The inner-shell ionization of selenophene at 120 eV produces a rich array of fragmentation dynamics, including many originating from Auger-Meitner processes. In this report, three-dimensional velocity-map imaging and covariance analysis were used to identify and characterize over 50 distinct selenophene fragmentation channels. The majority resulted in two or three ‘heavy’ products containing selenium or carbon, many of which had identical mass-to-charge ratios but different chemical compositions due to the degree of hydrogenation and the selenium isotope involved. Covariance analysis was used to isolate these reaction channels and to provide estimates of their relative yields. In combination with prior similar studies on thiophene and furan, the current results indicate that the nature of the heteroatom significantly influences the charge redistribution and bond cleavage dynamics induced by the Auger-Meitner process, and demonstrate the sensitivity of inner-shell ionization dynamics to the molecular and electronic structures of heterocyclic systems.

Halogenated nucleosides are known candidates for enhancing X-ray damage in radiotherapy of tumors and are also excellent model systems for investigating molecular and charge dynamics following deep inner-shell ionization. We carried out electron-multi-ion coincidence measurements with two gas-phase halogenated deoxyuridine derivatives - 5-iodo-4-thio-2'-deoxyuridine (ISDU) and 5-bromo-4-thio-2'-deoxyuridine (BrSDU). We report the fragmentation patterns and kinetic energies of ions in coincidence with the I 2p, Br 2p and S 1s photoelectrons, complementing the experiment with molecular mechanics simulations and developing the parametric, stochastic model further. This work demonstrates the applicability of the multiparticle energy- and momentum-resolved coincidence technique to the relatively unexplored regime of incomplete Coulomb explosion of a biomolecular system charged up by deep inner-shell Auger cascades. Simulations of the incomplete Coulomb explosions are carried out to compare with, and complement the experimental data. Their good overall agreement shows the expected trend of more energetic Coulomb explosions and an increased degree of atomization as deeper atomic inner-shells are ionized. The presented data on ion abundances and kinetic energies extend beyond these general predictions, providing essential input for modeling the subsequent damage propagation into the surrounding environment - an aspect directly related to the radiosensitizing properties. Some observables, such as the kinetic energies of halogen ions, are particularly sensitive to the interplay of the charge and nuclear dynamics and provide valuable benchmarks for model development.

Coulomb explosion imaging (CEI) is a powerful experimental technique that maps a molecule's geometric structure onto the momenta of ionic molecular fragments produced by rapid multiple ionization. Here, we apply CEI induced by pulses from an X-ray free-electron laser in order to image and distinguish complex hydrocarbon isomers with the chemical formula C7H8: toluene, cycloheptatriene, and 1,6-heptadiyne. The measured fragment-ion momentum distributions show discernible differences between the three isomers and provide signatures of specific carbon and hydrogen sites in the molecule. In contrast to previous work, we demonstrate that distinct 'marker atoms' are not strictly required for constructing a meaningful molecular frame of reference for the interpretation of the momentum-space data. Our work paves the way for tracking the ultrafast motion of nuclei during isomerization reactions in pure hydrocarbons.

Hydrogen migration is a ubiquitous phenomenon upon dissociation of organic molecules. Here we investigate the formation of a H3O+ fragment after core-level photoionization and Auger decay in aminobenzoic acid molecules - a process that requires the migration of at least two hydrogen atoms. Using photoelectron-photoion coincidence spectroscopy, the formation of a H3O+ fragment is observed to be more probable in ortho-aminobenzoic acid than in meta- and para-aminobenzoic acid. Energy-resolved Auger electron-photoion coincidences are measured for the ortho-isomer to investigate the internal energy dependence of the fragmentation channels, most notably of those producing H3O+. The corresponding fragmentation channels and their mechanisms are investigated by exploring the potential energy surface with ab initio quantum chemistry methods and molecular dynamics simulations. Excited-state modeling of dicationic ortho-aminobenzoic acid is used to interpret features in the Auger spectra and identify the electronic states contributing to the signals in the Auger electron photoion coincidence map. We show that populating low-energy excited states of the dication is sufficient to trigger hydrogen migration and produce H3O+ efficiently.

