Electron-ion recombination rate constant in dense gaseous Kr/CH4 mixtures

The rate constant of bulk electron-ion recombination is calculated for dense gaseous krypton doped with methane. In the calculations, the electron scattering is modeled by experimental, energy-dependent collision cross-sections. The recombination rate constant is found to increase with increasing methane concentration, due to efficient dissipation of electron energy in vibrationally inelastic e-CH₄ collisions.     

Ionization energies of argon clusters: a combined experimental and theoretical study

We have measured appearance energies of Ar(n)+, n相似文献   

Enhancement of the optical absorption in cholesteric liquid crystals due to photonic effects: an experimental study

An anomalous strong optical absorption was measured in a cholesteric liquid crystal (CLC) at both edges of its photonic band gap. The experiment was carried out by studying the luminescence generated by the CLC sample doped with a small amount of fluorescent dye. The material was excited with monochromatic light at different angles of incidence and polarisations. Clear peaks were found in the luminescence response at angles for which the pumping wavelength coincides with the positions of the gap edges. The effect is especially noticeable for excitation under circularly polarised light of the same handedness as that of the CLC helix, and it is the highest at the long-wavelength edge. The modification of the absorption is originated by the helicoidal (photonic) structure of the material, which drastically influences the propagation of electromagnetic waves at certain frequencies and polarisations. The results were analysed numerically using an extension of the Berreman method that incorporates absorption effects. Good agreement with the experiment was found.     

Influence of the potential on the intensity of depolarized light scattering ; in dense liquid argon

The depolarized light scattering intensity of argon at the triple point is calculated by molecular dynamics using the two-body Bobetic-Barker potential. The result differs from that obtained with a Lennard-Jones potential by no more than 15%.     

Electron recombination in ionized liquid argon: a computational approach based on realistic models of electron transport and reactions

In this work, we propose a new theoretical approach to modeling the electron-ion recombination processes in ionization tracks in liquid argon at 87 K. We developed a computer simulation method using realistic models of charge transport and electron-ion reactions. By introducing the concept of one-dimensional periodicity in the track, we are able to model very large cylindrical structures of charged particles. We apply our simulation method to calculate the electron escape probability as a function of the initial ionization density in the track. The results are in quantitative agreement with experiment for radiation tracks of relatively high ionization density. At low ionization densities, the simulation results slightly overestimate the experimental data. We discuss possible reasons for this disagreement and conclude that it can be explained by the role of δ tracks (short tracks of secondary electrons) in electron-ion recombination processes. We introduce an approximate model that takes into account the presence of δ tracks and allows the experimental data obtained from a liquid-argon ionization detector to be reproduced over a wide range of ionization density.     

Formation and characterization of the XeOO(+) cation in solid argon

This report presents the preparation and characterization of a xenon-containing cationic radical species, XeOO+. The XeOO+ cation was produced either by co-deposition of the reactive species generated by laser ablation of different transition metals with dioxygen and xenon mixtures in excess argon or by condensation of high-frequency discharged O2/Xe/Ar mixtures at 12 K and is identified by infrared absorptions. High-level quantum chemical calculations indicate that XeOO+ has a bent structure with direct xenon-oxygen dative bonding, and the doublet ground state is much more stable than the Xe + O2+ reactants.     

Isotope effects in tritium exchange between liquid pyrrole, pyrrolidine and gaseous hydrogen

The overall tritium separation factor between molecular hydrogen and liquid pyrrole and pyrrolidine has been measured between 280 and 325 K. The data are comparable with values of α measured for similar exchange reactions involving ammonia and methylamine. There is a visible correlation of the isotope effect with the energy of hydrogen bond formed by NH groups of liquid ammonia, methylamine, pyrrole and pyrrolidine.     

Estimation of the fraction of electrons escaping from recombination in the ionization of liquid argon with relativistic electrons and heavy ions

A new method is proposed to estimate the fraction of electrons escaping from recombination at zero electric field in liquid argon by using the electric field dependence of both scintillation and charge signals. The values obtained for the fraction are 0.35 for 1 MeV electrons and nearly zero for relativistic heavy ions.     

