R. Flesch

Dynamic Stabilization in 1sigmau-->1pig 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.

Cluster size effects in core excitons of 1s-excited nitrogen

Cluster size effects in core excitons below the N 1s ionization energy of nitrogen clusters are reported in the energy regime 405-410 eV. These results are compared to the molecular Rydberg states as well as the corresponding bulk excitons of condensed nitrogen. The experimental results are assigned using ab initio calculations. It is found that the lowest excitons (N 1s-->3ssigma and N 1s-->3ppi) are blueshifted relative to the molecular Rydberg transitions, whereas others (N 1s-->3dpi and N 1s-->4ppi) show a redshift. Results from ab initio calculations on (N(2))(13) clearly indicate that the molecular orientation within a cluster is critical to the spectral shift, where bulk sites as well as inner- and outer-surface sites are characterized by different inner-shell absorption energies. These results are compared to the experimental spectra as well as previous work on site-selectively excited atomic van der Waals clusters, providing an improved spectral assignment of core exciton states in weakly bound molecular clusters and the corresponding condensed phase.

A pump-probe photoionization mass spectrometer utilizing tunable extreme ultraviolet laser-produced-plasma radiation

An experimental device is reported that utilizes time-correlated nanosecond light pulses in combination with photoionization mass spectrometry. A primary light pulse is generated by a tunable dye laser in the ultraviolet regime, which photolyzes neutral gas targets under collision free conditions. Subsequently, a time-correlated extreme ultraviolet-light pulse comes from a laser-produced plasma that is monochromatized in the 10–25 eV regime. The photolysis products are ionized by one-photon absorption, so that the cations are finally detected by time-of-flight mass spectrometry. The performance of this experimental approach is characterized by investigating the primary photolysis products of chlorine dioxide. Finally, possible applications of this approach are briefly discussed.

Photoionization of the primary photoproducts of A(2∏)-excited ClO

Photoionization of the primary photofragments of chlorine monoxide (ClO) is reported. ClO is photolyzed in the X(2∏)→A(2∏)-regime, yielding Cl(2P) and O(3P,1D). The primary photolysis products, as well as the not photolyzed ClO, are subsequently probed by monochromatic, time-correlated vacuum-ultraviolet radiation from a laser produced plasma source. Autoionization is used for state-specific detection of the atomic photolysis products. The formation of O(3P) is exclusively observed above ≈264 nm. The threshold of O(1D) from A(2∏3/2)-excited ClO is found at 263.71±0.01 nm. The shape of the O(1D) yield near this threshold is discussed in terms of the rotational energy distribution and a rotational barrier of A(2∏3/2)-excited ClO. Direct (nonresonant) one-photon-ionization is used to establish the absolute photoionization cross sections of ClO(X(2∏)), Cl(2P), and O(1D) near 15 eV. Additional experiments on the UV-photolysis of Cl2, yielding Cl(2P), are consistent with the results on ClO. The present work is ...

C 1s →π* 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.

Sulfur 1s excitation of S2 and S8: Core–valence- and valence–valence–exchange interaction and geometry-specific transitions

Sulfur 1s excitation of S2 and S8 is reported. The experiments made use of monochromatic synchrotron radiation from the BW-1 beam line at the HASYLAB (Hamburg, Germany), where total cation yields and photoionization mass spectra were investigated as a function of the source temperature that was used for evaporation of α-sulfur. The near-edge structure undergoes characteristic changes as a function of the source temperature, reflecting significant size- and geometry-dependent changes in electronic structure of the sulfur clusters. The detailed analysis of the near-edge features is obtained from ab initio (MR-SDCI) calculations that focus on S8 and S2. The present results are discussed in comparison to previous work on inner-shell excited sulfur clusters as well as condensed sulfur. The near-edge structure of S2 is compared to that of O2 and Se2, yielding characteristic differences in core–valence and valence–valence–exchange interaction.