Tracking the motion of individual atoms during chemical reactions represents a severe experimental challenge, especially if several competing reaction pathways exist or if the reaction is governed by the correlated motion of more than two molecular constituents. Here we demonstrate how ultrashort X-ray pulses combined with coincident ion imaging can be used to trace molecular iodine elimination from laser-irradiated diiodomethane (CH2I2), a reaction channel of fundamental importance but small relative yield that involves the breaking of two molecular bonds and the formation of a new one. We map bending vibrations of the bound molecule, disentangle different dissociation pathways, image the correlated motion of the iodine atoms and the methylene group leading to molecular iodine ejection, and trace the vibrational motion of the formed product. Our results provide a quantitative mechanistic picture behind previously suggested reaction mechanisms and prove that a variety of geometries are involved in the molecular bond formation.

The MUSTACHE setup (MUlti-STep photofragmentation studies by Auger electron–ion Coincidences using High Energy photons) is a high-resolution electron–multi-ion coincidence system optimized for gas-phase experiments in the tender (∼2–10 keV) and hard (>5 keV) X-ray range. The system integrates a high-resolution hemispherical electron analyzer with a Wiley–McLaren-type ion time-of-flight (TOF) spectrometer, enabling coincidence measurements of Auger electrons and high-energy photoelectrons. Designed to overcome challenges in high-energy electron detection while maintaining excellent energy resolution, the setup covers a broad kinetic energy range up to 5 keV, allowing investigation of hard-X-ray-induced Auger cascades in molecules containing high-Z elements, where initial fluorescence decay is followed by Auger processes within this 5 keV detection window. The ion TOF spectrometer provides high-resolution ion mass and momentum analysis, essential for studying light and fast ions generated by deep-core ionization. System capabilities are demonstrated through test measurements on benchmark atomic and molecular systems, such as argon, nitrogen and carbon disulfide. These measurements demonstrate energy-resolved high-kinetic-energy photoelectron–ion coincidences and momentum-resolved multi-ion coincidences following deep-core ionization and Coulomb explosion. MUSTACHE enables investigations into deep-core ionization, Auger cascade processes and Coulomb explosion dynamics in isolated gas-phase species, offering insights into fundamental ionization and fragmentation processes. These results demonstrate that the MUSTACHE setup is a powerful tool for high-resolution electron–ion coincidence spectroscopy, extending advanced coincidence techniques into the hard X-ray regime and providing unprecedented opportunities for studying high-energy X-ray-induced phenomena.

Modern light sources such as free electron lasers allow tracking photoinduced events with unprecedented accuracy. Ion spectroscopy is a particularly useful tool, revealing for example momentum correlations in dissociation and Coulomb explosion patterns. Therefore, determining ion momenta and their relationship with the highest achievable accuracy is very valuable. Here, we develop a systematic approach to accurate ion momentum determination in Wiley-McLaren type ion time-of-flight spectrometers taking into account also field penetration effects. The developed analytical formulae at various levels of approximation are compared with ray tracing simulations and the effects are also illustrated in an experimental example.

The photodynamics of 1- and 2-iodopropane (1 and 2-IP) were studied in a time-resolved scheme incorporating ultraviolet (UV) excitation and extreme ultraviolet (XUV) probing, which initiates photoionization selectively from the I 4d core orbital. UV absorption in the A-band of both isomers leads to prompt C-I bond fission, with significant disposal of internal energy into the propyl radical product. Site-selective ionization enables a range of charge transfer (CT) processes between the nascent highly charged iodine ions and neutral propyl radicals, dependent on the interfragment distance at the instant of ionization. Subtle differences in the dynamics of these CT processes between the two isomers are observed. In 1-IP, the kinetic energies of iodine ions produced by UV photodissociation and subsequent XUV multiple ionization increased notably over the first few hundred femtoseconds, which could be understood in terms of differing gradients along the photodissociation coordinates of the neutral and polycationic states involved in the pump and probe steps, respectively. Led by a recent report of HI elimination in UV photoexcited 2-IP [Todt et al., Phys. Chem. Chem. Phys., 22(46), 27338 (2020)], we also model the most likely signatures of this process in the present experiment, and can identify signal in the 2-IP data (that is absent or significantly weaker in the data from the unbranched 1-IP isomer) that is consistent with such a process occurring on ultrafast timescales.

