Ultrafast X-ray spectroscopic investigations and molecular dynamics are now approachable with short pulses of laboratory, laser-produced high-order harmonics. Those X-rays probe transitions from localized inner shells of specific atomic sites in the molecules to valence orbitals, conveying new information about photochemical transformations. The interpretations of these spectra involve a new regime of core-to-valence X-ray probing that depends on energy shifts due to the surrounding electronic densities, spin coupling effects, energy shifts due to bond elongation with vibrational excitation, and even Jahn-Teller distortions. Coherent vibrational superpositions reveal different slopes of inner shell potentials with bond extension and Fermi resonance coupling, for the first time, in the X-ray. Open shell radicals have characteristic features of singly occupied orbitals and energetic shifts upon bond cleavage, which can be viewed from the localized atomic perspective. Corresponding theory work by collaborators provides a powerful assessment of the X-ray spectroscopic dynamics. Progress for revealing the full potential of time-resolved X-ray spectroscopy for the investigation of numerous novel features in molecular photochemistry is discussed.

In time resolved spectroscopy of molecular systems, spectral signatures are directly correlated with processes such as charge migration, intra and intermolecular vibrational relaxation, internal conversion, and intersystem crossing. However, the challenge of probing these analogous processes in material systems with surface sensitivity and ultrafast time resolution motivates the goal to extend a molecular-level understanding to dynamics at surfaces and interfaces. In this talk, we describe the recent ability to directly observe spin-polarized electron transport at semiconductor surfaces using XUV Magnetic Circular Dichroism (XUV-MCD) in a reflection geometry. The ability to produce spin polarized currents at interfaces underlies many promising applications ranging from spintronics to enantioselective photocatalysis, but designing materials capable of these applications requires an improved understanding of spin-dependent electron dynamics at interfaces. Towards this goal, XUV-MCD reflection-absorption spectroscopy provides direct observation of spin dynamics in magnetic materials with ultrafast time resolution and surface sensitivity. Yttrium iron garnet (Y₃Fe₅O₁₂, YiG) is a ferrimagnetic semiconductor, consisting of two sub-lattices based on octahedrally and tetrahedrally coordinated Fe(III) centers. A combination of linearly and circularly polarized XUV measurements at the Fe M_2,3-edge of YiG provides a detailed picture of these lattice-dependent electron dynamics, which give rise to spin polarized current at the YiG surface upon band gap excitation. These findings have important applications towards the development of spin selective photocatalysts as well as new platforms for light-induced control of ultrafast spin polarization at material interfaces.

A major channel of energy loss in solar energy conversion is nonradiative charge recombination, whereby photochemical or photovoltaic energy is lost to the surroundings as heat. Understanding the mechanism of charge recombination, particularly the timescale and coupling to nuclear and spin degrees of freedom, is critical for understanding how to promote long-lived charge separation. In this regard, bimetallic molecules with metal-to-metal charge transfer (MMCT) transitions are valuable model systems because the charge recombination reaction can be initiated with light by directly populating the charge transfer state. We employed femtosecond optical transient absorption (OTA) spectroscopy to monitor charge recombination following MMCT excitation in a heterobimetallic Fe(II)Co(III) complex. The measurements uncovered a long-lived excited state with a 500 ps lifetime. Time-dependent density functional theory (DFT) allowed for assignment of this state as a metal-centered high spin state. The combined experimental and theoretical approach pointed to an ultrafast intersystem crossing and charge recombination to a local, intermediate-spin, metal-centered excited state, followed by a slower intersystem crossing to the long-lived high-spin state. These results uncover the intricate mechanism of charge recombination in this molecule by elucidating the spectral signatures, lifetimes, assignments, energetics, and nuclear geometries of the states involved. The coupling of the electron transfer to vibrations and spin in this complex could account for the ultrafast timescale of the charge recombination and could be a target for promoting long-lived charge transfer states through synthetic tuning.

