Library

Notes: The energies and eigenfunctions of hyperfine levels belonging to the J=||K||=1 rotational manifold of the methylhalides have been calculated as a function of the strength of an applied electric (dc) field. From the eigenfunctions, the molecular orientational distribution function P(cos cursive-theta), where cursive-theta is the angle of the principal axis relative to the electric field, has been obtained. In the case of interest (J=1), the Legendre expansion of P(ρ), with ρ≡cos cursive-theta, can be characterized by two moments only: 〈P1〉 the orientation and 〈P2〉 the alignment. The calculated moments have been compared with data available from different molecular beam experiments, without recourse to a specific collision model. For the passage of the molecules through different electric fields, the adiabatic approximation has been assumed to apply, except for curve crossings with extremely small gaps. For methylfluoride and methylchloride, the calculated orientations are in agreement with the measured steric effects of the reactive collisions with Ca atoms, down to a field strength of a few volts per centimeter.At lower field strengths the orientation falls off more rapidly than predicted by the theory. This behavior is tentatively attributed to the variation in the direction of the electric field, and hence in the axis of quantization, across the reaction volume, at low field strengths. This may, in addition, cause a breakdown of the adiabatic approximation. In the case of methyliodide, quantitative agreement between the theoretical and the experimental asymmetry parameters of photodissociation is obtained if the latter ones are provisionally corrected for systematic errors. Contrary to earlier preliminary conclusions, the results show that within experimental error the adiabatic approximation is applicable. Up until now, no experimental data has been available for methylbromide. The calculations clearly demonstrate the inadequacy of restricting theoretical calculations to limiting cases, i.e., weak field and strong field, particularly because the Stark curves reveal the presence and the precise position of avoided crossings. Dependent on the field strength used for state selection and focusing, these crossings may, in principle, have a marked effect on the orientation obtained, given that the final orientation field at the reaction site remains the same. In addition, knowledge of the detailed Stark curves suggests a novel and feasible way to reverse the orientation of the molecules without interference with the detection system, which may be particularly useful if ionic products are formed.

Notes: The complexity of the absorption spectrum of NO2 can be attributed to a conical intersection of the potential energy surfaces of the two lowest electronic states, the electronic ground state of 2A1 symmetry and the first electronically excited state of 2B2 symmetry. In a previous paper we reported on the feasibility of using the hyperfine splittings, specifically the Fermi-contact interaction, to determine the electronic ground state character of the excited vibronic states in the region just above the conical intersection; 10 000 to 14 000 cm−1 above the electronic ground state. High-resolution spectra of a number of vibronic bands in this region were measured by exciting a supersonically cooled beam of NO2 molecules with a narrow-band Ti:Sapphire ring laser. The energy absorbed by the molecules was detected by the use of a bolometer. In the region of interest, rovibronic interactions play no significant role, with the possible exception of the vibronic band at 12 658 cm−1, so that the fine- and hyperfine structure of each rotational transition could be analyzed by using an effective Hamiltonian. In the previous paper we restricted ourselves to an analysis of transitions of the K−=0 stack. In the present paper we extend the analysis to transitions of the K−=1 stack, from which, in addition to hyperfine coupling constants, values of the A rotational constants of the excited NO2 molecules can be determined. Those rotational constants also contain information about the electronic composition of the vibronic states, and, moreover, about the geometry of the NO2 molecule in the excited state of interest. The results of our analyses are compared with those obtained by other authors. The conclusion arrived at in our previous paper that determining Fermi-constants is useful to help characterize the vibronic bands, is corroborated. In addition, the A rotational constants correspond to geometries that are consistent with the electronic composition of the relevant excited states as expected from the Fermi-constants. © 2000 American Institute of Physics.

Notes: The visible absorption spectrum of NO2 is very dense and irregular, and shows signs of a chaotic frequency and intensity distribution in the higher energy region. The complexity of the spectrum is related to a conical intersection of the potential energy surfaces of the two lowest electronic states. Above the conical intersection strong vibronic interactions lead to hybrid eigenstates, which can be viewed as mixtures of low vibrational levels of the electronically excited state and high vibrational levels of the electronic ground state. As a contribution to the elucidation of the nature of the vibronic bands of NO2 we have measured high-resolution spectra of a number of vibronic bands in the region between 10 000 and 14 000 cm−1 by exciting a supersonically cooled beam of NO2 molecules with a narrow-band Ti:Sapphire ring laser. The energy absorbed by the molecules was detected by a bolometer, and in some cases, laser-induced fluorescence was detected. The hyperfine structure is dominated by the Fermi-contact interaction and the magnitude of this interaction is a direct measure of the (electronic) composition of the hybrid eigenstates. In the present paper we have restricted our analysis to transitions of K−=0 stacks. The fine- and hyperfine structure of each rotational transition can be analyzed by using an effective Hamiltonian approach. The very good agreement that is found between the calculated transition strengths and the measured line intensities is evidence that in the spectral region studied, rovibronic interactions play a minor role. The composition of the hybrid eigenstates is compared with ab initio calculations reported in the literature, leading to the conclusion that measurements of the hyperfine structure are a helpful tool in characterizing vibronic bands. © 1998 American Institute of Physics.

