Electron Correlation as the Driving Force for Charge Transfer: Charge Migration Following Ionization in N-Methyl Acetamide

A theoretical prediction for generating isolated attosecond pulse in water window utilizing instantaneous frequency change of two-color driving laser pulse

This work reports on the theoretical generation of isolated soft-X ray 106 attosecond pulse within the water window spectral region, through interacting a chirped two-color femtosecond laser pulse with a hydrogen atom. To construct this pulse, a chirped 3.5 optical cycle (9.33 fs) laser pulse with a wavelength of 800 nm and an intensity of [Formula: see text] is used as the main field. A 37.32 fs laser pulse with a wavelength of 1600 nm and intensity of [Formula: see text], one-quarter of the main field intensity, is employed as the control field. A tangential hyperbolic function is used to impose chirp on the main field. The study simultaneously utilizes the lowest possible intensity and the highest possible pulse duration for both the main and the control field to reach the water window spectral region in high-order harmonic generation up to the 320th harmonic of the driving field.

Molecular Modes of Attosecond Charge Migration

First-principles calculations are employed to elucidate the modes of attosecond charge migration (CM) in halogenated hydrocarbon chains. We use constrained density functional theory (DFT) to emulate the creation of a localized hole on the halogen and follow the subsequent dynamics via time-dependent DFT. We find low-frequency CM modes (∼1  eV) that propagate across the molecule and study their dependence on length, bond order, and halogenation. We observe that the CM speed (∼4  Å/fs) is largely independent of molecule length, but is lower for triple-bonded versus double-bonded molecules. Additionally, as the halogen mass increases, the hole travels in a more particlelike manner as it moves across the molecule. These heuristics will be useful in identifying molecules and optimal CM detection methods for future experiments, especially for halogenated hydrocarbons which are promising targets for ionization-triggered CM.

Experimental Separation of Subcycle Ionization Bursts in Strong-Field Double Ionization of H_{2}

We report on the unambiguous observation of the subcycle ionization bursts in sequential strong-field double ionization of H_{2} and their disentanglement in molecular frame photoelectron angular distributions. This observation was made possible by the use of few-cycle laser pulses with a known carrier-envelope phase, in combination with multiparticle coincidence momentum imaging. The approach demonstrated here will allow sampling of the intramolecular electron dynamics and the investigation of charge-state-specific Coulomb distortions on emitted electrons in polyatomic molecules.

Competition between charge migration and charge transfer induced by nuclear motion following core ionization: Model systems and application to Li2. J Chem Phys 2019;151:124108. [PMID: 31575213 DOI: 10.1063/1.5117246]

Attosecond and femtosecond spectroscopies present opportunities for the control of chemical reaction dynamics and products, as well as for quantum information processing; we address the somewhat unique situation of core-ionization spectroscopy which, for dimeric chromophores, leads to strong valence charge localization and hence tightly paired potential-energy surfaces of very similar shape. Application is made to the quantum dynamics of core-ionized Li₂ ⁺. This system is chosen as Li₂ is the simplest stable molecule facilitating both core ionization and valence ionization. First, the quantum dynamics of some model surfaces are considered, with the surprising result that subtle differences in shape between core-ionization paired surfaces can lead to dramatic differences in the interplay between electronic charge migration and charge transfer induced by nuclear motion. Then, equation-of-motion coupled-cluster calculations are applied to determine potential-energy surfaces for 8 core-excited state pairs, calculations believed to be the first of their type for other than the lowest-energy core-ionized molecular pair. While known results for the lowest-energy pair suggest that Li₂ ⁺ is unsuitable for studying charge migration, higher-energy pairs are predicted to yield results showing competition between charge migration and charge transfer. Central is a focus on the application of Hush's 1975 theory for core-ionized X-ray photoelectron spectroscopy to understand the shapes of the potential-energy surfaces and hence predict key features of charge migration.

