This study presents a comprehensive investigation of the Auger spectra of three thionated uracils using a combination of theory and experiment. Non-resonant Auger spectra measured at the K-edges of carbon, nitrogen, and oxygen, as well as at the L2,3 edge of sulfur, are reported. Even though the final decay states (valence two-hole states) are the same for all edges, the resulting Auger spectra are strikingly different, highlighting the local nature of the Auger decay. Theoretical modeling, which employed a series of approaches incorporating varying levels of correlation and different treatments of the metastable states, provides a clear interpretation of the experimental spectra. The analysis highlights the crucial role of electron correlation, evident in both the density of the decay states and the spectra constructed using channel-specific partial decay widths, as well as the importance of spin-orbit coupling in shaping the L-edge spectra of thiouracils.

We present a combined experimental and theoretical study of the vibrationally resolved valence photoelectron spectra of the complete series of thiouracils (2-thiouracil, 4-thiouracil, and 2,4-dithiouracil) for binding energies between 8 and 17 eV. The theoretical spectra were calculated using equation-of-motion coupled cluster theory for ionization potential combined with the time-independent double-harmonic adiabatic Hessian approach. For all three thiouracils, the first ionization potential is found between 8.4 and 8.7 eV, which is 1 eV lower than for the canonical nucleobase uracil. Ionization bands up to 12 eV show strong vibrational progressions and are well reproduced by the calculations. These bands are attributed to the ionization of (primarily) sulfur- and oxygen-localized valence molecular orbitals. For higher binding energies, the calculations indicate that nonadiabatic couplings are important for the interpretation of the photoelectron spectra.

We present an experimental and computational study of the photoelectron spectrum of the astrochemical molecule isothiocyanic acid, HNCS. The spectrum is recorded up to a binding energy of 23 eV, and the vibrationally resolved bands of the cationic ground state (X̃+2Π) and the lowest excited cationic states (Ã+2A″, B̃+2A′, and 32A″) are reported. The spectrum exhibits a complex and rich vibronic structure, which was analyzed using advanced first-principles methodologies. The equilibrium geometries and the potential energy surfaces of the neutral and cationic ground state as well as of the lowest cationic excited states were computed using post-Hartree–Fock configuration interaction methods. For the photoionization populating the linear X̃+2Π state of HNCS+, an adiabatic ionization energy of 9.918 ± 0.008 eV is determined, which agrees well with our calculated value of 9.921 eV. For an accurate calculation of ionization energies of sulfur-containing molecules, previous explicitly correlated computational schemes were revised as tight d functions need to be included. The lowest photoelectron band is subject to Renner–Teller splitting and spin–orbit splitting and was analyzed using a perturbative approach. The bands corresponding to transitions populating the Ã+2A″ and B̃+2A′ cationic states show vibrationally resolved progressions that were also assigned, leading to adiabatic ionization energies of 13.361 ± 0.012 and 13.555 ± 0.012 eV. This work highlights the complex vibronic structure of the HNCS cation, and the data may help identify HNCS+ in astrophysical media.

In this study, we explore the influence of bromide anions concentration on the surface chemistry of colloidal gold nanoparticles synthesized via pulsed laser ablation in liquids (PLAL). Using X-ray photoelectron spectroscopy (XPS) in a controlled environment, by probing a beam of free-standing gold nanoparticles, we quantitatively characterize the surface composition of the nanoparticles, revealing that bromide adsorption significantly contributes to surface oxidation independently of counterion type and pH for alkaline solution. Additionally, our findings demonstrate the adjustability of halogen coverage post-synthesis, offering a versatile method for controlling nanoparticle properties.

We report resonant inelastic x-ray scattering (RIXS) spectra of neon atoms interacting with intense x-ray pulses generated using an x-ray free-electron laser (XFEL). We find that an unexpected peak emerges near the K α line of Ne, which does not coincide with any physical resonances of neon ions. We perform theoretical calculations based on a quantum-state-resolved rate-equation approach with x-ray-induced processes including possible resonant excitations. Our dynamics simulations demonstrate that a sequence of multiple resonant photoabsorption events are involved and the interplay of those multiple resonances in combination with the relatively large spectral bandwidth of XFEL radiation leads to the emergent resonance-like structure at a position where no resonances exist. Our finding offers critical guidance for future applications of high-intensity RIXS at XFEL facilities.