Lanthanum doped strontium titanate (LST) fuel electrode materials for SOFC application have been studied quite extensively. These materials have shown good stability, but relatively poor catalytic activity [1]. B-site doping of LST with Ni and other elements and exsolution of these has been used [2] to increase the catalytic activity of the surface of titanates. La 0.31 Sr 0.58 Ti 0.97 Ni 0.03 O 3- d (LSTN3) as one promising composition from this material group has been studied in this work. According to the results available in the literature, electrodes with very similar stoichiometry, as La 0.3 Sr 0.55 Ti 0.95 Ni 0.05 O 3−δ demonstrate high resistance to coking and ability to recover from sulfur poisoning [3]. In this work, pulsed laser deposited (PLD) thin film LSTN3 electrodes were studied in situ before and after electrochemical activation in different gas atmospheres and electrode polarisations using near ambient pressure XPS (NAP–XPS) method at TEMPO beamline (SOLEIL synchrotron) using a dual-chamber cell [4]. Additionally, the characteristics of the electrochemically activated surface were compared to the characteristics of as-prepared (before electrochemical activation) LSTN3 studied at Bessy 2 ISISS end station. Complementary electrochemical measurements at normal pressure (but otherwise at the same gas environments) with circular LSTN3 microelectrodes were performed in addition to the in situ EIS measurements to link the surface chemistry data from NAP–XPS experiments to electrochemical behavior of the surface (Pt current collectors enhanced the activity of the LSTN3 electrodes measured in situ during the NAP–XPS experiments [5]). The cell for dual chamber NAP–XPS experiment was produced as follows. First, porous highly active LSCT counter electrode was produced on one side of 15 mm diameter Kerafol 10Sc1CeSZ electrolyte and sintered. Then 200 nm Pt current collector with 6 µm wide stripes with 48 µm spacing were applied on the other side of the electrolyte using lift-off photolithography and magnetron sputtering of Pt. Then 2 µm thick LSTN3 film was pulsed laser deposited through Inconel mask onto the current collectors at 800 °C, 1.0·10 -2 mbar O 2 pressure and 3.0 J·cm -2 laser fluence. The composition of LSTN deposited on these conditions was determined with XRF and microwave plasma atomic emission spectroscopy (MP–AES). Next, the cell was mounted into our dual chamber NAP–XPS setup [4], wires connecting to LSCT counter electrode were connected with Pt-paste, glass seal connecting the cell to the holder was applied, the assembly was sintered at 870 °C for 10 h. The cell holder was then laser welded shut to seal off the counter electrode chamber. The cells used for electrochemical characterisation of LSTN3 microelectrodes had porous Pt-GDC counter electrodes prepared (GDC paste prepared according to [6]), LSTN3 microelectrodes were pulsed laser deposited through laser cut 60 µm thick stainless steel shadow mask with circular holes. For electrochemical measurements LSTN3 microelectrodes were contacted with tip of 70Pt30Ir wire and characterised at 2 electrode configuration in series with porous Pt-GDC electrode. The impedance spectra of the microelectrodes were fitted with simple R s (R low ||CPE) circuit model in the frequency range of 1 to 0.01 Hz to avoid over-parametrisation. More complex model describing partial ionic conductivity limitation through thin film mixed conductor [7] yielded to very similar R low values as the simplified model. R low describes LSTN3|gas surface electrochemical activity and the constant phase element parameter Q describes the chemical capacitance of LSTN3, R s describes the contact resistance between 70Pt30Ir probe and microelectrode. The influence of electrochemical activation on NAP–XPS spectra of LSTN3 model electrode was studied at two gas environments, 0.5% H 2 O + 99.5% H 2 and 30% H 2 O + 70% H 2 and at three different potentials: 0, +0.2 and −0.2 V vs OCV. At first, the XPS characterisation was carried out before electrochemical activations and then after different activations at 850 °C in 0.5% H 2 O + 99.5% H 2 atmosphere: (1) holding at OCV for 3 h; (2) polarising at −1 V for 150 s; (3) polarising at -1V for 450 s. XPS measurements at OCV were also performed in 0.5% H 2 O + 99.5% H 2 at 650 °C after the first and second activations. The same activation procedure was applied to 150 µm circular LSTN3 microelectrodes. The effect of activation on impedance spectra of LSTN3 microelectrode can be seen on fig 1. Holding the electrode at 850 °C for 3 h caused only a minimal decrease in R low . Most likely the temperature is too low to enable sufficient diffusion of Ni into Ni exsoluted particles. Electrochemical activation via polarising at −1 V for 150 s at 850 °C in 0.5% H 2 O + 99.5% H 2 caused the 4.1 times decrease of R low (measured 650 °C and OCV in 0.5% H 2 O + 99.5% H 2 gas environment) and activation with −1 V for 450 s at 850 °C caused R low to decrease 6.6 times compared to the values before activation. The effect of activation is highest when comparison of R low is carried out