Dissociation kinetics of excited ions: PEPICO measurements of Os3(CO)12 - The 7-35 eV single ionization binding energy region.

In this article, we study the photoinduced dissociation pathways of a metallocarbonyl, Os3(CO)12, in particular the consecutive loss of CO groups. To do so, we performed photoelectron-photoion coincidence (PEPICO) measurements in the single ionization binding energy region from 7 to 35 eV using 45-eV photons. Zero-energy ion appearance energies for the dissociation steps were extracted by modeling the PEPICO data using the statistical adiabatic channel model. Upon ionization to the excited ionic states above 13 eV binding energy, non-statistical behavior was observed and assigned to prompt CO loss. Double ionization was found to be dominated by the knockout process with an onset of 20.9 ± 0.4 eV. The oscillator strength is significantly larger for energies above 26.6 ± 0.4 eV, corresponding to one electron being ejected from the Os3 center and one from the CO ligands. The cross section for double ionization was found to increase linearly up to 35 eV ionization energy, at which 40% of the generated ions are doubly charged.

Solvation structure around ruthenium(II) tris(bipyridine) in lithium halide solutions.

The solvation of the ruthenium(II) tris(bipyridine) ion ([Ru(bpy)3](2+)) is investigated with molecular dynamics simulations of lithium halide solutions in polar solvents. The anion distribution around the [Ru(bpy)3](2+) complex exhibits a strong solvent dependence. In aqueous solution, the iodide ion forms a solvent shared complex with [Ru(bpy)3](2+), but not in the other solvents. Between Cl(-) and [Ru(bpy)3](2+), the strong hydration of the chloride ion results in a solvent separated complex where more than one solvent molecule separates the anion from the metal center. Hence, tailored solvation properties in electrolytes is a route to influence ion-ion interactions and related electron transfer processes.

Geometrical and energetical structural changes in organic dyes for dye-sensitized solar cells probed using photoelectron spectroscopy and DFT.

The effects of alkoxy chain length in triarylamine based donor-acceptor organic dyes are investigated with respect to the electronic and molecular surface structures on the performance of solar cells and the electron lifetime. The dyes were investigated when adsorbed on TiO2 in a configuration that can be used for dye-sensitized solar cells (DSCs). Specifically, the two dyes D35 and D45 were compared using photoelectron spectroscopy (PES) and density functional theory (DFT) calculations. The differences in solar cell characteristics when longer alkoxy chains are introduced in the dye donor unit are attributed to geometrical changes in dye packing while only minor differences were observed in the electronic structure. A higher dye load was observed for D45 on TiO2. However, D35 based solar cells result in higher photocurrent although the dye load is lower. This is explained by different geometrical structures of the dyes on the surface.

Ionization and photofragmentation of Ru3(CO)12 and Os3(CO)12.

In this paper, we use a combination of photoelectron spectroscopy, mass spectrometry, and density functional theory calculations to get a detailed understanding of valence single and double ionization and the subsequent dissociation processes. This is exemplified on benchmark systems, trimetallo-dodecacarbonyls M3(CO)12 with M = Ru, Os, where the energy remaining in the molecule after photoionization can be retrieved by measuring the degree of fragmentation of the molecular ion. The intensity of different mass peaks can thus be directly related to ionization cross sections obtained by photoelectron spectroscopy. We find that the M-CO dissociation energy rises as the number of CO ligands decreases due to dissociation. Moreover, ionization of the CO ligands has a higher cross section than that of the metal center for both single and double ionization. After advanced fragmentation, a CO bond can break and the carbon atom remains bonded to the metal core. In addition, we found that the valence ionization cross sections of M3(CO)12 are maximal at about 40 eV photon energy thus showing a more pronounced shape resonance than Ru and Os-complexes with a single metal atom center. Finally, an np → nd giant resonance absorption causes a significant increase of the ionization cross section above 50 eV for Ru3(CO)12.

Density functional investigation and some optical experiments on dye-sensitized quantum dots.

Dye-sensitized quantum dots (QDs) are promising candidates for dye-sensitized solar cells (DSSCs). Here, we report steady state (absorption and photoluminescence) optical measurements on several sizes of CdS QDs ligated with Coumarin 343 dye (C-343) and two different solvents, viz., chloroform and toluene. We further report detailed first principles density functional theory and time-dependent density functional theory studies of the geometric, electronic and optical (absorption and emission) properties of three different sized capped QDs, ligated with C-343 dye. The absorption spectrum shows a QD-size-independent peak, and another peak which shifts to blue with decrease in QD size. The first peak is found to arise from the dye molecule and the second one from the QD. Charge transfer using natural transition orbitals (NTOs) is found to occur from dye-to-QDs and is solvent-dependent. In the emission spectra, the luminescence intensity of the dye is quenched by the addition of the QD indicating a strong interaction between the QD and the dye.

Solvent-dependent structure of the I3(-) ion derived from photoelectron spectroscopy and ab initio molecular dynamics simulations.

