Orientation dependence of high-order harmonic generation in molecules

Published versio

Femtosecond laser time-of-flight mass spectrometry of labile molecular analytes: laser-desorbed nitro-aromatic molecules

Femtosecond laser time-of-flight mass spectra of solid samples of trinitrobenzene (TNB), trinitrotoluene (TNT) and trinitrophenol (TNP) have been recorded. Desorption of the solid samples was enacted by the fourth harmonic output (266 nm) of a 5 ns Nd:YAG laser. Subsequent femtosecond post-ionisation of the plume of neutral molecules was achieved using 800 nm laser pulses of 80 fs duration. Mass spectra have been recorded for desorption laser intensities from 2-6 × 109 W cm-2 with ionisation laser intensities between 2 × 1014 and 6 × 1015 W cm-2. Femtosecond laser ionisation has been shown to be capable of generating precursor and characteristic high-mass fragment ions for labile nitro-aromatic molecules commonly used in high-explosive materials. This feature is critical in the future development of femtosecond laser-based analytical instruments that can be used for complex molecular identification and quantitative analysis of environmentally important labile molecules. Furthermore, a comparison of femtosecond post-ionisation mass spectra with standard 70 eV electron impact data has revealed similarities in the spectra and hence the fragmentation processes

Finite element analysis of a shear punch test

The authors present new experimental data for shear punch tests and tensile tests using four different materials. The correlation between shear punch and tensile yield stresses must be established empirically, and the procedure for doing this is discussed. The elastic-plastic deformation in a shear punch test develops gradually with increasing punch displacement and is often assumed to be simple shear. This is examined using finite element analysis to simulate the development of plasticity during the early stages of punch displacement. The simulation results are used to rationalize the empirically established yield stress correlations

Ultrafast laser time-of-flight mass analysis of laser-desorbed atoms and molecules

Femtosecond time-of-fight mass spectra of laser-desorbed gallium arsenide (GaAs) and trinitrobenzene (TNB) reveal the characteristic features and differences of femtosecond atomic and molecular ionisation. Significant yields of multiply-charged atomic fragments, parent molecular ions and structure-specific fragment ions of labile molecules have been obtained. An optimum desorption laser intensity for the transfer of neutral intact parent molecules in to the gas phase has been observed to be molecule-dependent. This work demonstrates the potential of ultrafast laser ionisation and has significant implications for analytical and environmental studies of solid materials

An investigation of the angular distributions of fragment ions arising from the linear CS₂ and CO₂ molecules

The nonlinear interaction of the triatomic molecules CS<sub>2</sub> and CO<sub>2</sub> molecules with the intense field of a linearly polarized laser beam of femtosecond (fs) pulse duration, was used to study the ionization and dissociation of the parent molecule. The fragment ion angular distributions arising from the Coulomb explosion of the parent ions were also measured. For CS<sub>2</sub>, the angular distributions of CS<sub>2</sub>+, CS<sub>2</sub><sup>2</sup>+, CS<sub>2</sub><sup>3</sup>+, CS+, CS<sup>2</sup>+, S<sup>n</sup>+ (n less than or equal to 6) and C<sup>m+</sup> (m less than or equal to 4) ions are presented for a laser intensity of 1 x 10<sup>16</sup> W cm<sup>-2</sup> at a wavelength of 790 nm and pulse duration of 50 fs. The angular distributions of the parent molecular ions are all isotropic. The S<sup>n+</sup>fragments are peaked along the time-of- flight (TOF) axis, whereas the C<sup>m+</sup> fragments explode perpendicularly to this. Similar results for CO<sub>2</sub> are also presented for comparison. The S ion distributions do nor narrow as their ionic charge increases, and it is argued that the angular distributions for CS<sub>2</sub> are due mainly to the angular dependence of the ionization probability On the other hand, the distributions from the lighter CO<sub>2</sub> molecule are thought to be at least partly due to alignment via dipole moments induced by the laser, as in this case the O<sup>n+</sup> angular distributions are seen to narrow as their charge increases. The conclusion of these results is that the laser pulse may be too short for the CS<sub>2</sub> molecule to align in the pulse. Angular distributions are also presented for varying laser pulse durations, in the range of 50 fs to 300 ps. The dynamics of the ionization/dissociation mechanism are discussed in the context of the TOF mass spectra and angular distributions recorded for CS<sub>2</sub>

Differentiation of Colletotrichum gloeosporioides isolates by using total proteins and esterase electrophoretic patterns and extracellular enzymes production Diferenciação de isolados de Colletotrichum gloeosporioides por meio de padrões eletroforéticos de proteínas totais e isoesterase, e produção de enzimas extracelulares

Isolates of Colletotrichum gloeosporioides (ISO-1, ISO-2, ISO-3, ISO-4, ISO-5 and ISO-6), the causal agent of anthracnose disease on mango fruits, were characterized by electrophoretic patterns of total proteins and esterase in polyacrylamida gel, and also, by production of extracellular enzymes on specific solid substrate. The electrophoretic analysis showed variation in number, intensity of coloration and position of the bands in the gel at each studied system tested. In contrast to the monomorphic behavior to total proteins, high esterase polymorfism was observed indicating difference among isolates. All isolates showed the activity of extracellular enzymes such as amylase, lipase, and protease with some variation among them. The proteolitic activity seemed to be more accentuated than the two other enzymes studied.<br>Isolados de Colletotrichum gloeosporioides (ISO-1, ISO-2, ISO-3, ISO-4, ISO-5 e ISO-6), agente causal da antracnose em frutos de mangueira, foram caracterizados por meio de padrões eletroforéticos de proteínas totais e esterase, em gel de poliacrilamida e produção de enzimas extracelulares, em substratos sólidos específicos. A análise eletroforética mostrou variação no número, intensidade de coloração e posição das bandas no gel, dentro dos dois sistemas estudados. Verificou-se polimorfismo em relação a esterase, mostrando maior diferença entre os isolados, enquanto que nas proteínas totais, observou-se comportamento aparentemente uniforme. Quanto à produção de enzimas extracelulares: amilase, lípase e protease, todos os isolados, embora variando em comportamento, apresentaram atividade para essas enzimas, sendo aparentemente mais acentuada à atividade proteolítica