Maia Magrakvelidze

Understanding the role of phase in chemical bond breaking with coincidence angular streaking

Electron motion in chemical bonds occurs on an attosecond timescale. This ultrafast motion can be driven by strong laser fields. Ultrashort asymmetric laser pulses are known to direct electrons to a certain direction. But do symmetric laser pulses destroy symmetry in breaking chemical bonds? Here we answer this question in the affirmative by employing a two-particle coincidence technique to investigate the ionization and fragmentation of H₂ by a long circularly polarized multicycle femtosecond laser pulse. Angular streaking and the coincidence detection of electrons and ions are employed to recover the phase of the electric field, at the instant of ionization and in the molecular frame, revealing a phase-dependent anisotropy in the angular distribution of H⁺ fragments. Our results show that electron localization and asymmetrical breaking of molecular bonds are ubiquitous, even in symmetric laser pulses. The technique we describe is robust and provides a powerful tool for ultrafast science.

Complementary imaging of the nuclear dynamics in laser-excited diatomic molecular ions in the time and frequency domains

Experimentally, the bound and dissociative nuclear dynamics in small molecular ions can be resolved in time by using intense ultrashort pump in combination with delayed probe laser pulses. We discuss the modelling of related pump–probe-delay-dependent fragment kinetic-energy-release (KER) spectra for the laser-induced dissociative ionization of selected diatomic molecules and show how the quantum-mechanical simulation of measured KER spectra—in both the time domain and as a function of the beat frequency between molecular vibrational levels—reveals dissociation pathways and the characteristics of initially occupied molecular potential curves.

Field-Free Orientation of CO Molecules by Femtosecond Two-Color Laser Fields

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

IR-assisted ionization of helium by attosecond extreme ultraviolet radiation

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

Coherence of Auger and inter-Coulombic decay processes in the photoionization of Ar@C60 versus Kr@C60

Abstract For the asymmetric spherical dimer of an endohedrally confined atom and a host fullerene, an innershell vacancy of either system can decay through the continuum of an outer electron hybridized between the systems. Such decays, viewed as coherent superpositions of the single-center Auger and two-center inter-Coulombic (ICD) amplitudes, are found to govern leading decay mechanisms in noble-gas endofullerenes, and are likely omnipresent in this class of nanomolecules. A comparison between resulting autoionizing resonances calculated in the photoionization of Ar@C60 and Kr@C60 exhibits details of the underlying processes. Graphical abstract

Attosecond time delays in the valence photoionization of xenon and iodine at energies degenerate with core emissions

Study of photoemission dynamics in real time using coherent pump-probe experiments is a key for exploring electron correlations in matter. Such experiments may involve the conventional streaking or the standard reconstruction of attosecond beating by interference of two-photon transitions (RABITT) measurements. Since the additional delay introduced by the probe pulse from the Coulomb-laser coupling can be estimated independently and subtracted from the measured result, the RABITT approach is consistent with the Wigner-Smith (WS) route to determine emission time that involves energy differential of the photoamplitude phase [1]. We compute these quantum phases and resulting WS delays in the photoionization of valence electrons of Xe and I atoms, using a scheme of time-dependent local density approximation (TDLDA) [2]. Analysis of the results with the goal to temporally access details of electron correlations has been carried out. To identify two spectral features in the valence emission as induced due to its dynamical correlation with the degenerate core (4d) channel, we plot the 5s cross sections of Xe and I in the top panel of Figure 1. Note these features in each of the curves as (i) a strong shape resonance at lower energies and (ii) a Cooper minimum (CM) at higher energies. The bottom panel shows the 5s time delays for the atoms in the same energy range. Strong negative delays near the resonance indicate accelerated emissions of the electron for both Xe and I. This corroborates to the general expectation of a collective-type resonance environment in which electrons feel transient repulsion as the emission process becomes favored [3]. In contrast, an opposite temporal trend is displayed near the CM region where the delays appear positive. This slower emergence near CM dovetails with the time-behavior earlier seen in the Ar emission near a spectral minimum [2]. Also note that for Xe our result closely agrees with the streaking measurement for the delay of 4d relative to 5s [4]. The work presents first results for an openshell atom like I, although modeled in a spherical frame. Robust similarities between Xe and I, we hope, will encourage attosecond measurements on these atoms.

Attosecond delay of xenon 4d photoionization at the giant resonance and Cooper minimum

A Kohn-Sham time-dependent local-density-functional scheme is utilized to predict attosecond time delays of xenon 4d photoionization that involves the 4d giant dipole resonance and Cooper minimum. The fundamental effect of electron correlations to uniquely determine the delay at both regions is demonstrated. In particular, for the giant dipole resonance, the delay underpins strong collective effect, emulating the recent prediction at C60 giant plasmon resonance [T. Barillot et al, Phys. Rev. A 91, 033413 (2015)]. For the Cooper minimum, a qualitative similarity with a photorecombination experiment near argon 3p minimum [S. B. Schoun et al, Phys. Rev. Lett. 112, 153001 (2014)] is found. The result should encourage attosecond measurements of Xe 4d photoemission.

First prediction of inter-Coulombic decay of C60 inner vacancies through the continuum of confined atoms

Considering the photoionization of Ar@ and Kr@ endofullerenes, the decay of innershell excitations through the outershell continuum of the confined atom via the inter-Coulombic decay (ICD) pathway is detailed. Excitations to atom- hybrid states, when these states exist, can induce coherence between ICD and electron-transfer mediated decay (ETMD). This should be the dominant above-threshold decay process for a variety of confined systems, and the strength of these resonances is such that they should be amenable for study by photoelectron spectroscopy.

Auger-intercoulombic hybridized decay resonances in Kr@C60

We predict resonant hybridization of Auger and intercoulombic decay (ICD) processes in the photoionization of Kr@C60 using the time-dependent local density approximation (TDLDA) for the calculation.

Attosecond time delays in the photoionization of noble gas atoms studied in TDLDA

We perform time-dependent local density functional calculations of the quantum phase and time delays of valence photoionization of noble gas atoms. Results may be accessed by XUV-IR interferometric metrology.