Vibronic Correlations in Molecular Strong-Field Dynamics.

We investigate the ultrafast vibronic dynamics triggered by intense femtosecond infrared pulses in small molecules. Our study is based on numerical simulations performed with 2D model molecules and analyzed in the perspective of the renowned Lochfrass and bond-softening models. We give a new interpretation of the observed nuclear wave packet dynamics with a focus on the phase of the bond oscillations. Our simulations also reveal intricate features in the field-induced nuclear motion that are not accounted for by existing models. Our analyses assign these features to strong dynamical correlations between the active electron and the nuclei, which significantly depend on the carrier envelope phase of the pulse, even for relatively "long" pulses, which should make them experimentally observable.

Unraveling the variational breakdown of core valence separation calculations: Diagnostic and cure to the over relaxation error of double core hole states.

The core valence separation (CVS) approximation is the most employed strategy to prevent the variational collapse of standard wave function optimization when attempting to compute electronic states bearing one or more electronic vacancies in core orbitals. Here, we explore the spurious consequences of this approximation on the properties of the computed core hole states. We especially focus on the less studied case of double core hole (DCH) states, whose spectroscopic interest has recently been rapidly growing. We show that the CVS error leads to a systematic underestimation of DCH energies, a property in stark contrast with the case of single core hole states. We highlight that the CVS error can then be interpreted as an over relaxation effect and design a new correction strategy adapted to these specificities.

Alternative Pathway to Double-Core-Hole States.

Excited double-core-hole states of isolated water molecules resulting from the sequential absorption of two x-ray photons have been investigated. These states are formed through an alternative pathway, where the initial step of core ionization is accompanied by the shake-up of a valence electron, leading to the same final states as in the core-ionization followed by core-excitation pathway. The capability of the x-ray free-electron laser to deliver very intense, very short, and tunable light pulses is fully exploited to identify the two different pathways.

Anisotropic dynamics of two-photon ionization: An attosecond movie of photoemission.

Imaging in real time the complete dynamics of a process as fundamental as photoemission has long been out of reach because of the difficulty of combining attosecond temporal resolution with fine spectral and angular resolutions. Here, we achieve full decoding of the intricate angle-dependent dynamics of a photoemission process in helium, spectrally and anisotropically structured by two-photon transitions through intermediate bound states. Using spectrally and angularly resolved attosecond electron interferometry, we characterize the complex-valued transition probability amplitude toward the photoelectron quantum state. This allows reconstructing in space, time, and energy the complete formation of the photoionized wave packet.

Specific chemical bond relaxation unraveled by analysis of shake-up satellites in the oxygen single site double core hole spectrum of CO2.

We recently developed [A. Ferté, et al., J. Phys. Chem. Lett., 2020, 11, 4359] a method to compute single site double core hole (ssDCH or K-2) spectra. We refer to that method as NOTA+CIPSI. In the present paper this method is applied to the O K-2 spectrum of the CO2 molecule, and we use this as an example to discuss in detail its convergence properties. Using this approach, theoretical spectra in excellent agreement with the experimental one are obtained. Thanks to a thorough interpretation of the shake-up states responsible for the main satellite peaks and through comparison with the O K-2 spectrum of CO, we can highlight the clear signature of the two non-equivalent carbon oxygen bonds in the oxygen ssDCH CO2 dication.

Advanced Computation Method for Double Core Hole Spectra: Insight into the Nature of Intense Shake-up Satellites.

Double core hole spectroscopy is an ideal framework for investigating photoionization shake-up satellites. Their important intensity in a single site double core hole (ssDCH) spectrum allows the exploration of the subtle mix of relaxation and correlation effects associated with the inherent multielectronic character of the shake-up process. We present a high-accuracy computation method for single photon double core-shell photoelectron spectra that combines a selected configuration interaction procedure with the use of non-orthogonal molecular orbitals to obtain unbiased binding energy and intensity. This strategy leads to the oxygen ssDCH spectrum of the CO molecule that is in excellent agreement with the experimental result. Through a combined wave function and density analysis, we highlight that the most intense shake-up satellites are characterized by an electronic reorganization that opposes the core hole-induced relaxation.

Optimal Basis Set for Electron Dynamics in Strong Laser Fields: The case of Molecular Ion H2.

