Theory meets experiment for unravelling the C1s X-ray photoelectron spectra of pyridine, 2-fluoropyridine, and 2,6-difluoropyridine

Core Hole Effect to Valence Excitations: Tracking and Visualizing the Same Excitation in XPS Shake-Up Satellites and UV Absorption Spectra

Introducing a core hole significantly alters the electronic structure of a molecule, and various X-ray spectroscopy techniques are available for probing the valence electronic structure in the presence of a core hole. In this study, we visually demonstrate the influence of a core hole on valence excitations by computing the ultraviolet absorption spectra and the shake-up satellites in X-ray photoelectron spectra for pyrrole, furan, and thiophene, as complemented by the natural transition orbital (NTO) analysis over transitions with and without a core hole. Employing equivalent core hole time-dependent density functional theory (ECH-TDDFT) and TDDFT methods, we achieved balanced accuracy in both spectra for reliable comparative analysis. We tracked the same involved valence transition in both spectra, offering a vivid illustration of the core hole effect via the change in corresponding particle NTOs introduced by a 1s core hole on a Cα, Cβ, or O atom. Our analysis deepens the understanding of the core hole effect on valence transitions, a phenomenon ubiquitously observed in general X-ray spectroscopic analyses.

Vibrationally-Resolved X-ray Photoelectron Spectra of Six Polycyclic Aromatic Hydrocarbons from First-Principles Simulations

Vibrationally resolved C 1s X-ray photoelectron spectra (XPS) of a series of six polycyclic aromatic hydrocarbons (PAHs; phenanthrene, coronene, naphthalene, anthracene, tetracene, and pentacene) were computed by combining the full core hole density functional theory and the Franck-Condon simulations with the inclusion of the Duschinsky rotation effect. Simulated spectra of phenanthrene, coronene, and naphthalene agree well with experiments both in core binding energies (BEs) and profiles, which validate the accuracy of our predictions for the rest molecules with no high-resolution experiments. We found that three types of carbons i (inner C), p (peripheral C bonded to three C atoms), and h (peripheral C bonded to an H atom) show decreasing BEs. In linear PAHs (the latter four), h-type carbons further split into h1 or h2 (on inner or edge benzene ring) subtypes with chemical shifts of ca. 0.2-0.4 eV. All major Franck-Condon-active modes are characterized to be in-plane vibrations: low-frequency (<800 cm^-1) C-C ring deformation modes play an essential role in determining the peak asymmetries; and for each h-type carbon a high-frequency (ca. 3600 cm^-1) C*-H stretching mode is responsible for the high-energy tail. We found that core ionization leads to reduction of all C*-C and C*-H bond lengths and ring deformation with a definite direction. Based on theoretical spectra of four linear PAHs, we found asymptotic relations and anticipated possible spectral features for even larger linear PAHs. Our calculations provide accurate reference spectra for XPS characterizations of PAHs, which are useful in understanding the vibronic coupling effects in this family.

Vibrationally resolved valence and core photoionization and photoexcitation spectra of an electron-deficient trivalent boron compound: the case of catecholborane

Compounds containing trivalent boron (TB) as the electron-deficient site(s) find numerous practical uses ranging from Lewis bases in organic synthesis to high-tech industry, with a number of novel applications anticipated. We present an experimental and theoretical study of the gas-phase valence photoionization (VUV-PES), core photoionization (XPS) and photoexcitation (NEXAFS) spectra of a representative TB compound catecholborane (CB). For modelling and assigning the spectra we used the ΔDFT and restricted single excitation space TD-DFT methods for the XPS and NEXAFS, and OVGF and EOM-CCSD for the VUV-PES. The vibrationally resolved structure was computed in the Franck-Condon (FC) and Herzberg-Teller (FCHT) approximations generally resulting in a good agreement with the observed spectral features. For the prediction of core-electron binding energies (CEBEs) several density functionals were tested. The best performance overall was furnished by ωB97X-D suggesting that including the dispersion correction is beneficial. The FCHT vibronic intensities are in clear discrepancy with the B 1s NEXAFS spectrum if the harmonic approximation is used for the B-H wagging mode both in the ground and in the first core-excited state. Instead, a much better agreement is obtained if the excited state potential is approximated to a symmetric double-well. The observed vibronic pattern could be a general fingerprint of the presence of TB centre(s), specifically, the transfer of the (core) density to the vacant boron p-orbital in the excited state.

An efficient hybrid scheme for time dependent density functional theory

A hybrid approach able to perform Time Dependent Density Functional Theory (TDDFT) simulations with the same accuracy as that of hybrid exchange-correlation (xc-) functionals but at a fraction of the computational cost is developed, implemented, and validated. The scheme, which we name Hybrid Diagonal Approximation (HDA), consists in employing in the response function a hybrid xc-functional (containing a fraction of the non-local Hartree-Fock exchange) only for the diagonal elements of the omega matrix, while the adiabatic local density approximation is employed for the off-diagonal terms. HDA is especially (but not exclusively) advantageous when using Slater type orbital basis sets and allows one to employ them in a uniquely efficient way, as we demonstrate here by implementing HDA in a local version of the Amsterdam Density Functional code. The new protocol is tested on NH₃, C₆H₆, and the [Au₂₅(SCH₃)₁₈]^- cluster as prototypical cases ranging from small molecules to ligand-protected metal clusters, finding excellent agreement with respect to both full kernel TDDFT simulations and experimental data. Additionally, a specific comparison test between full kernel and HDA is considered at the Casida level on seven other molecular species, which further confirm the accuracy of the approach for all investigated systems. For the [Au₂₅(SCH₃)₁₈]^- cluster, a speedup by a factor of seven is obtained with respect to the full kernel. The HDA, therefore, promises to provide a quantitative description of the optical properties of medium-sized systems (nanoclusters) at an affordable cost, thanks to its computational efficiency, especially in combination with a complex polarization algorithm version of TDDFT.

Theoretical assessment of vibrationally resolved C1s X-ray photoelectron spectra of simple cyclic molecules

Vibrationally-resolved K-edge XPS simulation by the full/equivalent core-hole method is enabled and assessment of the C1s spectra of cyclic molecules shows excellent/acceptable agreement with the experiment.