Ultrafast Nonadiabatic Cascade and Subsequent Photofragmentation of Extreme Ultraviolet Excited Caffeine Molecule

Ultrafast XUV chemistry is offering new opportunities to decipher the complex dynamics taking place in highly excited molecular states and thus better understand fundamental natural phenomena as molecule formation in interstellar media. We used ultrashort XUV light pulses to perform XUV pump-IR probe experiments in caffeine as a model of prebiotic molecule. We observed a 40 fs decay of excited cationic states. Guided by quantum calculations, this time scale is interpreted in terms of a nonadiabatic cascade through a large number of highly correlated states. This shows that the correlation driven nonadiabatic relaxation seems to be a general process for highly excited states, which might impact our understanding of molecular processing in interstellar media.

Attosecond Pump-Probe Spectroscopy of Charge Dynamics in Tryptophan

Attosecond pump-probe experiments performed in small molecules have allowed tracking charge dynamics in the natural time scale of electron motion. That this is also possible in biologically relevant molecules is still a matter of debate, because the large number of available nuclear degrees of freedom might destroy the coherent charge dynamics induced by the attosecond pulse. Here we investigate extreme ultraviolet-induced charge dynamics in the amino acid tryptophan. We find that, although nuclear motion and nonadiabatic effects introduce some decoherence in the moving electron wave packet, these do not significantly modify the coherence induced by the attosecond pulse during the early stages of the dynamics, at least for molecules in their equilibrium geometry. Our conclusions are based on elaborate theoretical calculations and the experimental observation of sub-4 fs dynamics, which can only be reasonably assigned to electronic motion. Hence, attosecond pump-probe spectroscopy appears as a promising approach to induce and image charge dynamics in complex molecules.

Ultrafast Charge Migration in XUV Photoexcited Phenylalanine: A First-Principles Study Based on Real-Time Nonequilibrium Green's Functions

The early-stage density oscillations of the electronic charge in molecules irradiated by an attosecond XUV pulse takes place on femto- or subfemtosecond time scales. This ultrafast charge migration process is a central topic in attoscience because it dictates the relaxation pathways of the molecular structure. A predictive quantum theory of ultrafast charge migration should incorporate the atomistic details of the molecule, electronic correlations, and the multitude of ionization channels activated by the broad-bandwidth XUV pulse. We propose a first-principles nonequilibrium Green's function method fulfilling all three requirements and apply it to a recent experiment on the photoexcited phenylalanine amino acid. Our results show that dynamical correlations are necessary for a quantitative overall agreement with the experimental data. In particular, we are able to capture the transient oscillations at frequencies 0.15 and 0.30 PHz in the hole density of the amine group as well as their suppression and the concomitant development of a new oscillation at frequency 0.25 PHz after ∼14 fs.

Photoinduced Ultrafast Charge Transfer and Charge Migration in Small Gold Clusters Passivated by a Chromophoric Ligand

Because the development of attopulses, charge migration induced by short optical pulses has been extensively investigated. We report a computational purely electronic dynamical study of ultrafast few femtoseconds (fs) charge transfer and charge migration in realistic passivated stoichiometric Au₁₁ and Au₂₀ gold nanoclusters functionalized by a bipyridine ligand. We show that a net significant amount of electronic charge (0.1 to 0.4 |e| where |e| is the electron charge) is permanently transferred from the bipyridine chromophore to the gold cluster during the short 5-6 fs UV-vis strong pulse. This electron transfer to the metallic core is induced by the optical excitation of electronic states with a partial charge transfer character involving the chromophore before the onset of nuclei motion. In addition, the photoexcitation by the strong fs pulse builds a nonequilibrium electronic density that beats between the chromophore and the metallic core around the average of the transferred value. Modular systems made of a donor chromophore that can be photoexcited in the UV-vis range coupled to an efficient acceptor that could trap the charge are of interest for applications to nanodevices. Our study provides understanding on the very early, purely electronic dynamics built by the fs optical excitation and the initial charge separation step.

Pumping and probing vibrational modulated coupled electronic coherence in HCN using short UV fs laser pulses: a 2D quantum nuclear dynamical study

The coupled electronic-nuclear coherent dynamics induced by a short strong VUV fs pulse in the low excited electronic states of HCN is probed by transient absorption spectroscopy with a second weaker fs UV pulse. The nuclear time-dependent Schrodinger equation is solved on a 2D nuclear grid with several electronic states with a Hamiltonian including the dipole coupling to the pump and the probe electric fields. The two internal nuclear coordinates describe the motion of the light H atom. There is a band of several excited electronic states at about 8 eV above the ground state (GS) that is transiently accessed by the pump pulse. We tailored the pump so as to selectively populate the lowest 1A'' electronic state thereby the pulse creates an electronic coherence with the GS. Our simulations show that this electronic coherence is modulated by the nuclear motion and persists all the way to dissociation on the 1A'' state. Transient absorption spectra computed as a function of the delay time between the pump and the probe pulses provide a detailed probe of the electronic amplitude and its phase, as well as of the modulation of the electronic coherence by the nuclear motion, both bound and dissociative.