A clear understanding of the mechanisms that control the electron dynamics in a strong laser field is still a challenge that requires interpretation by advanced theory. Development of accurate theoretical and computational methods, able to provide a precise treatment of the fundamental processes generated in the strong field regime, is therefore crucial. A central aspect is the choice of the basis for the wave function expansion. Accuracy in describing multiphoton processes is strictly related to the intrinsic properties of the basis, such as numerical convergence, computational cost, and representation of the continuum. By explicitly solving the 1D and 3D time-dependent Schrödinger equation for H2+ in the presence of an intense electric field, we explore the numerical performance of using a real-space grid, a B-spline basis, and a Gaussian basis (improved by optimal Gaussian functions for the continuum). We analyze the performance of the three bases for high-harmonic generation and above-threshold ionization for H2+. In particular, for high-harmonic generation, the capability of the basis to reproduce the two-center interference and the hyper-Raman phenomena is investigated.

Laser-induced blurring of molecular structure information in high harmonic spectroscopy.

High harmonic spectroscopy gives access to molecular structure with Angström resolution. Such information is encoded in the destructive interferences occurring between the harmonic emissions from the different parts of the molecule. By solving the time-dependent Schrödinger equation, either numerically or with the molecular strong-field approximation, we show that the electron dynamics in the emission process generally results in a strong spectral smoothing of the interferences, blurring the structural information. However we identify specific generation conditions where they are unaffected. These findings have important consequences for molecular imaging and orbital tomography using high harmonic spectroscopy.

Self-Probing Spectroscopy of the SF6 Molecule: A Study of the Spectral Amplitude and Phase of the High Harmonic Emission.

We present characterizations of the attosecond pulse train produced in the high harmonic generation (HHG) from SF6 molecules irradiated by a strong pulsed laser field at 800 nm. At harmonic order 17, we observe a minimum in the amplitude of the emitted spectrum and a corresponding distortion in the phase. Our experimental results are compared to two models: a multicenter interference model focused on the effect of the structure of the SF6 molecule in HHG and a model focused on the interferences between multiple ionization channels in HHG. We find that the experimental results agree very well with the multiple ionization channels model, illustrating that HHG in molecules can be very complex and that it provides insights of the intramolecular electron dynamics during the interaction process.

Excited state dynamics in SO2. III. An ab initio quantum study of single- and multi-photon ionization.

We present an ab initio quantum study of the photoelectron spectra of sulfur dioxide, based on wavepacket propagations on manifolds of ionic, and excited/Rydberg states. We obtain excellent agreement for two different cases. First, the one photon ionization case where we can reproduce all details of the experimental spectrum and demonstrate the influence of the conical intersection between two of the ionic states. Then the multiphoton ionization regime, in which the dynamics of the wave packet on the two lowest singlet states is directly mapped in the spectra via a pump-probe scheme, as proposed in the experimental companion paper [I. Wilkinson et al., J. Chem. Phys. 140, 204301 (2014)].

Direct observation of spin-forbidden transitions through the use of suitably polarized light.

The study of excited triplet states of a molecular system is a difficult task because accessing them involves forbidden transitions from the singlet ground state. Nevertheless, absorption spectra of many molecules present, at low energies, the weak fingerprint of these triplet states. At higher energies this information is usually masked by the intense signal of the singlet states. Here we show, for the specific case of the sulphur dioxide molecule, that the combined use of polarized light and molecular alignment can enhance the triplet part of the spectrum, even making it the only absorption process.

Theoretical study of Raman chirped adiabatic passage by X-ray absorption spectroscopy: highly excited electronic states and rotational effects.

Raman Chirped Adiabatic Passage (RCAP) is an efficient method to climb the vibrational ladder of molecules. It was shown on the example of fixed-in-space HCl molecule that selective vibrational excitation can thus be achieved by RCAP and that population transfer can be followed by X-ray Photoelectron spectroscopy [S. Engin, N. Sisourat, P. Selles, R. Taïeb, and S. Carniato, Chem. Phys. Lett. 535, 192-195 (2012)]. Here, in a more detailed analysis of the process, we investigate the effects of highly excited electronic states and of molecular rotation on the efficiency of RCAP. Furthermore, we propose an alternative spectroscopic way to monitor the transfer by means of X-ray absorption spectra.

Communication: Theoretical prediction of the importance of the (3)B2 state in the dynamics of sulfur dioxide.

Even though the sulfur dioxide molecule has been extensively studied over the last decades, its photo-excitation dynamics is still unclear, due to its complexity, combining conical intersections, and spin-orbit coupling between a manifold of states. We present a comprehensive ab initio study of the intersystem crossing of the molecule in the low energy domain, based on a wave-packet propagation on the manifold of the lowest singlet and triplet states. Furthermore, spin-orbit couplings are evaluated on a geometry-dependent grid, and diabatized along with the different conical intersections. Our results show for the first time the primordial role of the triplet (3)B2 state and furthermore predict novel interference patterns due to the different intersystem crossing channels induced by the spin-orbit couplings and the shapes of the different potential energy surfaces. These give new insight into the coupled singlet-triplet dynamics of SO2.

