and quantum dynamics. We formulate also theory of time-dependent separable molecular potentials to model analytically the dynamics of realistic electronic wave packets for molecules in strong laser fields. We dedicate this work to the memory of Bertrand Carré, who passed away in March 2018 at the age of 60.

Probing ultrafast dynamics of chiral molecules using time-resolved photoelectron circular dichroism

Measuring the ultrafast dynamics of chiral molecules in the gas phase has been a long standing and challenging quest of molecular physics. The main limitation to reach that goal has been the lack of highly sensitive chiroptical measurement. By enabling chiral discrimination with up to several 10% of sensitivity, photoelectron circular dichroism (PECD) offers a solution to this issue. However, tracking ultrafast processes requires measuring PECD with ultrashort light pulses. Here we compare the PECD obtained with different light sources, from the extreme ultraviolet to the mid-infrared range, leading to different ionization regimes: single-photon, resonance-enhanced multiphoton, above-threshold and tunnel ionization. We use single and multiphoton ionization to probe the ultrafast relaxation of fenchone molecules photoexcited in their first Rydberg states. We show that time-resolved PECD enables revealing dynamics much faster than the population decay of the Rydberg states, demonstrating the high sensitivity of this technique to vibronic relaxation.

Controlling the multi-electron dynamics in the high harmonic spectrum from N2O molecule using TDDFT

In this study, high harmonic generation from a multi-atomic nitrous oxide molecule was investigated. A comprehensive three-dimensional calculation of the molecular dynamics and electron trajectories through an accurate time-dependent density functional theory was conducted to efficiently explore a broad harmonic plateau. The effects of multi-electron and inner orbitals on the harmonic spectrum and generated coherent attosecond pulses were analyzed. The role of the valence electrons in controlling the process and extending the harmonic plateau was investigated. The main issue of producing a super-continuum harmonic spectrum via a frequency shift was considered. The time-frequency representation by means of a wavelet transform of the induced dipole acceleration provided a good insight into the distorted effects from the nonlinear processes in high harmonic emission. The effect of the chirped laser pulse on the production of broadband amplitude was justified in this model. By adjusting the optimal laser parameters to an input intensity of 2.5 × 10¹⁴ W cm^-2, an isolated 68 as pulse was generated.

Sequential and direct ionic excitation in the strong-field ionization of 1-butene molecules

We study the Strong-Field Ionization (SFI) of the hydrocarbon 1-butene as a function of wavelength using photoion-photoelectron covariance and coincidence spectroscopy. We observe a striking transition in the fragment-associated photoelectron spectra: from a single Above Threshold Ionization (ATI) progression for photon energies less than the cation D0-D1 gap to two ATI progressions for a photon energy greater than this gap. For the first case, electronically excited cations are created by SFI populating the ground cationic state D0, followed by sequential post-ionization excitation. For the second case, direct sub-cycle SFI to the D1 excited cation state contributes significantly. Our experiments access ionization dynamics in a regime where strong-field and resonance-enhanced processes can interplay.

Rotational Echoes: Rephasing of Centrifugal Distortion in Laser-Induced Molecular Alignment

We study and demonstrate the rephasing property of the echo response in a multilevel rotational system of iodomethane via long time-resolved optical birefringence measurements. The strong centrifugal distortion of iodomethane is utilized as a dephasing mechanism imprinted on the echo signal and is shown to rephase throughout its evolution. The dependence of the echo signal amplitude on the driving pulses' intensities is theoretically and experimentally explored. The analogy to Hahn's spin echoes is discussed, and a quantum-mechanical version of Hahn's track runners is provided for the case of multilevel rotational system.