Ab initio molecular dynamics (MD) simulations of the solvation of LiI3 in four different solvents (water, methanol, ethanol, and acetonitrile) are employed to investigate the molecular and electronic structure of the I3(-) ion in relation to X-ray photoelectron spectroscopy (XPS). Simulations show that hydrogen-bond rearrangement in the solvation shell is coupled to intramolecular bond-length asymmetry in the I3(-) ion. By a combination of charge analysis and I 4 d core-level XPS measurements, the mechanism of the solvent-induced distortions has been studied, and it has been concluded that charge localization mediates intermolecular interactions and intramolecular distortion. The approach involving a synergistic combination of theory and experiment probes the solvent-dependent structure of the I3(-) ion, and the geometric structure has been correlated with the electronic structure.

Solvent dependence of the electronic structure of I(-) and I3(-).

We present synchrotron-based I4d photoelectron spectroscopy experiments of solutions from LiI and LiI3 in water, ethanol, and acetonitrile. The experimentally determined solvent-induced binding energy shifts (SIBES) for the monatomic I(­) anion are compared to predictions from simple Born theory, PCM calculations, as well as multiconfigurational quantum chemical spectral calculations from geometries obtained through molecular dynamics of solvated clusters. We show that the SIBES for I(­) explicitly depend on the details of the hydrogen bonding configurations of the solvent to the I(­) and that static continuum models such as the Born model cannot capture the trends in the SIBES observed both in experiments and in higher-level calculations. To extend the discussion to more complex polyatomic anions, we also performed experiments on I3(­) and I(­)/I3(­) mixtures in different solvents and the results are analyzed in the perspective of SIBES. The experimental SIBES values indicate that the solvation effects even for such similar anions as I(­) and I3(­) can be rather different in nature.

From ligand fields to molecular orbitals: probing the local valence electronic structure of Ni(2+) in aqueous solution with resonant inelastic X-ray scattering.

Bonding of the Ni(2+)(aq) complex is investigated with an unprecedented combination of resonant inelastic X-ray scattering (RIXS) measurements and ab initio calculations at the Ni L absorption edge. The spectra directly reflect the relative energies of the ligand-field and charge-transfer valence-excited states. They give element-specific access with atomic resolution to the ground-state electronic structure of the complex and allow quantification of ligand-field strength and 3d-3d electron correlation interactions in the Ni(2+)(aq) complex. The experimentally determined ligand-field strength is 10Dq = 1.1 eV. This and the Racah parameters characterizing 3d-3d Coulomb interactions B = 0.13 eV and C = 0.42 eV as readily derived from the measured energies match very well with the results from UV-vis spectroscopy. Our results demonstrate how L-edge RIXS can be used to complement existing spectroscopic tools for the investigation of bonding in 3d transition-metal coordination compounds in solution. The ab initio RASPT2 calculation is successfully used to simulate the L-edge RIXS spectra.

Collective hydrogen-bond dynamics dictates the electronic structure of aqueous I3(-).

The molecular and electronic structures of aqueous I3(-) and I(-) ions have been investigated through ab initio molecular dynamics (MD) simulations and photoelectron (PE) spectroscopy of the iodine 4d core levels. Against the background of the theoretical simulations, data from our I4d PE measurements are shown to contain evidence of coupled solute-solvent dynamics. The MD simulations reveal large amplitude fluctuations in the I-I distances, which couple to the collective rearrangement of the hydrogen bonding network around the I3(-) ion. Due to the high polarizability of the I3(-) ion, the asymmetric I-I vibration reaches partially dissociated configurations, for which the electronic structure resembles that of I2 + I(-). The charge localization in the I3(-) ion is found to be moderated by hydrogen-bonding. As seen in the PE spectrum, these soft molecular vibrations are important for the electronic properties of the I3(-) ion in solution and may play an important role in its electrochemical function.

Ab Initio Calculations of X-ray Spectra: Atomic Multiplet and Molecular Orbital Effects in a Multiconfigurational SCF Approach to the L-Edge Spectra of Transition Metal Complexes.

A new ab initio approach to the calculation of X-ray spectra is demonstrated. It combines a high-level quantum chemical description of the chemical interactions and local atomic multiplet effects. We show here calculated L-edge X-ray absorption (XA) and resonant inelastic X-ray scattering spectra for aqueous Ni(2+) and XA spectra for a polypyridyl iron complex. Our quantum chemical calculations on a high level of accuracy in a post-Hartree-Fock framework give excellent agreement with experiment. This opens the door to reliable and detailed information on chemical interactions and the valence electronic structure in 3d transition-metal complexes also in transient excited electronic states. As we combine a molecular-orbital description with a proper treatment of local atomic electron correlation effects, our calculations uniquely allow, in particular, identifying the influence of interatomic chemical interactions versus intra-atomic correlations in the L-edge X-ray spectra.