Raman chirped adiabatic passage probed by X-ray spectroscopy.

We report a theoretical study of the selective vibrational excitation of a HCl molecule achieved by Raman chirped adiabatic passage (RCAP) and probed by X-ray photoelectron spectroscopy (XPS). It is demonstrated that HCl can be prepared in any vibrational level up to ν = 9 with nearly complete population inversion. We explore the effects of both the rotation of the molecule and of the temperature on the RCAP process, which is proved to be very robust. Furthermore, we emphasize that XPS spectra at the chlorine K-shell threshold show characteristic signatures of the populated vibrational level, allowing us to follow the RCAP process.

Ab initio quantum study of the photodynamics and absorption spectrum for the coupled 1(1)A2 and 1(1)B1 states of SO2.

The nonadiabatic photoinduced dynamics occurring in the coupled 1(1)A(2) and 1(1)B(1) excited states of SO(2) is investigated using ab initio quantum dynamical methods. To this end, large scale calculations of the potential energy surfaces have been carried out at the multireference configuration interaction level. All vibrational degrees of freedom of the molecule are considered in the potential energy surface calculations and the quantum dynamical treatment. To deal with the symmetry-allowed conical intersection which occurs between the potential energy surfaces, we use the diabatic picture in the framework of regularized diabatic states. Wave-packet propagation on the coupled surfaces was performed and allowed to reproduce with good accuracy the complex absorption band observed experimentally in the 29,000-42,000 cm(-1) range. This provides a basis for a subsequent theoretical treatment of the high order harmonic spectra of SO(2).

Attosecond resolved electron release in two-color near-threshold photoionization of N2.

We have simulated two-color photoionization of N(2) by solving the time-dependent Schrödinger equation with a simple model accounting for the correlated vibronic dynamics of the molecule and of the ion N(2)(+). Our results, in very good agreement with recent experiments [Haessler et al., Phys. Rev. A 80, 011404 (2009)], show how a resonance embedded in the molecular continuum dramatically affects the phases of the two-photon transition amplitudes. In addition, we introduce a formal relation between these measurable phases and the photoelectron release time, opening the way to attosecond time-resolved measurements, equivalent to double-slit experiments in the time domain.

Low field laser ionization of argon clusters: the remarkable fragmentation dynamics of doubly ionized clusters.

We have investigated the fission following a Coulomb explosion in argon clusters (up to Ar800) irradiated by a femtosecond infrared laser with moderate intensity IL approximately 10(13) W cm(-2). We report the a priori surprising observation of well-defined velocity distributions of the ionized fragments Ar+n<50. This is interpreted by the formation of a valence shell excited charged ion, followed by relaxation, charge transfer by autoionizing collision at very short distance, and asymmetric fission.

N-K near-edge x-ray-absorption fine structures of acetonitrile in gas phase.

The dynamic processes of N(1s) core-hole excitation in gas-phase CH3CN molecule have been studied at both Hartree-Fock and hybrid density-functional theory levels. The vibrational structure is analyzed for fully optimized core-excited states. Frank-Condon factors are obtained using the linear coupling model for various potential surfaces. It is found that the vibrational profile of the N-K absorption can be largely described by a summation of two vibrational progressions: a structure-rich profile of nu(CN) stretching mode and a large envelope of congestioned vibrational levels related to the strong (-C-CN) terminal bending bond. Excellent agreement between theoretical and experimental spectra is obtained.

Strong field atomic ionization: origin of high-energy structures in photoelectron spectra.

Two distinct interpretations have been proposed to account for conspicuous enhancements of the ionization peaks in the high energy part of above-threshold ionization spectra. One of them ascribes the enhancement to a multiphoton resonance involving an excited state, while other analysis performed for zero-range model potential link it to "channel closings, " i.e., to the change in the number of photons needed to ionize the atom when the laser intensity increases. We report the results of model calculations that confirm the existence of a resonant process in atoms and shed light on why short-range potential models can mimic the experimental observations.

Direct and indirect pathways in strong field atomic ionization dynamics.

With the help of a suitably chosen momentum-space analysis, we study some of the basic mechanisms governing the physics of the processes occurring when atoms are submitted to intense infrared laser pulses, with peak intensities 10(14) W cm(-2)