Mesoporous silica originating from a gaseous ammonia epoxide ring opening and the thermodynamic data on some divalent cation adsorptions

An organofunctionalized mesoporous HMS-like compound has been synthesized by reacting the silylating agent 3-glycidoxypropyltrimethoxysilane with gaseous ammonia. The reaction path leads to the opening of the three membered epoxide ring to incorporate ammonia to give the modified silylating agent. This new silylating agent was used to synthesize a mesostructure inorganic-organic hybrid through the neutral template directing agent, dodecylamine, using a co-condensation process, and exploring the ability of the silicon source tetraethoxysilane. The final solid named HMS-NH has been characterized through elemental analysis, X-ray powder diffraction, nitrogen gas adsorption, infrared spectroscopy and solid state NMR for the 29Si nucleus. An amount of 1.06+/-0.10 mmol of pendant groups is covalently bonded to the inorganic backbone. The attached basic centers adsorbed divalent cations to give the maxima adsorption capacity of 0.74+/-0.03, 0.55+/-0.06, 0.53+/-0.05 and 0.51+/-0.06 mmolg(-1) for copper, nickel, zinc and cobalt, respectively. From calorimetric determinations the quantitative thermal effects for all these cation/basic center interactions gave exothermic enthalpy, negative Gibbs free energy and positive entropy. These thermodynamic data confirmed the energetically favorable condition of such interactions at the solid/liquid interface for all systems.     

Absorption spectra and photochemistry of the toluene cation and benzyl radical in solid argon

Toluene diluted in argon subjected to continuous argon discharge radiation during condensation at 21 K revealed absorptions at 310.5 and 449.6 nm due to benzyl radical, and 317 nm due to a C₇7H₉ radical. A photosensitive 430 nm band, in agreement with photodissociation spectra of the toluene parent cation, is assigned to this species.     

Dissociative recombination of the weakly bound NO-dimer cation: cross sections and three-body dynamics

Dissociative recombination (DR) of the dimer ion (NO)(2) (+) has been studied at the heavy-ion storage ring CRYRING at the Manne Siegbahn Laboratory, Stockholm. The experiments were aimed at determining details on the strongly enhanced thermal rate coefficient for the dimer, interpreting the dissociation dynamics of the dimer ion, and studying the degree of similarity to the behavior in the monomer. The DR rate reveals that the very large efficiency of the dimer rate with respect to the monomer is limited to electron energies below 0.2 eV. The fragmentation products reveal that the breakup into the three-body channel NO+O+N dominates with a probability of 0.69+/-0.02. The second most important channel yields NO+NO fragments with a probability of 0.23+/-0.03. Furthermore, the dominant three-body breakup yields electronic and vibrational ground-state products, NO(upsilon=0)+N((4)S)+O((3)P), in about 45% of the cases. The internal product-state distribution of the NO fragment shows a similarity with the product-state distribution as predicted by the Franck-Condon overlap between a NO moiety of the dimer ion and a free NO. The dissociation dynamics seem to be independent of the NO internal energy. Finally, the dissociation dynamics reveal a correlation between the kinetic energy of the NO fragment and the degree of conservation of linear momentum between the O and N product atoms. The observations support a mechanism in which the recoil takes place along one of the NO bonds in the dimer.     

Calorimetry by immersion into liquid nitrogen and liquid argon: a better way to determine the internal surface area of micropores

The aim of this work is to assess the internal surface area of a set of samples (either carbons or oxides, either porous or nonporous, either microporous or mesoporous) by microcalorimetry via immersion into liquid nitrogen or argon. We have made use of an isothermal, heat-flux microcalorimeter, initially designed and built in our laboratory for the sake of gas adsorption experiments at 77 or 87 K. It seems that immersion calorimetry into liquid nitrogen and argon makes it possible to go one step further in the determination of the internal surface area of micropores.     