Site-Dependent Spectral Shifts in Core-to-π* Excitations of Pyridine Clusters †

Site- and element-selective core-to-pi* excitation in free pyridine clusters is investigated. The experimental results indicate the occurrence of site- and size-dependent spectral shifts in the C 1s and N 1s --> pi* excitation regime. Specifically, we observe in the C 1s regime a substantial and site-dependent redshift of the low energy slopes of the C 1s --> pi* band by 90 meV in clusters relative to the bare molecule, whereas the high energy slopes of this band remain almost unchanged. In contrast, a size-dependent blueshift of the same order of magnitude is found for the entire N 1s --> pi* band. This is distinctly different from previous results on van der Waals clusters, where exclusively redshifts in 1s --> pi* transitions are observed. The experimental results are compared to ab initio calculations, which serve to simulate the 1s --> pi*( v = 0) transitions. These results clearly indicate that the spectral shifts are primarily a result of electrostatic interactions between the molecular moieties and that an antiparallel orientation of molecular units preferably dominates in variable-size pyridine clusters.

Elastic light scattering from free sub-micron particles in the soft X-ray regime

We report the first experimental results on angle-resolved elastic light scattering in the soft X-ray regime, where free sub-micron particles in the size regime between 150 and 250 nm are studied in the gas phase by using a continuous particle beam. Two different types of studies are reported: (i) Angle-resolved elastic light scattering experiments provide specific information on the scattering patterns in the regime of element-selective inner-shell excitation near the Si 2p-edge (80-150 eV). In addition to intense forward scattering, we observe distinct features in the angle-resolved scattering patterns. These are modelled by using Mie theory as well as a model that includes contributions from diffuse and specular reflection. The results are primarily attributed to scattering from soft X-rays in the surface layer. (ii) Spectroscopic experiments are reported, where the photon detector is placed at a given scattering angle while scanning the photon energy near the Si 2p-absorption edge. These results are also analyzed by a Mie model, yielding accurate information of the size distribution.

Core-multishell nanocarriers: Transport and release of dexamethasone probed by soft X-ray spectromicroscopy

Label-free detection of core-multishell (CMS) nanocarriers and the anti-inflammatory drug dexamethasone is reported. Selective excitation by tunable soft X-rays in the O 1s-regime is used for probing either the CMS nanocarrier or the drug. Furthermore, the drug loading efficiency into CMS nanocarriers is determined by X-ray spectroscopy. The drug-loaded nanocarriers were topically applied to human skin explants providing insights into the penetration and drug release processes. It is shown that the core-multishell nanocarriers remain in the stratum corneum when applied for 100min to 1000min. Dexamethasone, if applied topically to human ex vivo skin explants using different formulations, shows a vehicle-dependent penetration behavior. Highest local drug concentrations are found in the stratum corneum as well as in the viable epidermis. If the drug is loaded to core-multishell nanocarriers, the concentration of the free drug is low in the stratum corneum and is enhanced in the viable epidermis as compared to other drug formulations. The present results provide insights into the penetration of drug nanocarriers as well as the mechanisms of controlled drug release from CMS nanocarriers in human skin. They are also compared to related work using dye-labeled nanocarriers and dyes that were used as model drugs.

Selective Probing of the Penetration of Dexamethasone into Human Skin by Soft X-ray Spectromicroscopy

Selective probing of dexamethasone in excised human skin using soft X-ray spectromicroscopy provides quantitative concentration profiles as well as two-dimensional drug distribution maps. Element- and site-selective excitation of dexamethasone at the oxygen K-edge with the lateral step width adjusted to 1 μm provides detailed information on the location of the drug in the different skin layers. The key of this work is to probe dexamethasone selectively at the carbonyl site (C3) by the O 1s → π* transition, providing also a most efficient way to quantify the drug concentration as a function of penetration depth in correlation with structural properties of the skin containing carboxyl and amide oxygen sites occurring at higher transition energy than dexamethasone. Following drug exposure for 4 h, the glucocorticoide is located in about equal amounts in the stratum corneum, the outermost horny layer of skin, and in the viable epidermis, whereas in the dermis no dexamethasone is detected. In the stratum corneum, most of the lipophilic drug is found in regions between corneocytes, where epidermal lipids are dominating.