Nuclear Motion Driven Ultrafast Photodissociative Charge Transfer of the PENNA Cation: An Experimental and Computational Study

Ultrafast nuclear driven charge transfer prior to dissociation is an important process in modular systems as was demonstrated experimentally in the bifunctional molecule 2-phenylethyl-N,N-dimethylamine (PENNA) in work by Lehr et al. ( J. Phys. Chem. A 2005 , 109 , 8074 ). The ultrafast dynamics of PENNA photoexcited to the three lowest electronic states of the cation (D₀, D₁, and D₂) was studied using quantum chemistry and surface hoping. We show that a conical intersection, localized in the Franck-Condon region, between the D₀ and the D₁ states, leads to an ultrafast charge transfer, computed here to be on a time scale of 65 fs, between the phenyl and the amine charged subunits. On the D₀ ground state, the dissociation proceeds on the 60 ps time scale through a 19 kcal/mol late barrier. The computed kinetic energy release is in good agreement with a new experimental measurement of PENNA ionization by an 800 nm 30 fs intense laser pulse.

Ultrafast Spin-State Dynamics in Transition-Metal Complexes Triggered by Soft-X-Ray Light

Recent advances in attosecond physics provide access to the correlated motion of valence and core electrons on their intrinsic timescales. For valence excitations, processes related to the electron spin are usually driven by nuclear motion. For core-excited states, where the core hole has a nonzero angular momentum, spin-orbit coupling is strong enough to drive spin flips on a much shorter time scale. Here, unprecedented short spin crossover is demonstrated for L-edge (2p→3d) excited states of a prototypical Fe(II) complex. It occurs on a time scale, which is faster than the core-hole lifetime of about 4 fs and can be manipulated by the excitation conditions. A detailed analysis of such phenomena will help to gain a fundamental understanding of spin-crossover processes and establish the basis for their control by light.

Decoherence, control and attosecond probing of XUV-induced charge migration in biomolecules. A theoretical outlook

The sudden ionization of a molecule by an attosecond pulse is followed by charge redistribution on a time scale from a few femtoseconds down to hundreds of attoseconds. This ultrafast redistribution is the result of the coherent superposition of electronic continua associated with the ionization thresholds that are reached by the broadband attosecond pulse. Thus, a correct theoretical description of the time evolution of the ensuing wave packet requires the knowledge of the actual ionization amplitudes associated with all open ionization channels, a real challenge for large and medium-size molecules. Recently, the first calculation of this kind has come to light, allowing for interpretation of ultrafast electron dynamics observed in attosecond pump–probe experiments performed on the amino acid phenylalanine [Calegari et al., Science 2014, 346, 336]. However, as in most previous theoretical works, the interpretation was based on various simplifying assumptions, namely, the ionized electron was not included in the description of the cation dynamics, the nuclei were fixed at their initial position during the hole migration process, and the effect of the IR probe pulse was ignored. Here we go a step further and discuss the consequences of including these effects in the photoionization of the glycine molecule. We show that (i) the ionized electron does not affect hole dynamics beyond the first femtosecond, and (ii) nuclear dynamics has only a significant effect after approximately 8 fs, but does not destroy the coherent motion of the electronic wave packet during at least few additional tens of fs. As a first step towards understanding the role of the probe pulse, we have considered an XUV probe pulse, instead of a strong IR one, and show that such an XUV probe does not introduce significant distortions in the pump-induced dynamics, suggesting that pump–probe strategies are suitable for imaging and manipulating charge migration in complex molecules. Furthermore, we show that hole dynamics can be changed by shaping the attosecond pump pulse, thus opening the door to the control of charge dynamics in biomolecules.