Excited state wavepacket dynamics in NO2 probed by strong-field ionization

We present an experimental femtosecond time-resolved study of the 399 nm excited state dynamics of nitrogen dioxide using channel-resolved above threshold ionization (CRATI) as the probe process. This method relies on photoelectron-photoion coincidence and covariance to correlate the strong-field photoelectron spectrum with ionic fragments, which label the channel. In all ionization channels observed, we report apparent oscillations in the ion and photoelectron yields as a function of pump-probe delay. Further, we observe the presence of a persistent, time-invariant above threshold ionization comb in the photoelectron spectra associated with most ionization channels at long time delays. These observations are interpreted in terms of single-pump-photon excitation to the first excited electronic X̃ ²A₁ state and multi-pump-photon excitations to higher-lying states. The short time delay (<100 fs) dynamics in the fragment channels show multi-photon pump signatures of higher-lying neutral state dynamics, in data sets recorded with higher pump intensities. As expected for pumping NO₂ at 399 nm, non-adiabatic coupling was seen to rapidly re-populate the ground state following excitation to the first excited electronic state, within 200 fs. Subsequent intramolecular vibrational energy redistribution results in the spreading of the ground state vibrational wavepacket into the asymmetric stretch coordinate, allowing the wavepacket to explore nuclear geometries in the asymptotic region of the ground state potential energy surface. Signatures of the vibrationally "hot" ground state wavepacket were observed in the CRATI spectra at longer time delays. This study highlights the complex and sometimes competing phenomena that can arise in strong-field ionization probing of excited state molecular dynamics.

The importance of Rydberg orbitals in dissociative ionization of small hydrocarbon molecules in intense laser fields

Much of our intuition about strong-field processes is built upon studies of diatomic molecules, which typically have electronic states that are relatively well separated in energy. In polyatomic molecules, however, the electronic states are closer together, leading to more complex interactions. A combined experimental and theoretical investigation of strong-field ionization followed by hydrogen elimination in the hydrocarbon series C₂D₂, C₂D₄ and C₂D₆ reveals that the photofragment angular distributions can only be understood when the field-dressed orbitals rather than the field-free orbitals are considered. Our measured angular distributions and intensity dependence show that these field-dressed orbitals can have strong Rydberg character for certain orientations of the molecule relative to the laser polarization and that they may contribute significantly to the hydrogen elimination dissociative ionization yield. These findings suggest that Rydberg contributions to field-dressed orbitals should be routinely considered when studying polyatomic molecules in intense laser fields.

Frequency analysis of the laser driven nonlinear dynamics of HCN

We study the vibrational dynamics of a model for the HCN molecule in the presence of a monochromatic laser field. The variation of the structural behavior of the system as a function of the laser frequency is analyzed in detail using the smaller alignment index, frequency maps, and diffusion coefficients. It is observed that the ergodicity of the system depends on the frequency of the excitation field, especially in its transitions from and into chaos. This provides a roadmap for the possibility of bond excitation and dissociation in this molecule.

Molecular frame photoemission by a comb of elliptical high-order harmonics: a sensitive probe of both photodynamics and harmonic complete polarization state

Due to the intimate anisotropic interaction between an XUV light field and a molecule resulting in photoionization (PI), molecular frame photoelectron angular distributions (MFPADs) are most sensitive probes of both electronic/nuclear dynamics and the polarization state of the ionizing light field. Consequently, they encode the complex dipole matrix elements describing the dynamics of the PI transition, as well as the three normalized Stokes parameters s₁, s₂, s₃ characterizing the complete polarization state of the light, operating as molecular polarimetry. The remarkable development of advanced light sources delivering attosecond XUV pulses opens the perspective to visualize the primary steps of photochemical dynamics in time-resolved studies, at the natural attosecond to few femtosecond time-scales of electron dynamics and fast nuclear motion. It is thus timely to investigate the feasibility of measurement of MFPADs when PI is induced e.g., by an attosecond pulse train (APT) corresponding to a comb of discrete high-order harmonics. In the work presented here, we report MFPAD studies based on coincident electron-ion 3D momentum imaging in the context of ultrafast molecular dynamics investigated at the PLFA facility (CEA-SLIC), with two perspectives: (i) using APTs generated in atoms/molecules as a source for MFPAD-resolved PI studies, and (ii) taking advantage of molecular polarimetry to perform a complete polarization analysis of the harmonic emission of molecules, a major challenge of high harmonic spectroscopy. Recent results illustrating both aspects are reported for APTs generated in unaligned SF₆ molecules by an elliptically polarized infrared driving field. The observed fingerprints of the elliptically polarized harmonics include the first direct determination of the complete s₁, s₂, s₃ Stokes vector, equivalent to (ψ, ε, P), the orientation and the signed ellipticity of the polarization ellipse, and the degree of polarization P. They are compared to so far incomplete results of XUV optical polarimetry. We finally discuss the comparison between the outcomes of photoionization and high harmonic spectroscopy for the description of molecular photodynamics.

