An atom or molecule interacting with an intense, ultrafast laser pulse is a fundamental problem in modern physics. At intensities that are approximately one-tenth an atomic unit of field (50 V/A) the physics is well described by a semi-classical 3-step model where an electron tunnel ionizes, driven by the strong-field and then rescatters with its parent core. The consequence of this physics has opened the areas of attosecond science and spatial-temporal molecular imaging. However in a strong field experiment, the exponential rate of tunnel ionization fixes the release phase of the electron wave packet (EWP) at the extreme of the laser field. In this talk, we will describe a method that allows for more precise studies of the strong field process. The approach simulates the 3-step model by replacing the tunneling step with single-photon ionization by an attosecond XUV pulse. A phase-locked intense low-frequency field drives the EWP mimicking steps (2) and (3) but with little or no ionization. We will present both experimental and theoretical results demonstrating the viability of this approach.

Cobalt dioxolene complexes undergoing photoinduced valence tautomerism (VT) are potential candidates for molecular electronic devices. Thermal or photoinduced electron transfer between a dioxolene ligand and the metal center reversibly switch the complex between a paramagnetic or diamagnetic species. A low-spin Co^II intermediate is believed to be involved in the photogeneration of the paramagnetic state from the diamagnetic state, but has not been spectroscopically observed yet. Our group has developed a tabletop M-edge XANES spectrometer which generates femtosecond extreme ultraviolet pulses via high-harmonic generation (HHG). M-edge spectroscopy has been shown to be sensitive to changes in oxidation state, spin state and/or ligand field changes at metal centers, and the spectra can be predicted using ligand field multiplet theory. In this work, we will present temperature dependent and time-resolved spectra of a cobalt dioxolene VT compound and highlight the ultrafast electron transfer and spin state changes. The theoretical tools developed to predict and interpret the M-edge spectra of metal-centered excited states will also be discussed. This work demonstrates that HHG-based XUV absorption spectroscopy is an accessible alternative to synchrotron-based X-ray absorption spectroscopy for elucidating the electronic structure and photoinduced dynamics of transition metal complexes.

Spin crossover (SCO) metal-organic complexes have seen recent advances for their applicability in spintronic devices by exhibiting bidirectional stability between high-spin (HS) and low-spin (LS) states upon application of external stimuli. However, adsorbing these molecular complexes onto surfaces can dramatically change their SCO characteristics so alternative materials are being explored. Cobalt ferrite (CFO, CoFe₂O₄) thin films are presently studied for their ability to induce magnetism but may exhibit SCO the same way Fe/Co Prussian Blue molecular analogs SCO after photoexcitation. To investigate the potential SCO dynamics of CFO, we employ reflection-absorption extreme ultraviolet (XUV-RA) spectroscopy which is element and oxidation state specific and sensitive to changes in the local geometry and spin state. Photoexciting CFO initiates an electron transfer from Co²⁺ to Fe³⁺, reducing iron and oxidizing cobalt. By comparing time-averaged experimental spectra to charge-transfer multiplet simulations, we find excellent agreement with LS Co³⁺ CFO indicating a SCO from native HS Co²⁺ after photoexcitation. Kinetic analysis using a two-state sequential model produces a SCO time constant of 245 ± 30 fs and the initial and final state solutions agree well with simulated HS and LS Co³⁺ CFO spectra respectively. The driving mechanism for SCO is small hole polaron formation evident by changes in the oxygen L₁-edge signature. Hole polarons compress the oxygen lattice around photoexcited Co³⁺ increasing the crystal field splitting of hybrid Co³⁺ 3d orbitals and enabling the initially blocked oxygen 2s → 2p transition. These results demonstrate the ability of XUV-RA to capture the femtosecond spin dynamics of metal oxide materials and provides insight into the SCO characteristics of CFO for use in spintronic devices.

r0pt Figure The tetranuclear metal-oxo complex [Co^III₄O₄](OAc)₄(py)₄ is a homogeneous water oxidation catalyst and structural mimic of the oxygen evolving complex in photosynthetic organisms. The catalytic cycle involves a highly-oxidized intermediate the nature of which is controversial: leading formulations argue for either [Co^IV₂Co^III₂O₄] or [Co^VCo^III₃O₄]. We report the sensitization of the [Co^III₄O₄] cubane with a perylene bisimide dye thus enabling photoinduced electron transfer upon green light illumination and effecting a transient oxidation of the cobalt center(s). The resultant high-valent states and their dynamics are probed by ultrafast transient absorption and extreme ultraviolet (XUV) spectroscopies. XUV spectroscopy, by dint of its oxidation-state specificity, has the potential to reveal the true distribution of holes among cobalt centers within the oxidized cluster.