Notes: The vibrational and rotational energy distributions of ground state SrF(X 2Σ) formed in the reactions of electronically excited Sr(3P1) with methylfluoride, ethylfluoride, and 1,1-difluoroethane have been studied by laser-induced fluorescence. Although the reactions of ground state Sr with these reactants are exothermic, no SrF products are observed for those reactions in this study. The fraction of available energy disposed into the sum of rotational and vibrational energy of the SrF(X 2Σ) product is approximately the same for all three reactions, i.e., 40%. The reaction of Sr(3P1) with CH3F results in very low vibrational excitation in the SrF reaction product. The product vibration increases in going to C2H5F and C2H4F2. It is concluded that the alkyl group influences the energy disposal mechanism in these reactions, and some suggestions are given for a partial explanation of the observations. © 1999 American Institute of Physics.

Notes: Laser-induced fluorescence spectra reveal the internal energy distributions of SrF(X 2Σ) formed in the reactions of electronically excited Sr(3P1) with various unsaturated fluorohydrocarbons, i.e., CHF(Double Bond)CH2, CF2(Double Bond)CH2, CHF(Double Bond)CHF, and C6H5F. The internal energy distribution of the ground state diatomic product typically shows less vibrational excitation, without inversion, and somewhat lower rotational excitation than the reactions of Sr(3P1) with HF and saturated hydrocarbons. The different behavior of the two groups of reactants is rationalized by a simple MO picture, assuming that an electron from Sr is transferred to a σ* orbital in HF and the saturated fluorohydrocarbons and to a π* orbital in the unsaturated fluorohydrocarbons with a subsequent transfer to a σ* orbital of the C–F bond. The latter transfer constitutes an extension of the reaction path, leading to less vibrational excitation. This would explain why the energy disposal in the reaction with C6H5F behaves similar to that in the reactions with the fluoroethenes. Even if the shape of the vibrational distribution of the SrF product is the same for all unsaturated fluorohydrocarbons studied, the degree of vibrational excitation varies strongly. This even holds when comparing cis- and transCHF=CHF, where the distributions can be characterized by distinct surprisal parameters. © 2002 American Institute of Physics.

Notes: Abstract Chara vulgaris L. growing in a brackish water lake was investigated in a field study during the main growth season (May to October 1985). Sucrose content and the ionic concentrations of the cations Na+, K+, Mg2+ and Ca2+ and the anions Cl− and SO42- of the vacuolar sap were estimated. Sucrose concentration in the vacuolar sap of vegetative growing plants was negligible, but with the beginning of the sexual reproduction period (fructification) the sucrose content increased from about 2 mol m−3 to 110 mol m−3. This level remained constant until the end of the fructification period. In spite of the increase of the sucrose concentration the osmotic potential of the vacuolar sap was constant. This was achieved by changing the ionic concentrations accordingly; in old or vegetative growing plants the ionic content accounted for about 80% of the vacuolar osmotic potential, but was about 63% during fructification. Sucrose is considered as a major photosynthate to supply the developing antheridia and oogonia and to serve as a precursor for the starch stored in the eggs.

Notes: 1. The effect of phosphate on species composition in biofilms was studied under three different phosphate regimes (0.5, 5 and 50 μm) in two different multi species communities: one composed of the four diatom species Melosira varians, Nitzschia perminuta, Navicula trivialis and Achnanthes lanceolata and one containing these diatom species plus the two cyanobacterial species Leptolyngbya foveolarum and Cylindrospermum stagnale.2. Algal growth in monocultures and mixtures was measured as chlorophyll a and PAM fluorimetry was applied to document density and physiological condition of the two main groups of photosynthetic organisms in mixed cultures.3. In phosphate-replete communities, a single species dominated the community (N. perminuta in the diatom mixture and L. foveolarum in the all species mixture), while in the phosphate-deprived communities several species persisted, in spite of severe phosphate limitation.4. We conclude that high supply of phosphate enables the species L. foveolarum, and to a lesser extent N. perminuta, to overgrow biofilm consortia, facilitated by their filamentous growth form, motility or the excretion of inhibitors. The persistence of several species under a low phosphate regime is explained by a less intense interspecific interaction in low-density biofilms. This clarifies field observations published previously.

Notes: Background : There is much debate about the influence of pre-treatment with a proton pump inhibitor on Helicobacter pylori eradication. The few studies investigating the influence of pre-treatment on triple and quadruple therapies did not find differences in eradication rates. However, the high eradication rates make it difficult to study factors associated with therapy failure in small populations. In order to overcome this problem we performed a meta-analysis.Methods : The literature was searched in order to identify randomized clinical trials comparing modern triple/quadruple therapies for H. pylori eradication without pre-treatment with a proton pump inhibitor with exactly the same regimen with pre-treatment. The overall risk difference (with − without pre-treatment) was calculated by pooling the risk differences of the individual studies weighted by the inverse of their variances.Results : Nine studies, investigating a total of 773 patients, were identified. There was considerable variation regarding therapy regimen and duration. Pooled eradication rates were 81.3% (312 of 384) for patients with pre-treatment and 81.2% (316 of 389) for patients without pre-treatment. The (weighted) overall risk difference was 0.1% (95% CI: −5%; 5%).Conclusion : Pre-treatment with a proton pump inhibitor does not influence H. pylori eradication.