Evaluation of different implementations of the Thomson liquid drop model: comparison to monovalent and divalent cluster ion experimental data

The Thomson model, used for calculating thermodynamic properties of cluster ions from macroscopic properties, and variations of this model were compared to each other and to experimental data for both hydrated mono- and divalent ions. Previous models that used the Thomson equation to calculate sequential binding thermodynamic values of hydrated ions, either continuously or discretely including an ion-dipole interaction term, were compared to a discrete model that includes the excluded volume of an impurity ion. All models, given their limitations, provided reasonable agreement to data for monovalent ions. For divalent cluster ions, the continuous model, and a discrete model that includes the ion-exclusion volume provide significantly better agreement to both the binding enthalpy and the binding entropy data as compared to the model that includes an ion-dipole term. A systematic deviation in the continuous model resulted in significantly lower binding enthalpies than the discrete model for clusters with fewer than about nine and 19 water molecules for mono- and divalent ions, respectively, but this difference became negligible for larger clusters. Previous investigations of the various Thomson model implementations used parameters for bulk water at 313 K. Using parameters at 298 K has a negligible effect at small cluster sizes, but at larger sizes, the binding enthalpies are 0.2 kcal/mol higher than with the 313 K parameters. Although small, the effect is significant for ion nanocalorimetry experiments in which thermochemical information is obtained from the number of water molecules lost upon activating large clusters.     

Temperature selectivity effects in reversed-phase liquid chromatography due to conformation differences between helical and non-helical peptides

In order to characterize the effect of temperature on the retention behaviour and selectivity of separation of polypeptides and proteins in reversed-phase high-performance liquid chromatography (RP-HPLC), the chromatographic properties of four series of peptides, with different peptide conformations, have been studied as a function of temperature (5-80 degrees C). The secondary structure of model peptides was based on either the amphipathic alpha-helical peptide sequence Ac-EAEKAAKEX(D/L)EKAAKEAEK-amide, (position X being in the centre of the hydrophobic face of the alpha-helix), or the random coil peptide sequence Ac-X(D/L)LGAKGAGVG-amide, where position X is substituted by the 19 L- or D-amino acids and glycine. We have shown that the helical peptide analogues exhibited a greater effect of varying temperature on elution behaviour compared to the random coil peptide analogues, due to the unfolding of alpha-helical structure with the increase of temperature during RP-HPLC. In addition, temperature generally produced different effects on the separations of peptides with different L- or D-amino acid substitutions within the groups of helical or non-helical peptides. The results demonstrate that variations in temperature can be used to effect significant changes in selectivity among the peptide analogues despite their very high degree of sequence homology. Our results also suggest that a temperature-based approach to RP-HPLC can be used to distinguish varying amino acid substitutions at the same site of the peptide sequence. We believe that the peptide mixtures presented here provide a good model for studying temperature effects on selectivity due to conformational differences of peptides, both for the rational development of peptide separation optimization protocols and a probe to distinguish between peptide conformations.     

Coriolis interaction in the raman spectrum of gaseous and liquid fluoroform

A study on the depolarization ratio of Raman spectrum in the region of the ν₂ and ν₅ fundamentals of CHF₃ shows clearly that both bands interact by strong Coriolis coupling. The effect is very obvious in the gas, but is also demonstrated for liquid CH F₃.     

Effect of standard phase differences between gas and liquid and the resulting experimental bias in the analysis of gaseous volatile organic compounds

Liquid- or gas-phase standards can be used for the analysis of VOCs in air. Once the accuracy is secured in the standard preparation stage, the use of gas-phase standard should be more reliable with the least matrix effect. However, it is not difficult to find that the liquid-phase standard is used more preferably in many laboratories for several reasons (e.g., low expense, easy handling, etc.). As such, one needs to accurately evaluate any possible bias stemming from the use of different standard phases. To this end, standards for 8 VOCs consisting of 4 aromatic compounds (benzene (B), toluene (T), styrene (S) and p-xylene (p-X)) and 4 others (methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK), butyl acetate (BuAc), and isobutyl alcohol (i-BuAl)) were prepared in both liquid and gas phases. Each standard was analyzed by the initial collection on the adsorption tube and by the combined application of thermal-desorption–gas chromatography–mass spectrometry (TD/GC/MS). The results indicated that experimental bias between the two phases, if expressed in terms of percent difference (PD), was very low in many target VOCs (B (1.09%), T (2.41%), p-X (3.64%), MEK (6.76%), and MIBK (0.17%)), while it was not in some targets (e.g., >10%: e.g., S, i-BuAl, and BuAc). In an ancillary experiment, biases were evaluated further by (1) calibrating gaseous samples against liquid phase standard and via (2) comparison between two different types of gas phase standards. In conclusion, treatment of different standards (e.g., between the same or different phases) will inevitably induce biases in most VOCs, although certain volatiles (e.g., benzene, MIBK, etc.) are virtually unaffected by such variables in a practical sense.     