Attosecond Hole Migration in Benzene Molecules Surviving Nuclear Motion

Hole migration is a fascinating process driven by electron correlation, in which purely electronic dynamics occur on a very short time scale in complex ionized molecules, prior to the onset of nuclear motion. However, it is expected that due to coupling to the nuclear dynamics, these oscillations will be rapidly damped and smeared out, which makes experimental observation of the hole migration process rather difficult. In this Letter, we demonstrate that the instantaneous ionization of benzene molecules initiates an ultrafast hole migration characterized by a periodic breathing of the hole density between the carbon ring and surrounding hydrogen atoms on a subfemtosecond time scale. We show that these oscillations survive the dephasing introduced by the nuclear motion for a long enough time to allow their observation. We argue that this offers an ideal benchmark for studying the influence of hole migration on molecular reactivity.

Single-photon laser-enabled auger spectroscopy for measuring attosecond electron-hole dynamics

We propose and simulate a new type of attosecond time-resolved spectroscopy of electron-hole dynamics, applicable particularly to ultrafast hole migration. Attosecond ionization in the inner-valence region is followed by a vacuum ultraviolet probe inducing single-photon laser-enabled Auger decay, a one-photon-two-electron transition filling the inner-valence vacancy. The double ionization probability as a function of the attosecond pump-vacuum ultraviolet probe delay captures efficiently the ultrafast inner-valence hole dynamics. Detailed ab initio calculations are presented for inner-valence hole migration in glycine.

Imaging orbitals with attosecond and Ångström resolutions: toward attochemistry?

The recently developed attosecond light sources make the investigation of ultrafast processes in matter possible with unprecedented time resolution. It has been proposed that the very mechanism underlying the attosecond emission allows the imaging of valence orbitals with Ångström space resolution. This controversial idea together with the possibility of combining attosecond and Ångström resolutions in the same measurements has become a hot topic in strong-field science. Indeed, this could provide a new way to image the evolution of the molecular electron cloud during, e.g. a chemical reaction in 'real time'. Here we review both experimental and theoretical challenges raised by the implementation of these prospects. In particular, we show how the valence orbital structure is encoded in the spectral phase of the recombination dipole moment calculated for Coulomb scattering states, which allows a tomographic reconstruction of the orbital using first-order corrections to the plane-wave approach. The possibility of disentangling multi-channel contributions to the attosecond emission is discussed as well as the necessary compromise between the temporal and spatial resolutions.

Length effects in VUV photofragmentation of protonated peptides

We have studied photoionization of protonated synthetic peptides YG(n)F (n = 0, 1, 3, 5, 10). Photon energies ranging from 8 to 30 eV were used. For YG(n)F peptides up to n = 5 small fragment ions related to the sidechains of the aromatic terminal amino acids Y and F dominate the fragmentation patterns. The associated yields scale with total photoabsorption cross section, demonstrating efficient hole migration towards the terminal amino acids upon photoionization of the peptide backbone. For n = 10 the side-chain loss channel is quenched and a series of large dications appear.

Radiation generated by the ultrafast migration of a positive charge following the ionization of a molecular system

Electronic many-body effects alone can be the driving force for an ultrafast migration of a positive charge created upon ionization of molecular systems. Here we show that this purely electronic phenomenon generates a characteristic IR radiation. The situation when the initial ionic wave packet is produced by a sudden removal of an electron is also studied. It is shown that in this case a much stronger UV emission is generated. This emission appears as an ultrafast response of the remaining electrons to the perturbation caused by the sudden ionization and as such is a universal phenomenon to be expected in every multielectron system.

Probing Ultrafast Purely Electronic Charge Migration in Small Peptides

A pump–probe experiment that can examine a pure charge migration on a time scale short compared to the onset of nuclear motion is discussed. The mass spectrometric studies of Schlag et al. suggest that short peptide terminated by an aromatic amino acid are particularly suitable test compounds. The pump pulse needs to ionize the molecule on a time scale short compared to the period of the electronic motion, typically sub-fs. However, ionization occurs preferentially when the electrical field of the light is maximal so that the duration of the pulse envelope can be somewhat longer. Detection by photoelectron spectrometry of the peptide cation, to produce a dication, is shown to be able to probe the electronic rearrangement.