at anodic polarisation. At −0.2 V cathodic polarisation the effect is similar to the behaviour at OCV. The increase in surface activity is likely to be caused by the Ni exsolution (confirmed by HR–SEM) at extremely reducing conditions in MIEC material (pO 2 is 1.5 ∙10 -40 bar) caused by the high cathodic polarisation. The effect of activation gradually decreases with time. The CPE parameter Q remains unaffected by the electrochemical activation, which is unexpected, because the highly cathodic polarisation affect the whole LSTN3 film and therefore activation should cause change in stoichiometry of the perovskite troughout the film, although in the case of highly A-site deficient titanates, surface is preferred location for exolutes [8]. Unaffected Q value indicates that the bulk LSTN3 is unable to exsolve Ni or the exolution has only tiny effect on chemical capacitance or the activation time is too short to create the defined condition (pO 2 is 1.5 ∙10 -40 bar) in whole electrode (limited oxide ion conductivity) and only surface of LSTN3 electrode is significantly affected. Highest changes caused by activation could be seen in Ti 2p XPS spectra. Surface sensitive scans with 510 eV photon energy are shown on Fig 2. Ti signal intensity on the surface increases with electrochemical activations (note that the spectra before first activation is missing on the figure). The activation causes the reduction of fraction of Ti 4+ to Ti 3+ , i.e. the Ti 3+ peak becomes visible after the 150 s activation and is well pronounced after the 450 s activation. Anodic polarisation at 650 °C with +0.2 V after activations had no effect on the Ti 2p spectra, thus the surface states are more or less “freezed in” at typical operating temperatures. Spectral changes caused by electrochemical activation could be also observed in the XPS spectra of La, Sr, Ni and O. Ni 3p lines show increase in Ni signal on the surface (no chance in bulk signal) as a result of activations, this indicates clearly that exsolution likely took place only on the MIEC│gas surface and agrees well with unchanged Q value. The increased Ni content on the surface remains unchanged during following tests at 650 °C. In O spectra, the most visible effect of activations is the disappearance of CO x species as a result of electrochemical activation. La 3d spectra shows only slight changes. All changes are analysed in detail and will be presented in this work. Acknowledgements This work was supported by Estonian Research Council grant PRG551 and by the project „Increasing the knowledge intensity of Ida-Viru entrepreneurship“ (ÕÜF2) co-funded by the European Union, and by Estonian Ministry of Education and Research (TK210). Part of the research was conducted using the NAMUR+ core facility supported by the Estonian Research Council (TT 13). References [1] X. Zhou, N. Yan, K.T. Chuang, J. Luo, Progress in La-doped SrTiO 3 (LST)-based anode materials for solid oxide fuel cells, RSC Adv 4 (2014) 118–131. https://doi.org/10.1039/C3RA42666A. [2] D. Neagu, G. Tsekouras, D.N. Miller, H. Menard, J.T.S. Irvine, In situ growth of nanoparticles through control of non-stoichiometry ´, Nat. Chem. 5 (2013) 916–923. https://doi.org/10.1038/nchem.1773. [3] P. Steiger, D. Burnat, H. Madi, A. Mai, L. Holzer, J. Van Herle, O. Kröcher, A. Heel, D. Ferri, Sulfur Poisoning Recovery on a Solid Oxide Fuel Cell Anode Material through Reversible Segregation of Nickel, Chem. Mater. 31 (2019) 748–758. https://doi.org/10.1021/acs.chemmater.8b03669. [4] K. Kooser, T. Käämbre, M. Vestli, U. Joost, S. Urpelainen, M. Kook, F. Bournel, J.J. Gallet, E. Lust, E. Kukk, G. Nurk, Operando high-temperature near-ambient pressure X-ray photoelectron spectroscopy and impedance spectroscopy study of Ni−Ce0.9Gd0.1O2−δ solid oxide fuel cell anode, Int. J. Hydrog. Energy 45 (2020) 25286–25298. https://doi.org/10.1016/j.ijhydene.2020.06.228. [5] M. Ainsar, K. Kooser, M. Kodu, T. Romann, G. Nurk, Electrochemical Study of La0.31Sr0.58Ti0.97N0.03O3-δ Thin Film and Pt-10Sc1CeSZ Electrodes, ECS Trans. 111 (2023) 419–428. https://doi.org/10.1149/11106.0419ecst. [6] A. Nenning, J. Fleig, Electrochemical XPS investigation of metal exsolution on SOFC electrodes: Controlling the electrode oxygen partial pressure in ultra-high-vacuum, Surf. Sci. 680 (2019) 43–51. https://doi.org/10.1016/j.susc.2018.10.006. [7] R. Huang, C.G. Carr, C.B. Gopal, S.M. Haile, Broad Applicability of Electrochemical Impedance Spectroscopy to the Measurement of Oxygen Nonstoichiometry in Mixed Ion and Electron Conductors, ACS Appl. Mater. Interfaces 14 (2022) 19629–19643. https://doi.org/10.1021/acsami.2c05417. [8] D. Neagu, J.T.S. Irvine, Structure and Properties of La 0.4 Sr 0.4 TiO 3 Ceramics for Use as Anode Materials in Solid Oxide Fuel Cells, Chem. Mater. 22 (2010) 5042–5053. https://doi.org/10.1021/cm101508w. Fig. 1 . The effect of electrochemical activation on impedance spectra in 0.5% H 2 O + 99.5% H 2 at 650 °C, 1 bar, at OCV. Fig. 2. The effect of electrochemical activation on Ti 2p XPS spectra in 0.5% H 2 O + 99.5% H 2 at 650 °C, 3 mbar. Photon energy is 510 eV. Figure 1

