Nonthermal desorption of molecular ions of polyatomic molecules induced by uv laser radiation

The paper presents the results of studies into ion desorption from the surface of adenine and anthracene molecular crystals and anthracene molecules adsorbed on a metal as the surface is irradiated by an uv laser at λ=249, 308, and 337 nm. A sharp increase in molecular ion yield has been revealed with a decrease in radiation wavelength down to 249 nm. At this wavelength the effect of molecular ion desorption is observed at the radiation intensity of 20∶50 kW/cm² and pulse duration of 20 ns, it cannot be explained by thermal mechanisms. The initial velocity distribution of photoions is nonthermal by character, with maximum velocity of 2·10⁵ cm/s in case of adenine. The mechanisms responsible for the effect observed are discussed.     

Coupling of the phase and relaxation transitions in polymer mixtures

The coupling and the competition of two phase transitions (demixing and crystallization) and the coupling of both demixing and vitrification are investigated during phase transformations under far-from-equilibrium conditions. The coupling leads to formation of the different non-equilibrium morphologies and to kinetic pecularities of phase transformations.     

Phase separation and glass transition point versus composition diagrams of binary mixtures with covalent bond between components

The geometric thermodynamics approach has been used for investigation of the possible glass transition point versus composition curves and their dependence on various parameters for both mixtures and systems with covalent bond between the components (block-, graftand star-polymers) in which phase separation is possible. Predicted relationships are compared with the experiment. Conditions have been determined under which glass transition hinders the liquid-liquid separation.List of principle symbols and abbreviations T _ps phase separation (or annealing) temperature - T _ps1,T _ps2... two-phase region annealing temperature - T _{ps 0} one-phase region annealing temperature - T _g1,T _g2 glass transition temperature of the first and second component - T _g,T _g glass transition temperature of phases with the compositionx andx - T _g ⁰ glass transition temperature of one-phase system - T _b temperature bordering the two-phase region at which the glass transition affects the phase separation - T_bin temperature of the liquid-liquid phase transition - M ₁,M ₂ molecular mass of 1st (rigid) and 2nd (soft) components, correspondingly - x, x compositions (fraction of the second component) in the first and second phases - x_trunc the value of the fraction of the second component at which the concentration profile is truncated by the glass transition - x _ent,M _ent the composition and molecular mass of the entrance beneath the binodal surface - x _cr,M _cr the critical composition and molecular mass - x _ent,x _ent the compositions of the first and second phases at the point of the entrance of the composition curve beneath the binodal surface - x_ex M _ex the composition and molecular mass of the composition curve exit from under the binodal surface - volume fraction - CPC cloud point curve - GTD glass transition diagram - GTCSS glass transition curve of a single phase system - LCP lower critical point - UCP upper critical point     

Morphology resulting from coupling of phase separation and glass transition in polymer blends

At temperatures near glass transition temperature, off-critical binary polystyrene/polybutadiene (PS/PB) mixtures were found to experience phase separation through the mechanism of slow hydrodynamic coarsening. A bicontinuous phase structure was produced on quenching the PS/PB-blends under cloud point curve. Different coarsening regimes of this percolation structure depending on degrees of supersaturation were found. At late stages of coarsening, secondary phase separation was observed.     

The effect of glass transition on phase separation in binary off-critical blends: different coarsening regimes near the nonequilibrium percolation threshold

The nonequilibrium percolation threshold was shown to take an intermediate position between binodal and mean-field spinodal. Below the nonequilibrium percolation threshold, a bicontinuous phase structure was produced. This percolation structure, depending on supersaturation, breaks down to ramified clusters slowly assuming a spherical droplet form or contracts to the center of the sample. In the latter case, at late stages, secondary phase separation is observed. Accepted: 21 September 1998     

Photoionization mass-spectrometry of benzene and benzaldehyde molecules with an excimer KrF laser

A laser photoionization time-of-flight mass-spectrometer has been developed and used to investigate the photoionization and photofragmentation of benzene and benzaldehyde molecules by an excimer KrF laser radiation at the wavelength of 249 nm in the intensity range from 5·10⁴ to 5·10⁹ W/cm². It has been found that at low laser intensity ions formed by two-step photoionization are most abundant in mass spectra. By increasing laser intensity an extensive fragmentation of molecules, up to C⁺ ions, was observed. The maximum ionization yield of benzaldehyde comes to 0.1% of the number of molecules in the photoionization volume and, according to calculations, to 10% for benzene molecules, when the radiation intensity is 5·10⁹ W/cm².