Claire Vallance

Using intense femtosecond soft x-ray pulses above the sulfur 2p edge produced by the Free-electron LASer in Hamburg (FLASH), we investigate the multiple ionization and fragmentation dynamics of CS2. We focus on the three-ion fragmentation pathways of CS2 polycations with total charges of +4 to +10, which can only be reached through absorption of multiple x-ray photons. By coupling three-dimensional velocity map imaging with covariance analysis, we determine the relative momentum distribution of all ions produced in each fragmentation channel at high ion count rates per shot. Deviations between these measured relative ion momentum distributions and those predicted by classical Coulomb explosion simulations, assuming an instantaneous charge buildup, grow with increasing total charge state, indicative of nuclear motion during the multiple ionizations that occur within the x-ray pulse duration—as confirmed by comparison with the main fragmentation channel of the trication, which can be formed following single-photon interaction. These dynamics can be modeled using a gradual charge buildup picture of sequential single-photon single-ionization events occurring during the pulse, following the initial core ionization. Our study underscores the importance of complete and channel-resolved measurements, the advances enabled by covariance analysis in high-count-rate experiments, and the critical role of charge buildup and nuclear dynamics during the x-ray free-electron laser pulse in Coulomb explosion imaging. The impacts of this nuclear motion can be controlled and minimized by utilizing shorter x-ray pulses, as will be of great importance in using time-resolved x-ray Coulomb explosion to probe the fastest dynamics in molecular photochemistry.

The chemical shifts of inner-shell atomic orbitals are highly sensitive to chemical bonding and molecular structure. In evolving systems, however, the spectra of distinct molecular species and geometries overlap, obscuring the underlying chemical dynamics. Here we demonstrate the use of electron–ion covariance analysis to combine the structural sensitivity of Coulomb explosion imaging with inner-shell spectroscopy, yielding geometry-specific spectra of transient and product species. We apply this approach to the excited state dynamics of CS2 probed by ionization above the S 2p edge. Electron–ion covariance with time- and momentum-selected S+ and S2+ ions isolates distinct S 2p photoelectron spectra for ground-state CS2, bent photoexcited CS2, the CS photoproduct, and bare atomic sulfur ─ species whose spectra overlap strongly in the channel-averaged measurement. Clear chemical shifts are observed in the covariance photoelectron spectrum for each of these species, all of which are consistent with high-level calculations. By extracting the atomic S contribution to the photoelectron spectrum in a finely time-resolved manner we can disentangle this contribution to the overall time-resolved photoelectron spectrum as the photodissociation proceeds. These results demonstrate the promise of electron–ion covariance as a general approach to geometry-resolved inner-shell spectroscopy, opening a route to tracking structural evolution through chemical shifts in complex photoexcited molecules.

The effects of methyl substitution on the photodynamics of 1,2-dithiane, a model cyclic disulfide, and the manifestation of these effects in an experimental observable, are investigated theoretically. We employ trajectory surface-hopping simulations initiated on the first excited-state, S₁, and compare the dynamic evolution of electronic state populations and nuclear geometries over 1 ps for 1,2-dithiane and its dimethylated analogue, cis-4,5-dimethyl-1,2-dithiane. Additional simulations are performed to disentangle steric and inertial effects on the dynamics in the substituted molecule. Finally, time-resolved valence photoelectron spectra are computed using the Dyson norm approximation. Our study predicts a delayed ring-closing event following initial S-S bond homolysis and an enhanced quantum yield of thiyl radical recombination on the ground-state for the dimethylated system.

Two-dimensional “crushed” velocity-map images of photoinduced or electron-induced reaction products can be Abel-inverted to reconstruct the full three-dimensional scattering distribution. Here, we develop the equivalent of the inverse Abel transform for covariance-map images, which are increasingly used in the field of velocity-map imaging to uncover correlations between the velocities of two or more scattered products. To formulate the inverse problem, we first derive the forward transformation, which maps the underlying cylindrically symmetric scattering distribution to its corresponding covariance-map measurement. We express the unknown scattering distribution as an expansion in an orthonormal basis and reformulate the inverse problem as a linear least-squares problem, which we solve via QR decomposition. This approach provides flexible control over the reconstruction resolution and supports natural regularization through both basis truncation and data smoothing. We validate the method on synthetic test problems, demonstrating accurate recovery of both local and extended structures, and show convergence using error metrics. We then apply our method to the experimental data on the dissociation products of CF3I2+ generated in electron–molecule collisions. For a simple binary fragmentation channel, the method recovers a localized point-source distribution that is consistent with physical understanding. For a more complex channel, we show that appropriate regularization obtains stable, interpretable results. Our results demonstrate that this method is robust, flexible, and well-suited for analyzing high-quality covariance-map scattering data.

We present the results from a detailed study of the fragmentation dynamics of CS22+ and CS23+, formed in intense femtosecond soft x-ray pulses above the sulfur 2p edge, primarily through single core photoionization from the S 2p site, and subsequent Auger–Meitner decay(s). By combining three-dimensional velocity map imaging with covariance analysis, we determine the relative momenta of the ions produced in each two- and three-body fragmentation channel, at significantly higher ion count rates than conventional coincidence measurements. We shed new light on the wide range of fragmentation channels observed from the CS2 dication and trication, including channels that involve ionization-induced bond formation and fragmentations producing undetected neutral cofragments. In the latter case, a “native frames” approach is used to isolate contributions from concerted and sequential fragmentations and extract dynamical information about each step of a concerted fragmentation process. While dications often fragment sequentially, the trication is dominated by concerted fragmentation. The main trication fragmentation channel into S+ + C+ + S+ can be well-approximated by classical Coulombic simulations of the ground-state geometry distribution, reflecting both the nature of the trication potential energy surface and the rapid multiple ionization prior to substantial structural dynamics. This study demonstrates ways in which fundamental insights into the fragmentation dynamics of polycations following x-ray ionization may be extracted, which will be beneficial to future studies that employ time-resolved x-ray Coulomb explosion imaging to study ultrafast photochemistry.

Introduction: Analysis of small molecule metabolites found in blood plasma of patients undergoing treatment for STEMI has the potential to be used as a clinical diagnostic and prognostic tool, capable of predicting disease progression, risk of negative outcomes, and response to treatment. Methods: Rapid mass spectrometry has been used to measure the metabolite profiles of coronary aspirate blood plasma from 288 STEMI patients enrolled in the Oxford Acute Myocardial Infarction (OxAMI) study. Supervised machine learning applied to the mass spectra was used to stratify patients based on clinically relevant variables related to health and treatment response. Results: In this small proof-of-concept study, patient mortality and microvascular obstruction (MVO) were predicted with over 80% accuracy; heart failure diagnosis, ischemic time, peak troponin, and thrombus score were predicted with over 76% accuracy, and creatinine and index of microcirculatory resistance (IMR) were predicted with over 70% accuracy. Using feature-reduction methods, we were able to identify key mass-to-charge (m/z) peaks in the mass spectra that correlated with the assignment to particular patent groups. These may potentially be used in the future as mass spectrometric biomarkers in the development of a diagnostic and prognostic test for STEMI risk.

The photochemistry of isolated molecules of 1- and 2-iodopropane is investigated in an ultrafast pump–probe scheme, exploring the dynamics associated with I atom or molecular HI production.

The effect of spin–orbit coupling on the “Newton's cradle”-type photodynamics in the cyclic disulfide 1,2-dithiane (C₄H₈S₂) is investigated theoretically.

We assess the impact of various experimental factors on ASAP-MS spectra, and make a series of recommendations to optimise the reliability and reproducibility of such measurements in clinical settings.