Surface photochemistry: The photo-chlorination of aromatic hydrocarbons with ferric chloride on alumina

Irradiation of a number of aromatic hydrocarbons adsorbed on alumina in the presence of ferric chloride leads to monochlorination.     

Nanofoaming in the surface of biopolymers by femtosecond pulsed laser irradiation

In this work, the nanostructuring induced in femtosecond (fs) laser irradiation of biopolymers is examined in self-standing films of collagen and gelatine. Irradiation by single 90 fs pulses at 800, 400 and 266 nm is shown to result in the formation of a modified layer with submicrometric size structures. The size and uniformity of the observed features are strongly dependent on irradiation wavelength and on the characteristics of the biopolymer (water content and mechanical strength). Examination of the films by laser induced fluorescence serves to assess the chemical modifications induced by laser irradiation, revealing changes in the emission bands assigned to the aromatic amino acid tyrosine and its degradation products. The results are discussed in the framework of a mechanism involving the generation of large free-electron densities, through multiphoton and avalanche ionization, which determine the temperature and stress distribution in the irradiated volume.     

Model properties relevant to laser ablation of moderately absorbing polymers

A set of polymers including biopolymers and polymers from renewable resources are studied, with attention paid to their capability to form a foamy surface layer by laser irradiation. A model of laser-induced pressure wave is discussed, with its tensile tail giving rise to a fast and dense nucleation of cavities that can expand to microbubbles when filled with ablation gas. The intensity of the pressure wave has a maximum for an absorption coefficient of ∼1000 cm⁻¹. Polyvinyl acetate, studied as a prototype polymer experimentally and by modeling, allows discussing the role of the viscosity drop in the dynamics of the laser-induced cavitations. In the Zeldovich frequency factor, a T (temperature) and P (pressure) dependent model of viscosity, and a T-dependent model of surface tension are introduced. It is further suggested that the well-known free-volume nanoholes existing in the material before the irradiation can constitute the nuclei of importance and that their concentration is one of the factors controlling the pre-exponential factor in the nucleation rate law.     

Nanofoaming dynamics in biopolymers by femtosecond laser irradiation

We have recently shown that irradiation of self-standing films of the biopolymers collagen and gelatine with single femtosecond laser pulses produces a nanofoaming layer with regular bubble size which can be controlled by wavelength selection. Following these initial studies, here we report on the temporal evolution of the foaming effect by measurements in situ and in real time of the change in the transmittance of a cw probe HeNe laser through the irradiated films. Self standing films of the biopolymers were irradiated with 90 fs laser pulses at 800, 400, and 266 nm. For fluences below and above the modification threshold a permanent attenuation of the transmission occurs (increasing with fluence). The initial decay of the transmission is fast (around few tens of ns), and is followed by dynamics in the longer timescale (micro and milliseconds). The temporal evolution of the transmission measured upon fs laser irradiation is similar with that determined in the irradiation of the biopolymer films at 248 nm with 25 ns laser pulses. The method allows separating in time the different processes occurring after irradiation that lead to a permanent nanofoaming structure, while the results allow us to understand the mechanisms of femtosecond laser processing of the biopolymers and their interest in biomedical applications.     

Changes in the etch rate of photosensitive polymers as a function of the pulse number

The ablation rates of a polyimide and a triazene polymer were studied gravimetrically by a quartz micro balance for 248-nm and 308-nm irradiation. Special care was taken to examine the dependence of the ablation rate at constant fluences for single pulses and the influence of consecutive pulses at the same position. A clear trend was observed in these measurements, i.e., that the mass loss after the first pulse is always different from values for the following pulses. This implies that it is very difficult to determine true ablation rates, which are the foundation of most ablation models. The differences of the mass loss between the first pulse and the following pulses is most probably due to carbonization of the material, resulting in varying ablation rates for the following pulses. The ablation rates are thus not a real material property but a superposition of the material ablation rates with the ablation rates of carbon and carbonized material. PACS 52.38.Mf; 71.20.Rv; 07.10.Lw     

A synthesis of (−)hibaene

A total synthesis of (?)hibaene 1 is described. The key steps are a photochemical cycloaddition of vinyl acetate on the α-β unsaturated ketone 2 and a Wagner-Meerwein type rearrangement of the diols 14 and 15, which contain a highly strained cyclobutane ring. A selective esterification of the rearranged diols 22a or 22b leads to intermediates which can conveniently be transformed into (?)hibaene.     

BIPHASIC PHOTOCHEMISTRY: THE FIRST ACTINOMETRIC REACTION ON A SOLID SUPPORT

Abstract Measurements were performed to determine the action spectrum and dose dependence for photoreactivation of E. coli B_s-1 cells after γ-irradiation. The similarities between photoreactivation after UV- and after γ-irradiation in action spectra, kinetics, and other characteristics indicate that the increased survival of γ-irradiated cells after illumination with photoreactivating light is the result of true photoenzymatic repair.     

Sur les équations de la magnétothermoélasticité

Summary Writing the MTE equations in non-dimensional variables, the author points out the essential parameters and the structure of the equations. The equation of the MTE system characteristics is determined, as well as the displacement velocities of the waves of weak discontinuity. Furthermore, the equation which is satisfied by the current density vectorJ is determined.The paper considers the Cauchy problem for the infinite perfectly conductive plane, in the presence of a temperature and of a magnetic field. The general solution for the current density and the magnetic field are determined from Maxwell's equation. The solution obtained permits one to determine the displacement and the temperature fields by means of the classical thermoelasticity.The method of solving the Cauchy problem is based upon the representation of the general solution by a continous superposition of plane waves.     

13C nmr spectroscopy of tetracarbocyclic diterpenes and related substances1

A caarbon shift analysis of hibaene, phyllocladene, cafestol, cafamarine, mascaroside and hilbane-like substances is presented.