Vibrationally resolved CH3NO2 and CD3NO2 photoelectron spectra and angular distribution parameters, β, have been measured in the photon energy range 20-80 eV. This allows a comprehensive investigation of conflicting literature interpretations of the two outermost photoelectron bands, complemented by high-level calculations characterizing the three O lone-pair-based orbital ionizations expected in this region. Franck-Condon simulations allow the regular vibrational progressions observed at the beginning of the first and second bands to be assigned as predominantly modes v5 and v6 of the D0 and D2 cation states, respectively. Irregular structuring observed midband in both bands is attributed to avoided crossings between the adiabatic D0/D1 and D1/D2 states as identified in potential surface cuts taken along the v5 and v6 normal mode coordinates. The associated D0/D1 and D1/D2 conical intersections were located and characterized, allowing possible vibronic coupling and its impact on the photoelectron spectra of the D0, D1, and D2 states to be discussed. This helps rationalize the apparently missing D1 ionization in this region, although some contribution by this state may be identifiable in the β-parameter spectra. Theoretical analysis of the complete valence region spectrum identifies the early breakdown of the independent electron model beyond these first two bands.

We investigate isothiocyanic acid, HNCS, by resonant and nonresonant Auger electron spectroscopy at the K-edge of carbon and nitrogen, and the L2,3-edge of sulfur, employing soft X-ray synchrotron radiation. The C1s and N1s ionization energies as well as the S2s and S2p ionization energies are determined and X-ray absorption spectra reveal the transitions from the core to the virtual orbitals. Final states for all normal Auger electron spectra and the resonant ones recorded at the carbon and nitrogen edge are assigned and rationalized with theoretical spectra obtained with a wave-function based protocol. The latter is based on ab initio configuration-interaction representations of the bound part of the electronic wave functions, the one-center approximation for Auger intensities, and a moment theory for band shapes. The computed spectra are in very good agreement with the experimental data and most of the relevant signals are assigned. The double ionization energy is derived from the S2p1/2 spectrum and in good agreement with a recently determined value. The Auger electron spectra are compared with those of the congener HNCO. A similar shape of the normal Auger electron spectra was found for the low binding energy final states, while intensities differed. Similarities are less pronounced in the resonant Auger electron spectra.

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.

Short-lived core-ionized neon atoms were investigated by measuring under resonant Raman conditions the Auger decay following the excitation of a second core electron into a Rydberg state. Making use of intense and narrow bandwidth x-ray free-electron laser pulses, the photoexcitation spectrum of the femtosecond-lived Ne + 1s 0 2s 2 2p 6 np series was characterized. Energy position and lifetimes of the lower-lying Rydberg states were determined and the final state configurations following the decay of the Ne + 1s 0 2s 2 2p 6 3p double-core hole resonance were partially resolved.

We present a comprehensive, combined experimental and theoretical study of the core-level photoelectron and near-edge x-ray absorption fine structure (NEXAFS) spectra of 2-thiouracil, 4-thiouracil, and 2,4-dithiouracil at the oxygen 1s, nitrogen 1s, carbon 1s, and the sulfur 2s and 2p edges. X-ray photoelectron spectra were calculated using equation-of-motion coupled-cluster theory (EOM-CCSD), and NEXAFS spectra were calculated using algebraic diagrammatic construction and EOM-CCSD. For the main peaks at O and N 1s as well as the S 2s edge, we find a single photoline. The S 2p spectra show a spin-orbit splitting of 1.2 eV with an asymmetric vibrational line shape. We also resolve the correlation satellites of these photolines. For the carbon 1s photoelectrons, we observe a splitting on the eV scale, which we can unanimously attribute to specific sites. In the NEXAFS spectra, we see very isolated pre-edge features at the oxygen 1s edge; the nitrogen edge, however, is very complex, in contrast to the XPS findings. The C 1s edge NEXAFS spectrum shows site-specific splitting. The sulfur 2s and 2p spectra are dominated by two strong pre-edge transitions. The S 2p spectra show again the spin-orbit splitting of 1.2 eV.