Theoretical and experimental studies of electrostatic effects in reversed-phase liquid chromatography

Electrostatic effects are important in many reversed-phase liquid chromatographic separations involving surfactants and ion-pair reagents. To give better understanding of such systems the Poisson-Boltzmann equation has been solved for two cases. The first is that of an infinitely long cylinder with charge density on the inner wall, and two phases in the interior, one in the center and the other around it. Salts with ions of various solubilities in each phase may be present. The solution is obtained for the low-potential case (Deltapsie / kT 1). The second case is that of a planar sandwich, wall/phase 1/phase 2/phase 1/wall, with the same chemistry as case 1 but without the low-potential restriction. In case 2, there is also a Langmuir adsorption isotherm for each ion. The theory shows that the information gained from the Stern-Gouy-Chapman theory, which is mathematically semi-infinite, only applies when the diffuse layer thickness is much smaller than the pore radius, an unlikely circumstance in chromatography. The volume-averaged electrostatic potential difference can be measured experimentally with the solutes ?(4)Si, ?(4)As(+), ?(4)B(-) (? = phenyl). The results are in agreement with theory. They show that the electrostatics of the system is dominated by fixed charges on the silica when the salt present is insoluble in the stationary phase, but when ions with some solubility in the stationary phase are used, mixed effects are obtained.     

On the use of different potential energy functions in rare-gas cluster optimization by genetic algorithms: application to argon clusters

We study the effect of the potential energy function on the global minimum structures of argon clusters arising in the optimization performed by genetic algorithms (GAs). We propose a robust and efficient GA which allows for the calculation of all of the putative global minima of Ar(N) (N = 3-78) clusters modeled with four different potentials. Both energetic and structural properties of such minima are compared among each other and with those previously obtained for the Lennard-Jones function. In addition, the possibility of obtaining global minima of one potential through local optimization over the corresponding cluster geometry given by other potentials was associated with some structural parameters. The influence of the contribution from the three-body (Axilrod-Teller-Muto) triple-dipole potential (including or not a damping function to describe its correct behavior at smaller interatomic distances) has also been analyzed.     

Temperature and structural changes of water clusters in vacuum due to evaporation

This paper presents a study on evaporation of pure water clusters. Molecular dynamics simulations between 20 ns and 3 micros of clusters ranging from 125 to 4096 molecules in vacuum were performed. Three different models (SPC, TIP4P, and TIP5P) were used to simulate water, starting at temperatures of 250, 275, and 300 K. We monitored the temperature, the number of hydrogen bonds, the tetrahedral order, the evaporation, the radial distribution functions, and the diffusion coefficients. The three models behave very similarly as far as temperature and evaporation are concerned. Clusters starting at a higher temperature show a higher initial evaporation rate and therefore reach the point where evaporation stop (around 240 K) sooner. The radius of the clusters is decreased by 0.16-0.22 nm after 0.5 micros (larger clusters tend to decrease their radius slightly more), which corresponds to around one evaporated molecule per nm(2). The cluster temperature seems to converge towards 215 K independent of cluster size, when starting at 275 K. We observe only small structural changes, but the clusters modeled by TIP5P show a larger percentage of molecules with low diffusion coefficient as t-->infinity, than those using the two other water models. TIP4P seems to be more structured and more hydrogen bonds are formed than in the other models as the temperature falls. The cooling rates are in good agreement with experimental results, and evaporation rates agree well with a phenomenological expression based on experimental observations.