The valence photoelectron spectra (PES) of gas-phase aminobenzoic acids (meta-, ortho- and para-isomers) were measured using synchrotron radiation and calculated from first principles using Density Functional Theory (DFT) with popular hybrid exchange-correlation functionals and many-body perturbation theory using the perturbative one-shot (G0W0) and eigenvalue self-consistent (GnW0) approaches within the GW approximation. The vibrational structures and line shapes found in the PES were modeled using Time-Dependent DFT. Theory can reproduce the experimental results very well. The photoelectron-photoion coincidence spectra of the ortho- and para-isomers were also measured. They reveal interesting differences in the fragmentation patterns and the influence of metastable states at the onset of fragmentation.

Time-resolved extreme ultraviolet spectroscopy was used to investigate photodissociation within the iodobenzene C-band. The carbon-iodine bond of iodobenzene was photolyzed at 200 nm, and the ensuing dynamics were probed at 10.3 nm (120 eV) over a 4 ps range. Two product channels were observed and subsequently isolated by using a global fitting method. Their onset times and energetics were assigned to distinct electron transfer dynamics initiated following site-selective ionization of the iodine photoproducts, enabling the electronic states of the phenyl fragments to be identified using a classical over-the-barrier model for electron transfer. In combination with previous theoretical work, this allowed the corresponding neutral photochemistry to be assigned to (1) dissociation via the 7B2, 8A2, and 8B1 states to give ground-state phenyl, Ph(X), and spin-orbit excited iodine and (2) dissociation through the 7A1 and 8B2 states to give excited-state phenyl, Ph(A), and ground-state iodine. The branching ratio was determined to be 87 ± 4% Ph(X) and 13 ± 4% Ph(A). Similarly, the corresponding amount of energy deposited into the internal phenyl modes in these channels was determined to be 44 ± 10 and 65 ± 21%, respectively, and upper bounds to the channel rise times were found to be 114 ± 6 and 310 ± 60 fs.