Ultrafast x-ray diffraction has been used to directly observe excited state electron density distributions in molecule upon photoexcitation (1). Theoretical studies have shown that its signal contains mixed elastic-inelastic coherence term originating from electronic coherence (2,3). In this study, we present a simulation study of valence-electron dynamics of oxazole using time-resolved off-resonant x-ray diffraction (4). A valence-state electronic wavepacket is prepared with an attosecond soft x-ray pulse through a stimulated resonant x-ray Raman process (5), and then probed with off-resonant single-molecule x-ray diffraction. We find that the time dependent diffraction signal originates solely from the electronic coherences and can be detected by existing experimental techniques. The present study thus provides a practical way of imaging electron dynamics in free molecules. In addition, the created electronic coherences and subsequent electron dynamics can be manipulated by resonant x-ray Raman excitations tuned to different core-excited states. (1) H. Yong, N. Zotev, J. M. Ruddock, B. Stankus, M. Simmermacher, A. Moreno Carrascosa, W. Du, N. Goff, Y. Chang, D. Bellshaw, M. Liang, S. Carbajo, J. E. Koglin, J. S. Robinson, S. Boutet, M. P. Minitti, A. Kirrander, P. M. Weber, Observation of the molecular response to light upon photoexcitation. Nat. Commun. 11 , 2157 (2020) (2) K. Bennett, M. Kowalewski, J. R. Rouxel, S. Mukamel, Monitoring molecular nonadiabatic dynamics with femtosecond x-ray diffraction. Proc. Natl. Acad. Sci. U.S.A. 115 , 6538-6547 (2018). (3) M. Simmermacher, N. E. Henriksen, K. B. Moller, A. Moreno Carrascosa, A. Kirrander, Electronic coherence in ultrafast x-ray scattering from molecular wave packets. Phys. Rev. Lett. 122, 073003 (2019). (4) H. Yong, S. M. Cavaletto, S. Mukamel, Ultrafast valence-electron dynamics in oxazole monitored by x-ray diffraction following a stimulated x-ray Raman excitation. J. Phys. Chem. Lett. 12, 9800-9806 (2021). (5) D. Healion, Y. Zhang, J. D. Biggs, N. Govind, S. Mukamel, Entangled valence electron-hole dynamics revealed by stimulated attosecond x-ray Raman scattering. J. Phys. Chem. Lett. 3, 2326-2331 (2012).

Molecular photodissociation is central to numerous photochemical processes relevant to atmospheric chemistry and photocatalysis. As platforms to understand the ultrafast excited state dynamics underlying complex molecular photodissociation mechanisms, we studied the ultraviolet photodissociation of two gas-phase molecules: acetyl iodideP. M. Kroger and S. J. Riley J. Chem. Phys. 67, 4483 (1977); https://doi.org/10.1063/1.434589 which is a photolytic precursor for the acetyl radical, and iron pentacarbonylM. Poliakoff and E. Weitz, Acc.Chem.Res. 1987, 20, 11, 408-414; https://doi.org/10.1021/ar00143a004 which is a model photocatalyst system. Using ultrafast extreme ultraviolet transient absorption spectroscopy, we followed the photodissociation dynamics of these molecules via core-to-valence transitions of their respective heavy atom constituents (I and Fe), giving access to atom-specific signatures of excited electronic states.
In acetyl iodide, we observe transient features with sub-100 fs lifetimes associated with the excited state wavepacket evolution prior to dissociation. These features then evolve to yield spectral signatures corresponding to the dissociation of the C-I bond. In iron pentacarbonyl, we observe transient features evolving on 100 fs to few-picosecond timescales due to excited state loss of carbonyl groups. We combine experimental findings with quantum chemical calculations to gain insight into the photodissociation dynamics. SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.
SAND No. SAND2022-2283 A

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Footnotes:

P. M. Kroger and S. J. Riley J. Chem. Phys. 67, 4483 (1977); https://doi.org/10.1063/1.434589, M. Poliakoff and E. Weitz, Acc.Chem.Res. 1987, 20, 11, 408-414; https://doi.org/10.1021/ar00143a004,

r0pt Figure Time-resolved photoelectron spectroscopy has emerged as one of the premier tools to study the complex coupled motion of electrons and nuclei that underlies ultrafast photochemical processes. To study the entire reaction pathway from reactants through intermediates to products, however, requires sufficiently energetic photons to ionise all species involved. The advent of high-flux high-harmonic generation sources now puts this within reach, and we present here femtosecond photoelectron spectroscopy studies using UV-pump XUV-probe pulses. We used this approach to probe the dynamics of dissociating CS₂ molecules across the entire reaction pathway upon excitation, Figure 1. Dissociation occurs either in the initially excited singlet manifold or, via intersystem crossing, in the triplet manifold. Both product channels are monitored and we show that, despite being more rapid, the singlet dissociation is the minor product and that triplet state products dominate the final yield. We will also show first results of our recent UV-pump XUV-probe studies of acetaldehyde photodissociation and aim to unravel the complex competing direct and roaming dissociation channels.

