Local electronic structure and nanolevel hierarchical organization of bone tissue: theory and NEXAFS study.

Theoretical and experimental investigations of native bone are carried out to understand relationships between its hierarchical organization and local electronic and atomic structure of the mineralized phase. The 3D superlattice model of a coplanar assembly of the hydroxyapatite (HAP) nanocrystallites separated by the hydrated nanolayers is introduced to account the interplay of short-, long- and super-range order parameters in bone tissue. The model is applied to (i) predict and rationalize the HAP-to-bone spectral changes in the electronic structure and (ii) describe the mechanisms ensuring the link of the hierarchical organization with the electronic structure of the mineralized phase in bone. To check the predictions the near-edge x-ray absorption fine structure (NEXAFS) at the Ca 2p, P 2p and O 1s thresholds is measured for native bone and compared with NEXAFS for reference compounds. The NEXAFS analysis has demonstrated the essential hierarchy induced HAP-to-bone red shifts of the Ca and P 2p-to-valence transitions. The lowest O 1s excitation line at 532.2 eV in bone is assigned with superposition of core transitions in the hydroxide OH-(H2O) m anions, Ca2+(H2O) n cations, the carboxyl groups inside the collagen and [PO4]2- and [PO4]- anions with unsaturated P-O bonds.

Gas-to-solid shift of C 1s-excited benzene.

The gas-to-solid shift of benzene is reported in the C 1s-core level regime, where the C 1s → π*-transition is investigated between 284.0 eV and 286.5 eV. Simultaneous experiments on the gas phase and condensed species are used to determine the gas-to-solid shift within an accuracy of ±5 meV. Specifically, it is observed that the vibrationally resolved C 1s → π*-transition in solid benzene is red-shifted by 55 ± 5 meV relative to the transition of the isolated molecule. Contrary to previously reported experimental data and estimates this gas-to-solid shift is somewhat smaller than the gas-to-cluster shift. It is significantly smaller than that determined in previous work on gaseous and condensed benzene. These results are discussed in terms of structural properties of molecular clusters and solid benzene by involving ab initio calculations as well as processes leading to spectral shifts of core-excited variable size matter. Finally, changes in the shape of the C 1s → π*-band upon the formation of solid benzene and benzene clusters are discussed.

C 1s -->pi* excitation in variable size benzene clusters.

The C 1s -->pi* transition in molecular benzene and benzene clusters is investigated by photoion yields at high energy resolution. The vibrationally resolved band shows the same shape in clusters as in the bare molecule, but it is redshifted by 50 meV in small clusters, i.e. near the threshold of cluster formation. This redshift increases to 70 meV with increasing cluster size. The results are assigned in comparison with ab initio calculations on model structures of dimers, trimers, and tetramers. These indicate that different carbon sites in the molecular moieties give rise to distinct spectral shifts, where carbon sites that are pointing to the pi-system of another molecule show a larger redshift than the other ones. Such structural properties are found in solid benzene, so that the gas-to-solid shift of C 1s -->pi* excited benzene is derived to be a redshift which is of the order of 100-180 meV.

Shape resonances in molecular clusters: the 2t(2g) shape resonances in S 2p-excited sulfur hexafluoride clusters.

Sulfur hexafluoride clusters are investigated in the S 2p excitation regime. For the first time special emphasis is put on high-energy resolution spectroscopy of shape resonances in free clusters. We have investigated the 2t(2g)-shape resonance occurring above the S 2p threshold as one typical example to study size effects that are related to shape resonances. A small redshift of this resonance of 30 +/- 5 meV occurs in clusters relative to the free molecule and changes in line shape are observed. A double-barrier optical potential model is applied for the analysis of intra- and intermolecular effects occurring in SF6 clusters, which is suitable for rationalizing the experimentally observed spectral changes. The experimental and theoretical results are briefly discussed in comparison to previous work on core-excited van der Waals clusters containing diatomic molecules.

Symmetry-dependent multielectron excitations near the C 1s ionization threshold and distortion of the shape resonance in CO(2).

Satellite bands accompanying the C 1s photoline for the CO2 molecule parallel to the electric vector of the incident radiation E are found to be more intense than those for CO2 perpendicular to E in the shape resonance region. This indicates that multielectron excitations are caused in part by the interaction of the outgoing C 1s photoelectron with the valence electrons. The photoelectron-impact valence excitations couple with the C 1s single-hole ionization and distort the shape resonance significantly. We assign the broad resonance at approximately 312 eV to a distorted Sigma(u) shape resonance.

Progress on inner-shell excitations of molecular van der Waals clusters.

Recent progress on core-level excitation of molecular van der Waals clusters is reported. Resonant excitation near element K edges of isolated and clustered molecules gives rise to small spectral shifts that can only be detected with high spectral resolution. This requires the use of state-of-the-art storage-ring facilities along with insertion devices and high-resolution soft X-ray monochromators. Selected experimental results on carbon monoxide clusters are reported. For the vibrationally resolved C 1s --> pi* (v = 0) band of clustered CO, these indicate characteristic line broadening as well as a small red shift of 2 +/- 1 meV compared with the isolated molecule. The results are discussed within the framework of the quasi-atomic approach with respect to intermolecular interactions, freezing of molecular rotations in clusters, and dynamic localization of resonant core-to-valence excitations.

Dynamic stabilization in 1sigma(u)-->1pi(g) excited nitrogen clusters.

High-resolution 1s near-edge spectra of molecular nitrogen and variable size nitrogen clusters obtained using monochromatic synchrotron radiation from the high brilliance BESSY-II storage ring facility are reported. The vibrationally resolved 1sigma(u)-->1pi(g) core-to-valence excitation band of clusters shows a distinct redshift of 6+/-1 meV relative to the isolated molecule, but the vibrational structure and linewidths are essentially unchanged. This shift is assigned to dynamic stabilization of 1sigma(u)-->1pi(g) excited molecules in clusters, arising from the dynamic dipole moment generated by core-hole localization in the low-symmetry cluster field. This leads to changes in intermolecular interactions compared to the ground-state cluster. Such spectral shifts are expected to occur generally in molecular clusters and in the corresponding condensed phase.