Role of Excited States In High-order Harmonic Generation

We investigate the role of excited states in high-order harmonic generation by studying the spectral, spatial, and temporal characteristics of the radiation produced near the ionization threshold of argon by few-cycle laser pulses. We show that the population of excited states can lead either to direct extreme ultraviolet emission through free induction decay or to the generation of high-order harmonics through ionization from these states and recombination to the ground state. By using the attosecond lighthouse technique, we demonstrate that the high-harmonic emission from excited states is temporally delayed by a few femtoseconds compared to the usual harmonics, leading to a strong nonadiabatic spectral redshift.

Bicircular High-Harmonic Spectroscopy Reveals Dynamical Symmetries of Atoms and Molecules

We introduce bicircular high-harmonic spectroscopy as a new method to probe dynamical symmetries of atoms and molecules and their evolution in time. Our approach is based on combining a circularly polarized femtosecond fundamental field of frequency ω with its counterrotating second harmonic 2ω. We demonstrate the ability of bicircular high-harmonic spectroscopy to characterize the orbital angular momentum symmetry of atomic orbitals. We further show that breaking the threefold rotational symmetry of the generating medium-at the level of either the ensemble or that of a single molecule-results in the emission of the otherwise parity-forbidden frequencies 3qω  (q∈N), which provide a background-free probe of dynamical molecular symmetries.

Role of tunnel ionization in high harmonic generation from substituted benzenes

We theoretically study high-harmonic generation in toluene, ortho-xylene and fluorobenzene driven by a 1.8 μm ultrashort pulse. We find that the chemical substitutions have a strong influence on the amplitude and phase of the emission from the highest occupied molecular orbital, despite having a small influence on the orbital itself. We show that this influence is due to the tunnel ionization step, which depends critically on the sign and amplitude of the asymptotic part of the wave function. We discuss how these effects would manifest in phase-sensitive high-harmonic generation spectroscopy experiments.

Multidimensional high harmonic spectroscopy of polyatomic molecules: detecting sub-cycle laser-driven hole dynamics upon ionization in strong mid-IR laser fields

High harmonic generation (HHG) spectroscopy has opened up a new frontier in ultrafast science, where electronic dynamics can be measured on an attosecond time scale. The strong laser field that triggers the high harmonic response also opens multiple quantum pathways for multielectron dynamics in molecules, resulting in a complex process of multielectron rearrangement during ionization. Using combined experimental and theoretical approaches, we show how multi-dimensional HHG spectroscopy can be used to detect and follow electronic dynamics of core rearrangement on sub-laser cycle time scales. We detect the signatures of laser-driven hole dynamics upon ionization and reconstruct the relative phases and amplitudes for relevant ionization channels in a CO₂ molecule on a sub-cycle time scale. Reconstruction of channel-resolved complex ionization amplitudes on attosecond time scales has been a long-standing goal of high harmonic spectroscopy. Our study brings us one step closer to fulfilling this initial promise and developing robust schemes for sub-femtosecond imaging of multielectron rearrangement in complex molecular systems.

