Volume 216, Issue 5 - 5

First measurements of fluorescence excitation spectra of the recently synthesized rigid stilbene analogue 1,1´-bi(benzocyclobutylidene) in asupersonic jet expansion show that, in contrast to the parent compound, both the trans - and the cis -conformer fluoresce under these conditions. The excitation energy dependence of fluorescence lifetimes indicates the onset of an efficient non-radiative decay channel above energy thresholds of 1340cm -1 and 990cm -1 for the trans - and cis -form, respectively, which is assigned to photoisomerization in the singlet state. From an RRKM analysis of the microcanonical rate coefficients an estimate of the high pressure limit of the thermal photoisomerization rate coefficient is obtained and compared with photoisomerization rate coefficients measured in low viscosity solution and in thermal vapor. There are strong indications that for this compound there are no dynamic or static solvent induced effects that lead to an anomalous acceleration of the reaction in solution.

The feasibility of time-resolved structural studies using pulsed X-ray radiation from a 3rd generation synchrotron is discussed and compared with scattering experiments which might be possible with a future free electron laser (X-FEL). As an example, it is shown that the structural changes during the isomerisation process of trans-stilbene into cis-stilbene can be observed using time-resolved X-ray diffraction in the gas-phase as well as in solution.

Ultrafast time resolved fluorescence anisotropy decay measurements were performed to gain insight into the energy gap dependence of donor-acceptor inter-chromophoric coupling within one supra-molecule. Three new compounds, each consisting of two semi-rigidly linked and strongly coupled chromophores, were designed and synthesized for this study. Their general structure is donor-spacer-acceptor, where "donor" is phenylamino, pyrenylamino, or benzanthronylamino moiety, and acceptor is aminobenzanthrone. While being similar structurally, the compounds differ significantly in the excitation energy difference of the two chromophores in a supra-molecule. Experimental data show an ultrafast initial fluorescence emission anisotropy decrease (within less then 1 ps) when the excited state energies of the interacting chromophores are close to each other or equal. No such fast fluorescence anisotropy dynamics is observed for a compound with a large energy gap.

The capture cross sections and rigidity factors for atom-linear rotor anisotropic interaction proportional to P 1 (cosγ) and P 2 (cosγ) and inverse powers of R are calculated in the sudden limit. Comparison with the infinite order sudden approximation (IOSA) shows that for large values of n the accuracy of IOSA yields the results which deviate from accurate sudden results within the expected limits. For smaller values of n , the IOSA performs better than anticipated; this is ascribed to the manifestation of the "remnant" integral of motion that exists for n =2.

The statistical thermodynamics of the van der Waals gas is treated by means of the particle entropies σ s = ka s . The non-dimensional number a s measures the particle entropy σ s in multiples of the Boltzmann constant k , which is the atomic entropy unit. a s can be calculated by an extended Schrödinger equation containing the temperature T . The attraction between the particles is introduced by perturbation theory using the mean field approximation, and the particle volume is considered by suitable boundary conditions in solving the eigenvalue problem. Eigenvalues and eigenfunctions are determined by the translational quantum numbers. – The occupation numbers f s of the entropy levels s are given by the Boltzmann distribution, which is rewritten in terms of the non-dimensional particle entropy a s and the α-function. The latter has the physical meaning of a dimensionless chemical potential, α=μ/ kT . In the case of a closed system α can be determined from the entropy quanta a s and the condition of the conservation of the particle number N . The α-function is the connecting link between the entropy quanta a s and the Helmholtz free energy F of the system, which can be calculated by integrating the α-function. From the thermodynamic potential F we deduce all the other relevant thermodynamic quantities as derivatives of F , e.g. the equation of state of the van der Waals gas, its entropy S and internal energy E . – The critical properties of the van der Waals gas are also deduced from the α-function. For that reason we expand α at the critical point c in terms of the small quantities τ=( T - T c )/ T c and Δρ=ρ-ρ c ; T c , ρ c critical temperature and particle density respectively. The discussion can be simplified by introducing the order parameter ξ=(ρ-ρ c )/ρ c and the conjugate thermodynamic field Δα=α(ξ,τ)-α(0,τ). The spontaneous symmetry breaking at the critical point c, the jump Δ C V of the heat capacity C V , the susceptibility χ and its relation to the isothermal compressibility κ T , the critical exponents α, β, γ, δ and other topics are discussed in some detail.

The process of carbon nanoparticles formation following the pyrolysis of carbon suboxide C 3 O 2 in the wide temperature range 1700-3700 K behind shock waves was investigated by measuring the extinction and emission profiles in the visible and IR ranges of spectra. The temperature dependence of the optical density shows two bell-shaped curves with the maxima at 1600 K and 3200 K. It is remarkable, that the ratio of extinction in the IR range (1.31 µ) to HeNe-laser (633 nm) as well as the ratio of the IR extinction to emissivity show the same temperature behavior. The possible correlation between the observed optical properties of the forming carbon particles and the change of their size and structure is discussed.

The simplest Quantum Cluster Equilibrium (QCE/3-21G level) theory of water is extended to isotopically substituted water. Microstructural composition and macroscopic properties are calculated for light, heavy and superheavy water. The main goal is to study qualitative aspects of cluster populations, phase diagrams and thermodynamical properties as a function of isotopic substitution. The QCE triple point is shifted to higher temperatures about 2.0 K for deuterated and 3.0 K for tritiated water, approximating the experimental shift of 3.8 K and 4.5 K, respectively. We find that isotopic substitution leads not only to the well known lowering of zero point energies and vibrational frequencies but also to changing cluster populations and thus to slightly different population-weighted structures. This result differs from the usual assumption that the structural properties of different isotopically substituted waters are nearly identical once a zero point energy induced thermal offset is taken into account, but is in agreement with results inferred from X-ray diffraction.

Under certain conditions the dynamics of spiral waves in active media can be described within a kinematical approach. We discuss the importance of curvature effects for the transition from rigidly rotating to outward meandering spirals. To include these effects into the kinematical description we propose a modified boundary condition for the curvature at the tip. This allows to recover a variety of rotation regimes with hypocycloid-like trajectories of the spiral tip corresponding to outward meandering spiral waves which are frequently observed experimentally in active media of quite different nature beyond the instability of rigid rotation.

We report experimental results on the formation of macroscopic concentration patterns in the polyacrylamide-methylene blue-oxygen-sulfide (PA-MBO) reaction. The patterns have the geometry of stripes or hexagons. The latter transform to more complicated structures within the gelled medium. For the case of hexagons, the pattern wavelength (here 1-5 mm) shows a nearly proportional dependence on the height of the reaction layer. The system generates colloidal particles (average diameter 30 µm) that we use to monitor the physical changes within the medium. Our results reveal three distinct phases of particle movement. The first one is caused by hydrodynamic flows within the pre-gel solution, whereas the third, and possibly the second, phase arises from localized volume changes within the gel matrix.

The relative electrical resistance of Pd-D system as a function of deuterium content at 278 K is presented in the total concentration range available so far. The data include results up to 0.7 n D / n Pd obtained by three different groups from 1938-69 and data achieved later in high pressures of gaseous deuterium up to 3.1 GPa from 1990-94. The high pressure results are discussed in respect to the probable deuterium concentration in the palladium lattice.