Monica de Simone

Hydrogen bonding in melamine is interpreted through a molecular-orbital framework directly linked to valence photoemission and N 1s NEXAFS, providing spectroscopic fingerprints of the underlying orbital interactions.

We determine with high accuracy the energy of the inner-shell transition 1 s 2 2 s 2 S 0 1 → 1 s 2 s 2 2 p 3 / 2 P 1 1 O K α 4 + 16 at 554.372(3) eV ( λ = 22.36480 ( 12 ) Å ) as well as its small shift of 2.2 ± 1.3 meV ( Δ λ = 0.089 ( 52 ) m Å ) for the O 18 isotope. This transition blends with a K α line of O 5 + used in astrophysical diagnostics, potentially affecting its reliability. In contrast to our experimental uncertainty of ± 3 meV, advanced electronic structure predictions for this four-electron system, including quantum electrodynamic (QED) corrections on the order of 100 meV, still scatter by more than ± 250 meV. Ions generated and stored in an electron beam ion trap were excited at the ELETTRA synchrotron facility with monochromatic soft x rays, with photon energies corrected by an additional spectrometer. Upon resonant excitation of O 4 + and their subsequent autoionization, we separate the photoions of each isotope by a time-of-flight measurement. This way, we resolve soft x-ray isotopic shifts of a few meV, obtain very accurate data on essential astrophysical ions, and test calculations down to the level of QED contributions.

The ultraviolet and vacuum ultraviolet (VUV) photo absorption spectra of fulvene were reconsidered by a combination of configuration interaction and density functional methods and extended to the newly acquired VUV photo absorption spectrum of the 6,6-dimethylfulvene derivative, where several Rydberg states have been identified. Singlet states of fulvene were studied using multi-root multi-reference configuration interaction with the H2C unit either coplanar with, or perpendicular to the ring. In contrast to ethylene, the lowest states are coplanar. The vibrational structure of the lowest (1B2) excited states of fulvene is well reproduced by calculated values. The second singlet state of fulvene, previously assigned as 1A1 on the basis of intensity, is incompatible with the calculated planar 1A1 state, which itself is a saddle point. This state shows significant quartic character on bending, and the best interpretation of the observed UV band of fulvene is of a bent form. The UV spectral state with low onset intensity is probably a 1B1 state of CS symmetry. Theoretical Rydberg states were determined for fulvene; the closest fit to the two known Rydberg states is to the 3p and 4p states (1B1). Comparison of the separation of the 2A2 and 2B1 states in the photoelectron spectra of the two compounds, with the threshold photoelectron spectrum of fulvene, shows that the 2B1 state vibrational structure is largely lost for both molecules. A reconsideration of the interaction between the X2A2 and A2B1 ionic states has led to the identification of the 4A2 quartic state of fulvene.

We present α-Al2O3 XAS, XES and RIXS measurements across the Al L2/L3 edges at about 79 eV excitation energy. In the emission spectra, we identify two fluorescence peaks, corresponding to electronic transitions into the 2p core hole from mixed states of Al 3s and Al 3d character, both mixed with O 2p orbitals. Even if the XAS spectrum shows more than one resonance, surprisingly only one clear RIXS signal with energy loss equal to 10.7 eV is present in the data. Nevertheless, this allows us to tentatively extract from the measured high-resolution data the linewidths for fluorescence and RIXS transitions, with the latter being almost a factor of two smaller than the former.

Two synchrotron-based studies on 4H-pyran-4-thione, photoelectron spectroscopy and vacuum ultraviolet (VUV) absorption spectra were performed. A highly resolved structure was observed in the photoelectron spectrum (PES), in contrast to an earlier PES study, where little structure was observed. The sequence of ionic states was determined using configuration interaction and coupled cluster methods. The vibrational structure of the lowest three PES bands was analyzed by configuration interaction and density functional calculations, providing a detailed explanation of the observed profiles. Several vibrational bands in the VUV absorption spectrum showed a similar structure to the bands in the PES and were identified as Rydberg states.

The electronic structure of 2(5H)-thiophenone in the gas phase was investigated by ultraviolet photoelectron spectroscopy and x-ray photoemission spectroscopy (XPS) and near edge x-ray absorption fine structure (NEXAFS) spectroscopy at the C 1s, O 1s, and S 2p edges. All assignments of the experimental results are supported by both ab-initio electron propagator outer-valence Green’s function (OVGF) calculations for the valence photoemission bands and density functional theory (DFT) and relativistic time dependent DFT calculations for the core levels XPS and NEXAFS spectra. Overall good agreement between experiment and theory is observed; this is especially true for core electron excitations which has permitted an unambiguous assignment of the observed spectral features in terms of single-particle excitations to virtual molecular orbitals. The assignment of the valence band spectra based on OVGF calculations, although satisfactory, points to the importance of electron correlations effects that partially break the single particle picture of ionization.

By exciting a series of 1 s 2 S 0 1 → 1 s n p P 1 1 transitions in heliumlike nitrogen ions with linearly polarized monochromatic soft x rays at the Elettra facility, we found a change in the angular distribution of the fluorescence sensitive to the principal quantum number n . In particular it is observed that the ratio of emission in directions parallel and perpendicular to the polarization of incident radiation increases with higher n . We find this n dependence to be a manifestation of the Hanle effect, which served as a practical tool for lifetime determinations of optical transitions since its discovery in 1924. In contrast to traditional Hanle effect experiments, in which one varies the magnetic field and considers a particular excited state, we demonstrate a “soft x-ray Hanle effect” which arises in a static magnetic field but for a series of excited states. By comparing experimental data with theoretical predictions, we were able to determine lifetimes ranging from hundreds of femtoseconds to tens of picoseconds of the 1 s n p P 1 1 levels, which find excellent agreement with atomic-structure calculations. We argue that dedicated soft x-ray measurements could yield lifetime data that are beyond current experimental reach and cannot yet be predicted with sufficient accuracy. Published by the American Physical Society 2024

The valence and core photoelectron spectra of three substituted disulfide systems, α-lipoic acid, trans-4,5-dihydroxy-1,2-dithiane, and di-Boc-cystamine, are presented alongside detailed theoretical analysis based on equation-of-motion coupled-cluster singles doubles for ionization potentials and the nuclear ensemble approach. A comparison of the linear and five- and six-membered ring cyclic structures reveals that the energetic separation of the non-bonding sulfur orbitals can be used to calculate a reliable estimate of the C–S–S–C dihedral angle, even for substituted disulfides, and that the sulfur 2p, oxygen 1s, and valence band photoelectron spectra are a useful site-specific probe of hydrogen bonding.

Experimental and calculated gas phase photoemission and photoabsorption spectra of DTDCTB small-molecule donor compared to DPTA and BTD building blocks.