We present results from a covariance ion imaging study, which employs extensive filtering, on the relationship between fragment momenta to gain deeper insight into photofragmentation dynamics. A new data analysis approach is introduced that considers the momentum partitioning between the fragments of the breakup of a molecular polycation to disentangle concurrent fragmentation channels, which yield the same ion species. We exploit this approach to examine the momentum exchange relationship between the products, which provides direct insight into the dynamics of molecular fragmentation. We apply these techniques to extensively characterize the dissociation of 1-iodopropane and 2-iodopropane dications prepared by site-selective ionization of the iodine atom using extreme ultraviolet intense femtosecond laser pulses with a photon energy of 95 eV. Our assignments are supported by classical simulations, using parameters largely obtained directly from the experimental data.

Imaging ultrafast atomic and molecular hydrogen motion with femtosecond time resolution is a challenge for ultrafast spectroscopy due to the low mass and small scattering cross section of the moving neutral hydrogen atoms and molecules. Here, we propose time- and momentum-resolved photoelectron diffraction (TMR-PED) as a way to overcome limitations of existing methodologies and illustrate its performance using a prototype molecular dissociation process involving the sequential ejection of a neutral hydrogen molecule and a proton from the methanol dication. By combining state-of-the-art molecular dynamics and electron-scattering methods, we show that TMR-PED allows for direct imaging of hydrogen atoms in action. More specifically, the fingerprint of hydrogen dynamics reflects the time evolution of polarization-averaged molecular-frame photoelectron angular distributions (PA-MFPADs) as would be recorded in X-ray pump/X-ray probe experiments with few-femtosecond resolution. We present the results of two precursor experiments that support the feasibility of this approach.

The primary and secondary fragmentation dynamics of iodobenzene following its ionization at 120 eV were determined using three-dimensional velocity map imaging and covariance analysis. Site-selective iodine 4d ionization was used to populate a range of excited polycationic parent states, which primarily broke apart at the carbon-iodine bond to produce I+ with phenyl or phenyl-like cations (CnHx (+) or CnHx (2 +) , with n = 1 - 6 and x = 1 - 5). The molecular products were produced with varying degrees of internal excitation and dehydrogenation, leading to stable and unstable outcomes. This further allowed the secondary dynamics of C6Hx (2+) intermediates to be distinguished using native-frame covariance analysis, which isolated these processes in their own centre-of-mass reference frames. The mass resolution of the imaging mass spectrometer used for these measurements enabled the primary and secondary reaction channels to be specified at the level of individual hydrogen atoms, demonstrating the ability of covariance analysis to comprehensively measure the competing fragmentation channels of aryl cations, including those involving intermediate steps.