Using high harmonic radiation to reveal the ultrafast dynamics of radiosensitiser molecules

5-Fluorouracil (5FU) is a radiosensitiser molecule routinely used in combined chemo- and radio-therapies to enhance and localize cancer treatments. We have employed ultra-short XUV pulses produced by high harmonic generation (HHG) as a pump pulse to study the dynamics underlying the photo-stability and the radiation damage of this molecule. This work shows that it is possible to resolve individual dynamics even when using unselected HH. By comparing the results with those obtained in the multiphoton absorption at 400 nm, we were able to identify the frequencies of the HH comb relevant to the recorded dynamics: HH5 and HH3. The latter excites a high-lying Rydberg state interacting with a valence state and its dynamics is revealed by a 30 fs decay signal in the parent ion transient. Our results suggest that the same photoprotection mechanisms as those conferring photostability to the neutral nucleobases and to the DNA appear to be activated: HH5 excites the molecule to a state around 10.5 eV that undergoes an ultrafast relaxation on a timescale of 30 fs due to nonadiabatic interactions. This is followed sequentially by a 2.3 ps internal conversion as revealed by the dynamics observed for another fragment ion. These dynamics are extracted from the fragment ion signals. Proton or hydrogen transfer processes are required for the formation of three fragments and we speculate that the time scale of one of the processes is revealed by a H⁺ transient signal.

Combined high-harmonic interferometries for vectorial spectroscopy

We present a new method to characterize transverse vectorial light produced by high-harmonic generation (HHG). The incoherent sum of the two components of the electric field is measured using a bi-dimensional transient grating while one of the components is simultaneously characterized using two-source interferometry. The combination of these two interferometric setups enables the amplitude and phase measurement of the two vectorial components of the extreme ultraviolet radiation. We demonstrate the potential of this technique in the case of HHG in aligned nitrogen, revealing the vectorial properties of harmonics 9-17 of a Ti:sapphire laser.

Discriminating between the Role of Phase Matching and that of the Single-Atom Response in Resonance Plasma-Plume High-Order Harmonic Generation

Resonance enhancement of high-order harmonic generation has recently been found in the interaction of intense ultrashort laser pulses with laser ablated plasma plumes. It is a promising route towards the production of an intense and coherent extreme ultraviolet radiation source. However, the mechanism of this resonance enhancement is still not clear. There are two possible explanations; one relies on a better recombination cross section through an autoionization state in the single-atom response. The other relies on improved phase matching conditions around the resonance. Here, we try to discriminate between these two conjectures by measuring coherence lengths of the harmonics, both on resonance and off resonance. Our findings support the single-atom response hypothesis.

Joint Measurements of Terahertz Wave Generation and High-Harmonic Generation from Aligned Nitrogen Molecules Reveal Angle-Resolved Molecular Structures

We report the synchronized measurements of terahertz wave generation and high-harmonic generation from aligned nitrogen molecules in dual-color laser fields. Both yields are found to be alignment dependent, showing the importance of molecular structures in the generation processes. By calibrating the angular ionization rates with the terahertz yields, we present a new way of retrieving the angular differential photoionization cross section (PICS) from the harmonic signals which avoids specific model calculations or separate measurements of the alignment-dependent ionization rates. The measured PICS is found to be consistent with theoretical predications, although some discrepancies exist. This all-optical method provides a new alternative for investigating molecular structures.

Dynamic Exchange in the Strong Field Ionization of Molecules

We show that dynamic exchange is a dominant effect in strong field ionization of molecules. In CO(2) it fixes the peak ionization yield at the experimentally observed angle of 45° between polarization direction and the molecular axis. For O(2) it changes the angle of peak emission and for N(2) the alignment dependence of yields is modified by up to a factor of 2. The effect appears on the Hartree-Fock level as well as in full ab initio solutions of the Schrödinger equation.

Polarization properties of below-threshold harmonics from aligned molecules H2+ in linearly polarized laser fields

We investigate the polarization properties of below-threshold harmonics from aligned molecules in linearly polarized laser fields numerically and analytically. We focus on lower-order harmonics (LOHs). Our simulations show that the ellipticity of below-threshold LOHs depends strongly on the orientation angle and differs significantly for different harmonic orders. Our analysis reveals that this LOH ellipticity is closely associated with resonance effects and the axis symmetry of the molecule. These results shed light on the complex generation mechanism of below-threshold harmonics from aligned molecules.