We theoretically study dynamic localization in molecules interacting with intense laser pulses. Mechanism is responsible for the effect of Charge Resonance Enhanced Ionization (CREI) studied for H₂⁺ and I₂⁺ for over 2 decades within the field of ultrafast intense laser AMO. Here we focus on the multielectron aspects and the attosecond electron dynamics. Calculations are performed for di- and polyatomic molecules at equilibrium internuclear distances and we discuss multielectron, or more precisely multi-orbital character of the process. CREI has been connected to the dynamic electron localization as well as to the multiple ionization bursts over one laser field cycle. We discuss the similarities and differences between CREI and results for multielectron molecules at equilibrium distances. Results obtained within TDDFT show that as expected if we use laser wavelength tuned to the resonance one could observe resonance enhancement of multiphoton ionization of valence orbitals, analogous to CREI. But calculations also reveal that in contrast to CREI studied for H₂⁺ and I₂⁺, the resonance one photon transition acts as a trigger for other excitations and leads to enhancement of ionization from multiple inner valence orbitals and the dynamical properties exhibit more complicated behavior than expected from simple '2-level'-H₂⁺ picture of CREI.

Coherence among several electronic states can produce electronic wavepackets. Due to the delocalized nature of electronic orbitals, electronic wavepackets initiated by strong field ionization have significant spatial evolution. However, the spatial evolution was not previously accessible to experimental investigations at the attosecond time scale. Using the two-electron-angular-streaking (2eAS) method, we carried out measurements on xenon and krypton, in which the yields of double ionization were measured with a time range between 0 and 2.4 fs. A clear difference in the time-resolved double ionization yield between xenon and krypton was observed: at around 1.3 fs, xenon shows a higher double ionization yield than that of krypton. At this time, the ionization site by the laser field is roughly about 180 degrees from that of the first ionization. This suggests that the second ionization is modulated by a dynamical process evolving at one femtosecond time scale. We attribute this to a spin-orbit electronic wave packet produced by the first ionization. A simulation using the time dependent configurational interaction with single excitation (TDCIS-IP-CAP) method was carried to model the ionization yield of a coherent superposition between two spin-orbit (SO) states. The calculation shows that due to the energy difference in SO splitting, the wavepackets evolves different temporally and spatially and the measured ionization yields have captured these detailed dynamics.

The advent of ultrashort laser pulses in the femtosecond to attosecond regime allows the study of ultrafast molecular dynamics with unprecedented time resolution [1,2]. These powerful modern light sources can result in the ionization of matter and thereby trigger electronic and nuclear dynamics [3,4]. In my talk, I will give an overview of the ongoing research efforts in the Worth group at UCL addressing the following fundamental questions: (i) Can we control photochemical processes by creating/manipulating a quantum superposition state with a laser pulse? (ii) Can we understand the coupled electron-nuclear motion and the associated ultrafast decoherence? (iii) Can we design laser pulses in a simple way to make use of the quantum interference pathways? (iv) Can we simulate an experimental photoelectron spectrum by developing simple theoretical models? These elementary aspects of laser-matter interactions are governed by quantum mechanics and therefore we solve the time-dependent Schrödinger equation using state-of-the-art quantum dynamics method, MCTDH [5], in combination with vibronic coupling Hamiltonian [6]. This further allows us to deal with the non-adiabatic coupling between the electrons and the nuclei [6]. The ionized electron is modeled explicitly by incorporating the continuum of free-electron states [7]. The QUANTICS suite of programs are used to run the dynamical simulations [8]. References [1] M. Nisoli, P. Decleva, F. Calegari, A. Palacios and F. Martín, Chem. Rev. 117, 10760 (2017). [2] H. H. Fielding and G. A. Worth, Chem. Soc. Rev. 47, 309 (2018). [3] V. Despré, N. V. Golubev and A. I. Kuleff, Phys. Rev. Lett. 121, 203002 (2018). [4] A. Henley, J. W. Riley, B. Wang and H. H. Fielding, Faraday Discuss. 221, 202 (2020). [5] M. H. Beck, A. Jäckle, G. A. Worth and H.-D. Meyer, Phys. Rep. 324, 1 (2000). [6] G. A. Worth and L. S. Cederbaum, Annu. Rev. Phys. Chem. 55, 127 (2004). [7] M. Seel and W. Domcke, J. Chem. Phys. 95, 7806 (1991). [8] QUANTICS, http://www2.chem.ucl.ac.uk/quantics/doc/index.html

Liquid water is important in nature and plays a critical role in numerous chemical and biological applications. The elementary reaction pathways for ionized water have been extensively studied, however, the short-lived reactive complex and its structural dynamic response after the proton transfer reaction remain illusive. Using a liquid-phase ultrafast electron diffraction technique to study the intermolecular oxygen-oxygen and oxygen-hydrogen bonds, we captured the short-lived radical-cation complex OH(H3O+) that was formed within 140 fs through a direct and fast oxygen-oxygen bond contraction and proton transfer, followed by the radical-cation pair dissociation and the subsequent structural relaxation of water shells within 250 fs. These studies provide direct evidence of this short-lived metastable radical-cation complex before separation, therefore improving our fundamental understanding of elementary reaction dynamics in ionized liquid water.