In sharp contrast to molecules, electronic states of clusters with an excited intermediate-shell electron can efficiently decay via an intermolecular Coulombic mechanism. Explicit examples are presented using large scale ab initio propagator calculations. The mechanism is illustrated and its generality is stressed.

We report a thorough theoretical and computational investigation of the effect of dye adsorption on the TiO₂ conduction band energy in dye-sensitized solar cells that is aimed at assessing the origin of the shifts induced by surface adsorbed species in the position of the TiO₂ conduction band. We thus investigate a series of working dye sensitizers and prototypical surface adsorbers and apply an innovative approach to disentangle electrostatic and charge-transfer effects occurring at the crucial dye–TiO₂ interface. We clearly demonstrate that an extensive charge rearrangement accompanies the dye–TiO₂ interaction, which amounts to transfer of up to 0.3–0.4 electrons from the dyes bound in a dissociative mode to the semiconductor. Molecular monodentate adsorption leads to a much smaller CT. We also find that the amount of CT is modulated by the dye donor groups, with the coumarin dyes showing a stronger CT. A subtle modulation of the semiconductor conduction band edge energy is found by varying the nature of the dye, in line with the experimental data from the literature obtained by capacitance and open circuit voltage measurements. We then decompose the total conduction band shift into contributions directly related to the sensitizer properties, considering the effect of the electric field generated by the dye on the semiconductor conduction band. This effect, which amounts to ca. 40% of the total shift, shows a linear correlation with the TiO₂ conduction band shifts. A direct correlation between the dye dipole and the observed conduction band shift is retrieved only for dyes of similar structure and dimensions. We finally found a near-exact proportionality between the amount of charge transfer and the residual contribution to the conduction band shift, which may be as large as 60% of the total shift. The present findings constitute the basis for obtaining a deeper understanding of the crucial interactions taking place at the dye–semiconductor interface, and establish new design rules for dyes with improved DSC functionality.

The CO stretching response upon coordination to a metal M to form [(L)_nM(CO)]^m complexes (L is an auxiliary ligand) is investigated in relation to the σ donation and π back-donation components of the M–CO bond and to the electrostatic effect exerted by the ligand–metal fragment. Our analysis encompasses over 30 carbonyls, in which the relative importance of donation, back-donation and electrostatics are varied either through the ligand in a series of [(L)Au(CO)]^0/+ gold(I) complexes, or through the metal in a series of anionic, neutral and cationic homoleptic carbonyls. Charge-displacement analysis is used to obtain well-defined, consistent measures of σ donation and π back-donation charges, as well as to quantify the σ and π components of CO polarization. It is found that all complexes feature a comparable charge flow of σ symmetry (both in the M–CO bonding region and in the CO fragment itself), which is therefore largely uncorrelated to CO response. By contrast, π back-donation is exceptionally variable and is found to correlate tightly with the change in CO bond distance, with the shift in CO stretching frequency, and with the extent and direction (C → O or C ← O) of the CO π polarization. As a result, we conclusively show that π back-donation can be an important bond component also in non-classical carbonyls and we provide the framework in which the spectroscopic data on coordinated CO can be used to extract quantitative information on the π donor properties of metal–ligand moieties.

The energies needed to create different types of double core vacancies as well as the resulting redistribution of the valence electrons are analyzed in comparison with single core vacancies. Numerical results are presented for CH₄ and in particular for the molecules C₂H₂, C₂H₄, and C₂H₆. A detailed perturbation theory analysis of the relaxation energies in terms of localized and delocalized molecular orbital pictures is presented. It is shown that the binding energies associated with double core vacancies where each of the two core holes is at a different atomic site sensitively probe the chemical environment of the atoms.

The Linac Coherent Light Source free electron laser is a source of high brightness x rays, $2\times {10}^{11}$ photons in a $\sim 5\mathrm{fs}$ pulse, that can be focused to produce double core vacancies through rapid sequential ionization. This enables double core vacancy Auger electron spectroscopy, an entirely new way to study femtosecond chemical dynamics with Auger electrons that probe the local valence structure of molecules near a specific atomic core. Using 1.1 keV photons for sequential x-ray ionization of impulsively aligned molecular nitrogen, we observed a rich single-site double core vacancy Auger electron spectrum near 413 eV, in good agreement with ab initio calculations, and we measured the corresponding Auger electron angle dependence in the molecular frame.