The translational energy of Dz desorbed from Si(100) and Si(111) surfaces was measured and found roughly equal to the thermal expectation at the surface temperature T,. Combining these results with previously measured internal state distributions, the total energy of the desorbed molecules is approximately equal to the equilibrium expectation at T,. Thus adsorption experiments, which suggest a large energetic barrier, are at variance with desorption experiments, which exhibit a trivial adsorption barrier, and the applicability of detailed balance for this system needs to be reexamined.

The photochemistry of O2 adsorbed on Pd(111) has been studied as a function of the energy of the exciting photons in the range of hν=3.9–6.4 eV. The obtained data are reproduced by a proposed kinetic model considering photostimulated desorption and dissociation, whereby the latter gives rise to additional displacement processes. This modeling yields rate constants and, hence, cross sections for the respective processes. It is found that the cross sections for all processes rise exponentially with photon energy. This result motivates a study of the distribution of hot electrons generated by laser irradiation, and its decay via electron-hole pair scattering events resulting in a broad secondary electron distribution. The energetic and spatial distribution of these electrons is calculated. The probability for these electrons to tunnel into an adsorbate affinity level is modeled and compared to the observed photo cross sections.

The infrared photodissociation of size-selected, small benzene clusters has been investigated in the region of the ν18 CH in-plane bend using a pulsed CO2 laser. By scattering the cluster beam with a secondary Ne beam and observing off-axis the effect of the laser irradiation with a rotatable mass spectrometer, cluster-specific spectroscopy is performed. The dependence of IR absorption and subsequent dissociation of (C6H6)n clusters has been investigated as a function of laser frequency and laser fluence for n=2, 3, and 4. The absorption profiles are structureless and show only little variation with cluster size. If, instead of He, Ne is used as carrier gas, the absorption profiles are distinctively narrower. This effect is attributed to a lower internal temperature achieved with Ne. In contrast to the benzene dimer and tetramer, the fluence dependence for the trimer dissociation is stronger than linear suggesting that more than one photon is needed to dissociate this cluster. In a computational approach, the structures of the benzene dimer, trimer, and tetramer have been calculated employing an energy minimization program. For the trimer a cyclic ring structure is determined. The computational results are in perfect agreement with the experimental findings.

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The experimental charge density distribution in 7-dispiro[2.0.2.1]heptane carboxylic acid has been determined using single-crystal X-ray diffraction data measured at 100 K. Multipole refinement converged at R-0.023 for 5539 reflections with I > 30-(/) and sin 0/A < 1.08 ]k -1 . Inclusion of hexadecapole functions for C atoms in the refinement is shown to be necessary. Experimental maps of the deformation density and of the Laplacian of p are in good agreement with those from ab initio calculations and also have some features specific for polyspirocyclopropanes. Analysis of the bond critical-point properties reveals that the effect of the 7r-acceptor hydroxycarbonyl substituent is more pronounced on the endo-side of the molecule, which can be related to the chemical behaviour of the compound. The influence of small conformational changes on the bond length distribution in the skeleton is also discussed.