r0pt Figure High-harmonics generated in molecules can contain information about electronic structure and time-dependent dynamics induced by the driving laser. In molecular ions, it was recently predicted that the usual odd harmonics can be accompanied by sidebands at non-integer multiples of the fundamental frequency, analogous to the appearance of Mollow triplets in quantum optics [see figure]. These sidebands result from competition between two nonperturbative processes, Rabi oscillation and high-harmonic generation, and therefore offer additional insight into the dynamics of the molecule in a strong laser field. While the Mollow sideband effect shows up in theoretical calculations of single-molecule harmonic generation, in principle the macroscopic response must also be calculated to obtain a complete description. We evaluate the macroscopic harmonic spectrum by combining many single-molecule calculations at different intensities, obtained from time-dependent density functional theory calculations of N₂⁺, and also from a 1D model potential. Our results suggest that the Mollow sidebands are approximately the same intensity as the main harmonics, and also they are radiated at wider angles, meaning they could be isolated more easily in an experiment. This work was supported by NSF JILA Physics Frontier Center (Grant No. PHY 1734006)

Strong-field ionization (SFI) has become a major workhorse of modern ultrafast spectroscopy and appears at the very heart of the high-harmonic generation process that is responsible for the production of extreme ultraviolet (XUV) laser pulses. When SFI is synchronized to another laser pulse through the pump-probe technique, this attosecond process can be used to arrest the intermediates of ultrafast photochemical reactions as well as explore the electronic structure of both atoms and molecules. In this talk, I will highlight our recent results where XUV transient absorption spectroscopy is used to measure the angular distributions of singly and doubly tunnel-ionized xenon atomic states via 4d core to 5p valence shell transitions between 55 and 60 eV. Orbital alignment measurements and theory are used to examine the role of electron correlation during atomic strong-field double ionization. The experimental M_J alignment distributions are compared to results of a rate-equation model based on sequential ionization and are now applied to account for the alignment prepared by tunneling ionization. The incredible energy and time resolution afforded through XUV spectroscopy is utilized to reveal entirely new information about the strong-field ionization mechanism, including new details about contrasting ionization timescales and orbital alignment of different spin-orbit states. The electron dynamics that operate during tunneling ionization reveal details about electron correlation that are fundamentally important for understanding light-matter interaction.

We present theoretical studies of the attosecond streaking time delay concept in photoionization via the investigation of the electron dynamics in the streaking field after the transition of the photoelectron into the continuum upon absorption of an extreme ultraviolet photon. Based on the results, a so-called finite range interpretation is introduced, that highlights that the delay is accumulated until the streaking pulse ends and, hence, over a finite range of the potential of the parent ion. We then summarize a few applications which provide insights into different aspects of the streaking time delay concept in photoionization, including the relation to the Wigner-Smith time delay, the role of the attochirp and the extension to two-photon processes. The work has been supported by DOE-BES (Award No. DE-SC0001771).

Strong-field ionization (SFI) using ultrashort, high-intensity laser pulses presents a unique opportunity for studying dynamics of molecular systems given its capability to generate coherent vibrational and electronic motion. Furthermore, the very interaction that drives the tunneling ionization is at the root of high harmonic generation of extreme ultraviolet light (XUV). Our homebuilt XUV spectrometer is uniquely suited to utilize SFI to study coherent, vibrational wave-packets of small molecules in the gas phase given its ability to observe transitions from localized core shells to delocalized valence shells to glean information about their larger molecular systems (e.g. oxidation state, spin state, magnetic quantum number, and local bonding environment). Its temporal resolution provides the capability to observe electronic motion and vibrational coherences that occur on the few-femtosecond time period. This presentation will focus on the techniques for generating few-cycle XUV pulses and our preliminary computational and experimental results on the dynamics and mechanisms behind the SFI of simple gas phase systems of fundamental importance for understanding light-matter interactions.