Observation of site-selective chemical bond changes via ultrafast chemical shifts

Chemical changes following photoexcitation can occur on femtosecond timescales, involving intricate interplay between electron rearrangement and nuclear motion. Pump-probe approaches are commonly used to investigate these dynamics, with shorter wavelengths (x-ray regime) offering improved temporal and spatial resolution. X-ray photoelectron spectroscopy (XPS) is a powerful tool, as core electron binding energies are sensitive to the chemical environment (chemical shift). While XPS has been largely confined to static studies, its potential to track ultrafast changes in chemical environment, even before nuclear motion, has been recognized. This work aims to apply the concept of chemical shifts to the few-femtosecond regime, utilizing a site-selective trigger to achieve higher temporal resolution and track transient systems via chemical shifts immediately after excitation. The study uses a combined experimental and theoretical approach to detect ultrafast changes in the chemical environment triggered by site-selective excitation in a hetero-site x-ray pump/x-ray probe scheme. This approach offers real-time observation of chemical environment changes around a specific atom during and after Auger processes, providing insights into fundamental molecular dynamics. The method's broad applicability to site-specific charge migration dynamics in complex systems, and its potential extension to the attosecond domain, is highlighted.

Previous studies have explored the use of ultrafast x-ray techniques to investigate electronic dynamics, including the use of pump-probe approaches with valence electron photoexcitation to drive molecules into out-of-equilibrium states. The transition to shorter x-ray wavelengths has enabled higher temporal and atomic spatial resolution. However, integrating local chemical information with high time resolution has been a significant challenge. Recent studies have demonstrated the potential of XPS to follow UV-excited state dynamics, but higher temporal resolution was required to track transient changes via chemical shifts directly after excitation. The present research builds upon these advances by adding site-selective excitation and providing a more complete understanding of ultrafast chemical processes.

The experimental setup employs a site-selective two-color femtosecond x-ray pump-probe scheme. An x-ray pump pulse excites a core electron from the oxygen atom to an unoccupied 2π* orbital, forming an unstable core-excited state CO(1s⁻¹ )2π*. This state decays via Auger processes, ejecting an Auger electron and forming multiple cationic states CO⁺. A time-delayed x-ray probe pulse ionizes a 1s electron from the carbon atom. Measuring the photoelectron kinetic energies provides binding energies and chemical shifts. The time delay between the pump and probe pulses was varied (-5 to 40 fs). To model the correlated electronic and nuclear dynamics, an accurate theoretical model was developed, treating both electron and nuclear motions at the quantum level. The experimental spectral and temporal resolutions were estimated to be 3.5 eV and ≤10 fs, respectively. The experimental data were processed and normalized; detailed data analysis routines are described in the Supplementary Materials. The theoretical model incorporates calculations at the CASSCF level for describing strong electron correlations in core-hole states, and Configuration Interaction level for highly excited satellite states. The Mulliken charges Qm were analyzed to determine local charge at each atomic center. Detailed descriptions of the theoretical model and experimental setup are given in the Methods section and Supplementary Materials.

Time-resolved C1s XPS spectra of the core-excited state and Auger states were measured and calculated. The core-excited state, with a calculated chemical shift of -2.4 eV above the ground state and a lifetime of 4.2 fs, decays rapidly through Auger processes. The experimental data show the core-excited state contribution overlaid with the ground state photoline, clearly identifiable in the difference signal. The transient signal promptly appears at 0 fs and decays in <10 fs, agreeing with the short lifetime. The Auger states exhibit a strong increase in binding energy compared to the neutral molecule, with two main peaks observed at ~310 eV and ~317 eV (theory ~320 eV). The lower energy peak shows a delayed rise (~5 fs), followed by decay; calculations show the residual signal at long lifetimes stems from bound Auger states. The higher energy peak, attributed to dissociative Auger states, displays a further delayed onset. The time-dependent chemical shifts are correlated with ultrafast changes in electron density and nuclear motion. Promotion of a localized O1s electron to a 2π* orbital induces changes in electron density, causing bond elongation and electron density withdrawal from the bond toward the oxygen site. This produces a calculated chemical shift of 2.4 eV driven by photoexcitation and core vacancy screening. The chemical shift further increases due to bond stretching in the transient state. The theory predicts a consistent shift that isn’t fully visible in the data, due to the use of SASE pulses in the experiment versus transform-limited pulses in the simulation. The Auger state dynamics are also connected to electron density shifts. In the dominant dissociative Auger state (Φ), the valence hole ends up trapped at the carbon site during molecular fragmentation, resulting in an increased C1s binding energy (~11 eV higher). The observed spectral intensity transfer from the lower to higher binding energy peak in the experimental data agrees with the calculated findings. Bound Auger states show different behavior, with multiple vibrational states populated and small bond distance variations.

The findings directly address the research question of tracking site-specific chemical bond changes with ultrafast resolution. The observed ultrafast changes in chemical shifts are directly linked to the dynamics of electron rearrangement and nuclear motion following core excitation and Auger decay. The excellent agreement between experimental and theoretical results validates the approach and underscores the importance of incorporating both electronic and nuclear dynamics in the model. The site-selective nature of the technique allows for direct observation of charge redistribution and hole trapping at specific atomic sites within a molecule. This capability opens new avenues for investigating charge and energy transport processes in more complex systems. The extension of the technique to more complex molecules and the attosecond regime could provide even deeper insights into fundamental chemical processes.

This study demonstrates a novel technique for observing site-selective chemical bond changes with few-femtosecond time resolution using a combined x-ray pump-probe and theoretical approach. The successful tracking of chemical shifts in CO during and after Auger decay showcases the method's ability to reveal intricate details of ultrafast molecular dynamics. Future research can extend this approach to more complex systems, probing charge migration and energy transport mechanisms with attosecond precision.

The experimental data are limited by statistical uncertainties and challenges in fully correcting for timing jitter and fluctuating pulse structure of the SASE pulses. The spectral resolution, while sufficient to observe the major features, could be improved to resolve finer details of chemical shifts predicted by theory. The theoretical model, while accurate, simplifies certain aspects of the excitation process, which might affect direct comparison with the experimental data on the time evolution of the chemical shifts. The focus on CO, while a good starting point, limits direct generalization to other molecules with different electronic structures and dynamics.