Search

The electron trapping or attachment cross section of carbon dioxide (CO2) condensed as thin films on a spacer of Ar is obtained using a simple model for electron trapping in a molecular film and then charge releasing from the same film by photon absorption. The measurements are presented for different electron exposures and impact energies, film thicknesses, and probing photon energies. The cross section for trapping an electron of incident energy between 0 and 5 eV reveals three different attachment processes characterized by a maximum at about 0.75 eV, a structured feature around 2.25 eV, and a shoulder around 3.75 eV. From the measurement of their dependence with the probing photon energy, the two lowest processes produce traps having a vertical electron binding energy of ∼3.5 eV, whereas the highest one yields a slightly higher value of ∼3.7 eV. The 0.75 eV maximum corresponds to the formation of vibrational Feshbach resonances in (CO2)n- anion clusters. The 2.25 eV feature is attributed to the formation of a vibrationally excited 2Πu anion in (CO2)n- clusters, followed by fast decay into its vibrational ground state without undergoing autodetachment. Finally, 3.75 eV shoulder is assigned to the well-known dissociative electron attachment process from 2Πu anion state producing the O- anion in the gas phase and the (CO2)nO- anions in clusters. [ABSTRACT FROM AUTHOR]

We propose an apparatus and method, based on the absorption of photon of energy ranging between 1.5 and 3.54 eV, to measure the absolute cross section for trapping low-energy electrons in a molecular film deposited on a metal substrate. A simple model is described to analyze electron trapping in the film and charge release from the same film by photon excitation. Measurements of the trapping of electrons in multilayer films of n-hexane are presented for different film thicknesses, electron exposures and incident energies, and probing photon fluxes and energies. The electron trapping cross section in a five-layer film of n-hexane is characterized by a large maximum reaching a value of (4.7±0.4)×10-17 cm2 at 10 eV and having a 2.3 eV full width at half maximum. This feature is ascribed to the formation of one or several core-excited resonances lying between 7 and 14 eV and leading, via dissociative electron attachment, to the formation of stable H- fragments, which stabilize within the first outerlayer of the film. [ABSTRACT FROM AUTHOR]

Aims: To analyse the exopolysaccharide (EPS) production by Lactobacillus helveticus ATCC 15807 in a chemically defined medium (CDM) and the effect of nutrients and stress culture conditions on cell growth and EPS formation. Methods and Results: Cultures were conducted in CDM: (i) containing essential and nonessential bases and vitamins; (ii) without nonessential bases and vitamins [Simplified CDM (SCDM)]; (iii) SCDM supplemented individually with vitamins and bases. The influence of carbohydrates, pH and osmotic culture conditions on growth and polymer formation was analysed. Adenine and lactose stimulated both growth and EPS production. Constant pH fermentations (4·5 and 6·2) did not improve EPS synthesis while NaCl and glycerol were detrimental for growth and polymer formation. In all media the EPS monomer composition was glucose and galactose (2·5 : 1). Conclusions: A SCDM containing adenine and lactose was optimal for cell growth and EPS formation by Lact. helveticus ATCC 15807. Controlled pH (6·2 and 4·5) and osmotic stress culture conditions did not improve polymer production. The EPS characteristics were identical in all media. Significance and Impact of the Study: This work provides a better knowledge on EPS synthesis by Lact. helveticus. A CDM to perform regulation studies on EPS production by Lact. helveticus species is now available. [ABSTRACT FROM AUTHOR]