Photoelectron recoil strongly modifies the high kinetic energy photoemission spectra from atoms and molecules as well as from surface structures. In most cases studied so far, photoemission from atomic-like inner-shell or core orbitals has been assumed to be isotropic in the molecular frame of reference. However, in the presence of molecular field splitting of p or d orbitals, this assumption is not justified per se. We present a general theoretical treatment, linking the orientational distribution of gas-phase molecules to the electron emission and detection in a certain direction in the laboratory frame. The approach is then applied to the S 2p photoemission from a linear molecule such as CS2 and we investigate, how the predicted orientational anisotropies due to molecular field splitting affect the photoelectron recoil excitations. Lastly, experimental S 2p high-kinetic-energy photoelectron spectra of SF6 and CS2 are analyzed using the modeled recoil lineshapes representing the anisotropy-affected recoil effects.

Radiation therapy uses ionizing radiation to break chemical bonds in cancer cells, thereby causing DNA damage and leading to cell death. The therapeutic effectiveness can be further increased by making the tumor cells more sensitive to radiation. Here, we investigate the role of the initial halogen atom core hole on the photofragmentation dynamics of 2-bromo-5-iodo-4-nitroimidazole, a potential bifunctional radiosensitizer. Bromine and iodine atoms were included in the molecule to increase the photoionization cross-section of the radiosensitizer at higher photon energies. The fragmentation dynamics of the molecule was studied experimentally in the gas phase using photoelectron-photoion-photoion coincidence spectroscopy and computationally using Born-Oppenheimer molecular dynamics. We observed significant changes between shallow core (I 4d, Br 3d) and deep core (I 3d) ionization in fragment formation and their kinetic energies. Despite the fact, that the ions ejected after deep core ionization have higher kinetic energies, we show that in a cellular environment, the ion spread is not much larger, keeping the damage well-localized. A study on photodissociation dynamics of 2-bromo-5-iodo-nitroimidazole - a model radiosensitizer - using coincidence spectroscopy and computational methods.

Double-core-hole (DCH) states in isolated-water and heavy-water molecules, resulting from the sequential absorption of two x-ray photons, have been investigated. A comparison of the subsequent Auger emission spectra from the two isotopes provides direct evidence of ultrafast nuclear motion during the 1.5-fs lifetime of these DCH states. Our numerical results align well with the experimental data, providing for various DCH states an in-depth study of the dynamics responsible of the observed isotope effect.

C-I bond extension and fission following ultraviolet (UV, 262 nm) photoexcitation of 2- and 3-iodothiophene is studied using ultrafast time-resolved extreme ultraviolet (XUV) ionization in conjunction with velocity map ion imaging. The photoexcited molecules and eventual I atom products are probed by site-selective ionization at the I 4d edge using intense XUV pulses, which induce multiple charges initially localized to the iodine atom. At C-I separations below the critical distance for charge transfer (CT), charge can redistribute around the molecule leading to Coulomb explosion and charged fragments with high kinetic energy. At greater C-I separations, beyond the critical distance, CT is no longer possible and the measured kinetic energies of the charged iodine atoms report on the neutral dissociation process. The time and momentum resolved measurements allow determination of the timescales and the respective product momentum and kinetic energy distributions for both isomers, which are interpreted in terms of rival 'direct' and 'indirect' dissociation pathways. The measurements are compared with a classical over the barrier model, which reveals that the onset of the indirect dissociation process is delayed by ∼1 ps relative to the direct process. The kinetics of the two processes show no discernible difference between the two parent isomers, but the branching between the direct and indirect dissociation channels and the respective product momentum distributions show isomer dependencies. The greater relative yield of indirect dissociation products from 262 nm photolysis of 3-iodothiophene (cf. 2-iodothiophene) is attributed to the different partial cross-sections for (ring-centred) π∗ ← π and (C-I bond localized) σ∗ ← (n/π) excitation in the respective parent isomers.