Resonant Auger processes provide a unique perspective on electronic interactions and excited vibrational and electronic states of molecular ions. Here, new data are presented on the resonant Auger decay of excited CH3I in the region just below the I 4d−1 ionization threshold. The resonances include the Rydberg series converging to the five spin–orbit and ligand-field split CH3I (I 4d−1) thresholds, as well as resonances corresponding to excitation from the I 4d5/2,3/2 orbitals into the σ* lowest unoccupied molecular orbital. This study focuses on participator decay that populates the lowest lying states of CH3I+, in particular, the X̃2E3/2 and 2E1/2 states, and on spectator decay that populates the lowest-lying (CH3I2+)σ* states of CH3I+. The CH3I (I 4d−1)σ* resonances are broad, and dissociation to CH3 + I competes with the autoionization of the core-excited states. Auger decay as the molecule dissociates produces a photoelectron spectrum with a long progression (up to v3+ ∼ 25) in the C–I stretching mode of the X̃2E3/2 and 2E1/2 states, providing insights into the shape of the dissociative core-excited surface. The observed spectator decay processes indicate that CH3I+ is formed on the repulsive wall of the lower-lying (CH3I2+)σ* potentials, and the photon-energy dependence of the processes provides insights into the relative slopes of the (4d−1)σ* and (CH3I2+)σ* potential surfaces. Data are also presented for the spectator decay of higher lying CH3I (I 4d−1)nl Rydberg resonances. Photoelectron angular distributions for the resonant Auger processes provide additional information that helps distinguish these processes from the direct ionization signal.

New data are presented on the resonant Auger decay of iodobenzene (C6H5I) in the region of the I 4d−1 ionization threshold. The excited molecules decay by participator and spectator processes to populate single-hole valence states and two-hole, one-particle excited states of the cation, providing new information on the structure of C6H5I+. Excitation of dissociative C6H5I (I 4d5/2,3/2−1)σ* resonances can, in principle, result in ultrafast dissociation to C6H5 + I** and the subsequent autoionization of I**, but no evidence for this process is observed. The results are compared with our recent study of the resonant Auger decay of methyl iodide (CH3I).

High-resolution photoelectron spectra of xenon (Xe) were measured in the 4p photoionization region. The anisotropy parameters were obtained for several photoelectron peaks with angle-resolved photoelectron measurements. The various complicated spectral features were observed due to the perturbed 4p-1 spin-orbit doublet and adjacent satellite structures. An analysis based on a relativistic multiconfiguration calculation has been carried out. The key role of the strong interactions between the 4p-1 core hole and the 4d-2nl configurations has been investigated in detail. Experimental peak energies and intensities are in good agreement with theory. We find that the many of the complicated features are 4d-2nl satellite lines, and their intensities are determined mostly by the amount of 4p-1 configurations that are mixed in. The experimental values agree quite well with theory, indicating that the photoelectron angular distributions are also strongly influenced by the 4p direct-ionization contributions.

We report a joint experimental and computational study of the photoelectron spectroscopy and the dissociative photoionization of fulminic acid, HCNO. The molecule is of interest to astrochemistry and astrobiology as a potential precursor of prebiotic molecules. Synchrotron radiation was used as the photon source. Dispersive photoelectron spectra were recorded from 10 to 22 eV, covering four band systems in the HCNO cation, and an ionization energy of 10.83 eV was determined. Transitions into the Renner-Teller distorted X+2Π state of the cation were simulated using wavepacket dynamics based on a vibronic coupling Hamiltonian. Very good agreement between experiment and theory is obtained. While the first excited state of the cation shows only a broad and unstructured spectrum, the next two higher states exhibit a well-resolved vibrational progression. Transitions into the excited electronic states of HCNO+ were not simulated due to the large number of electronic states that contribute to these transitions. Nevertheless, a qualitative assignment is given, based on the character of the orbitals involved in the transitions. The dissociative photoionization was investigated by photoelectron-photoion coincidence spectroscopy. The breakdown diagram shows evidence for isomerization from HCNO+ to HNCO+ on the cationic potential energy surface. Zero Kelvin appearance energies for the daughter ions HCO+ and NCO+ have been derived.

