Session Q10: Focus Session: Ultrafast Electron Dynamics

Photochemical processes: usual and less usual pathways

 The study of photoinduced dynamics in isolated molecules by time-resolved techniques has recently received a new impulse by the possibility of performing pump-probe experiments where the probe is high-resolution valence photoelectron spectroscopy, i.e. a technique which is sensitive to both electronic and geometrical structural changes. This new possibility has in particular been made available at the free-electron laser (FEL) FERMI, Elettra, Italy, which being a seeded source provides intense and highly monochromatized light pulses with negligible photon energy jitter, at variance with the majority of existing FELs

Two examples will be discussed. The first concerns the dynamics of photoexcitation-deexcitation of acetylacetone. By pumping with an optical laser and probing with valence photoelectron spectroscopy, the photochemical process is described in detail as consisting of a photoexcitation to a bright singlet state, followed by a conical intersection with a singlet dark state, and then another conical intersection with a triplet state [1].

The second example concerns a photochemical isomerization reaction, and namely the ring-opening reaction of 1,3 cyclohexadiene to hexatriene, which is possibly the most studied one with single-molecule advanced ultrafast techniques. The usual description of the reaction pathway in three steps (photoexcitation to the lowest-lying bright state followed by a conical intersection with the first dark state and then either another conical intersection with the open-chain isomer ground state or a return to the cyclic isomer ground state) is oversimplified. The overall picture is much more consistent if diabatic rather than adiabatic states are analyzed. In particular, we demonstrate that an initially high-lying double excited state is the gateway to the reaction [2].   

Attosecond electronic dynamics of core-excited states of N2O in the soft x-ray region

We present a combined experimental and theoretical study of the transient absorption spectroscopy of N₂O at the N K edge (400 eV) by employing an attosecond soft x-ray pulse and an overlapping intense femtosecond infrared pulse. We demonstrate that tunnel ionization of the core-excited states of neutral N₂O causes prominent half-cycle oscillations in the absorption signal. In addition, we observe the appearance of the N₂O⁺ signal with increasing delay as a result of the tunneling ionization of N₂O, thereby allowing us to follow the tunneling process in real time. We have developed theoretical models that allow us to interpret and reproduce the main trends of the experimental data   

Probing the dynamics of autoionizing states with Raman electron interferometry

Traditional pump-probe photoelectron wavepacket interferometry employs a broadband pulse to prepare a wavepacket and a delayed ionizing probe to produce quantum beats between overlapping pathways in the continuum. The kinetic energy resolution is however constrained by the probe pulse duration. We report an alternate approach that allows for both high temporal and spectral resolution by stimulating Raman transitions with one pulse and monitoring the differential changes in electron yield via delayed autoionization. We applied this new method to study the autoionizing wavepackets excited between the spin-orbit split ionization threshold of argon and obtained remarkable agreement with theoretical calculations. We extended this new method to study the autoionization dynamics in other atomic and molecular systems. Additionally, we report progress on the time and energy dependence of the photoelectron angular distribution parameters to understand the distinct phase relationship between signals from two ion core channels in argon. Experimental results are compared with multichannel quantum defect theory calculations to highlight the importance of many-electron interactions in photoionization.   

Probing Attosecond Electron Dynamics with X-Ray Free-Electron Lasers

We present recent experimental results using attosecond x-ray free electron laser pulses and pulse pairs to probe electron dynamics in small molecular systems.

Observing electron motion in molecules on its natural attosecond timescale is a frontier challenge for experimental science. Such measurements require attosecond duration light sources and atomic-site resolution, to localize the electronic charge as it moves around a molecule. Soft x-rays address transitions between core and valence electron orbitals in most of the abundant small atoms in living organisms (carbon, nitrogen, oxygen). Because core orbitals are well localized to their respective nuclei, these transitions offer atomic site specificity for creating and probing valence electron motion. X-ray free-electron lasers offer continuous wavelength tunability across the soft x-ray region and have recently produced attosecond pulses, close to the Fourier limit, with single-shot pulse energies up to hundreds of microjoules. We present time-resolved measurements of coherent electron dynamics and electron correlation effects in small molecules made with single attosecond x-ray pulses. The coherent evolution of a wavepacket of core-excited states in nitric oxide is probed by time-resolving the Auger-Meitner emission rate using angular streaking. Angular streaking is also used to measure the attosecond photoemission delay from the oxygen 1s orbital in nitric oxide. X-ray pulse pairs with controllable delay can also be produced and are used to probe coherent valence electron motion in the para-aminophenol molecule with attosecond time resolution.    

Probing molecular charge migration with high-harmonic sideband spectroscopy

Charge migration studies, which follow the coherent motion of electrons and holes in molecules in real time, are currently attracting much theoretical and experimental interest. In conjugated organic molecules, periodic charge migration motions emerge as attosecond solitons [1], with a localized hole that periodically travels back and forth across the target [2]. In this presentation, I will discuss recent theoretical efforts to probe these charge migration dynamics with high-harmonic sideband spectroscopy (HHSS) [3]. I will show how the time-dependent, migration-induced, modulation of the harmonic signal induces sidebands in high-harmonic spectra. In turn, these sidebands robustly encode information about the nature of the migration dynamics, which makes HHSS an all-optical, background-free, coherent probe. Finally, I will give an outlook on experimental applications of HHSS of charge migration.

[1] F. Mauger et al., Phys. Rev. Research 4, 013073 (2022).

[2] A.S. Folorunso et al., Phys. Rev. Lett. 126, 133002 (2021).

[3] K.A. Hamer et al., arXiv:2202.00730 (2022).   

Search for long-lasting electronic coherence using on-the-fly ab initio semiclassical dynamics

Using a combination of high-level ab initio electronic structure methods with efficient on-the-fly semiclassical evaluation of nuclear dynamics, we performed a massive scan of small polyatomic molecules searching for a long-lasting oscillatory dynamics of the electron density triggered by the outer-valence ionization. We observed that in most of the studied molecules, either the sudden removal of an electron from the system does not lead to the appearance of the electronic coherence or the created coherences become damped by the nuclear rearrangement on a time scale of a few femtoseconds. However, we report several so far unexplored molecules with the electronic coherences lasting up to 10 fs, which can be good candidates for experimental studies. In addition, we present the full-dimensional simulations of the electronic coherences coupled to nuclear motion in several molecules which were studied previously only in the fixed nuclei approximation.

    

Chemical Regulation of Attosecond Charge Migration

Charge migration (CM) is an attosecond process where a localized electron hole travels across a molecule in a coherent manner at time scales faster than nuclear motion. Despite much recent interest, many questions remain concerning the relationship between a molecule's chemical functionalization and the CM modes it supports. Understanding these structure/property relationships is critical for identifying molecules for attosecond experiments. In this work, we use time-dependent density functional theory simulations to elucidate the modes of attosecond charge migration (CM) in a range of functionalized conjugated linear and ring-shaped molecules. Here, the halogen acts as a site for initial hole creation via strong-field ionization, and distant electron/donating groups act as hole acceptors. We observe that for linear chains, 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 atomic number increases, the hole travels in a more particle-like manner as it moves across the molecule. Finally, the "hole affinity" of a functional group increases with the electron donating strength, leading to a Hammett-like series for CM. Together, these results form a set of heuristics that will be useful for guiding and interpreting identifying molecules for future charge migration experiments.