Using electronic absorption and fluorescence spectroscopic techniques, as well as quantum chemical calculations, we have studied the electronic spectra of thia-bridged stilbenophane (TSP) with close cofacial contact of two trans-stilbene (t-SB) units. Compared to the t-SB monomer, the experimental consequences of the cofacial arrangement are (i) a splitting of the main absorption band with a weakly allowed emitting state, and (ii) a strongly red-shifted, unstructured emission spectrum with long fluorescence decay times. According to the theoretical investigations, the two t-SB units are strongly bent in the electronic ground state (S 0 ), because of repulsive π-π overlap. In the first excited state (S 1 ), the t-SB units become almost planar, because of attractive π*-π* overlap. As a consequence, the symmetry-forbidden S 0 T S 1 transition couples strongly to interchromophore breathing modes of low frequency (ν 1 ) 67 cm -1 , ν 2 ) 117 cm -1 ), yielding structureless spectra with large Stokes shifts. The features of the calculated spectra are in good agreement with the experimental data. The results indicate that strong intermolecular vibronic coupling is also responsible for "excimer-like" emission in organic molecular crystals of cofacially arranged molecules. Furthermore, the different geometries in the S 0 and S 1 states of TSP give evidence for the mechanism of [2+2]photodimerization of t-SB in solutions.

We present neutron diffraction data for deuterium fluoride at two liquid and four supercritical states in the temperature range of 300–473 K and for pressures up to 320 bar, spanning a factor of 4 in sample density. The intra- and intermolecular parts of the measured structure factors were separated and conclusions about the temperature and density dependence of the intra- and intermolecular structure are deduced. © 2000 American Institute of Physics.

Neutron scattering experiments on carbon tetrafluoride (CF4) at high pressure were performed along the 370 K isotherm at three supercritical densities, covering a density range from ρ=1.07 to 1.26 g cm−3. The structure factors of the investigated thermodynamic states and the weighted sums of the atom pair correlation functions are presented. The variation of the density has only a weak effect on the structure factors. The experimentally obtained total atom pair correlation functions are interpreted with reverse Monte Carlo simulations. The atom pair correlation functions and angular distribution functions indicate a completely disordered arrangement of the molecules in fluid CF4 with no significant short-range orientational order, except for very close distances.

An apparatus for neutron diffraction experiments on corrosive liquids at high temperatures and high pressures is presented. The cell is constructed for temperatures up to 900 K and pressures up to 5000 bar and tested on molten zinc chloride at five different thermodynamic states at 723 K up to pressures of 4000 bar. The distinct terms and the total atom pair correlation functions of all five thermodynamic states are determined and their density dependences are discussed.