The determination of depth profiles across interfaces is of primary importance in many scientific and technological areas. Photoemission spectroscopy is in principle well suited for this purpose, yet a quantitative implementation for investigations of liquid-vapor interfaces is hindered by the lack of understanding of electron-scattering processes in liquids. Previous studies have shown, however, that core-level photoelectron angular distributions (PADs) are altered by depth-dependent elastic electron scattering and can, thus, reveal information on the depth distribution of species across the interface. Here, we explore this concept further and show that the experimental anisotropy parameter characterizing the PAD scales linearly with the average distance of atoms along the surface normal obtained by molecular dynamics simulations. This behavior can be accounted for in the low-collision-number regime. We also show that results for different atomic species can be compared on the same length scale. We demonstrate that atoms separated by about 1 Å along the surface normal can be clearly distinguished with this method, achieving excellent depth resolution.

Understanding the mechanisms of X-ray radiation damage in biological systems is of prime interest in medicine (radioprotection, X-ray therapy…). Study of low-energy rays, such as soft-X rays and light ions, points to attribute their lethal effect to clusters of energy deposition by low-energy electrons. The first step, at the atomic or molecular level, is often the ionization of inner-shell electrons followed by Auger decay in an aqueous environment. We have developed an experimental set-up to perform electron coincidence spectroscopy on molecules in a water micro-jet. We present here the first results obtained on sodium benzoate solutions, irradiated at the oxygen and carbon K-edges.

The valence ionization of uracil and mixed water-uracil clusters has been studied experimentally and by ab initio calculations. In both measurements, the spectrum onset shows a red shift with respect to the uracil molecule, with the mixed cluster characterized by peculiar features unexplained by the sum of independent contributions of the water or uracil aggregation. To interpret and assign all the contributions, we performed a series of multi-level calculations, starting from an exploration of several cluster structures using automated conformer-search algorithms based on a tight-binding approach. Ionization energies have been assessed on smaller clusters via a comparison between accurate wavefunction-based approaches and cost-effective DFT-based simulations, the latter of which were applied to clusters up to 12 uracil and 36 water molecules. The results confirm that (i) the bottom-up approach based on a multilevel method [Mattioli et al. Phys. Chem. Chem. Phys. 23, 1859 (2021)] to the structure of neutral clusters of unknown experimental composition converges to precise structure-property relationships and (ii) the coexistence of pure and mixed clusters in the water-uracil samples. A natural bond orbital (NBO) analysis performed on a subset of clusters highlighted the special role of H-bonds in the formation of the aggregates. The NBO analysis yields second-order perturbative energy between the H-bond donor and acceptor orbitals correlated with the calculated ionization energies. This sheds light on the role of the oxygen lone-pairs of the uracil CO group in the formation of strong H-bonds, with a stronger directionality in mixed clusters, giving a quantitative explanation for the formation of core-shell structures.

Fragmentation dynamics of core-excited isolated ammonia molecules is studied by two different and complementary experimental methods, high-resolution resonant Auger spectroscopy and electron energy-selected Auger electron-photoion coincidence spectroscopy (AEPICO). The combined use of these two techniques allows obtaining information on different dissociation patterns, in particular fragmentation before relaxation, often called ultrafast dissociation (UFD), and fragmentation after relaxation. The resonant Auger spectra contain the spectral signature of both molecular and fragment final states, and therefore can provide information on all events occurring during the core-hole lifetime, in particular fragmentation before relaxation. Coincidence measurements allow correlating Auger electrons with ionic fragments from the same molecule, and relating the ionic fragments to specific Auger final electronic states, and yield additional information on which final states are dissociative, and which ionic fragments can be produced in timescales either corresponding to the core-hole lifetime or longer. Furthermore, we show that by the combined use of two complementary experimental techniques we are able to identify more electronic states of the NH2+ fragment with respect to the single one already reported in the literature.

We studied N 1s(-1) inner-shell processes of the free base Phthalocyanine molecule, H2Pc, in the gas-phase. This complex organic molecule contains three different nitrogen sites defined by their covalent bonds. We identify the contribution of each site in ionized, core-shell excited or relaxed electronic states by the use of different theoretical methods. In particular, we present resonant Auger spectra along with a tentative new theoretical approach based on multiconfiguration self-consistent field calculations to simulate them. These calculations may pave the road towards resonant Auger spectroscopy in complex molecules.