Metastable ion studies: XIII—the measurement of appearance energies of metastable peaks

A simple, accurate and reproducible method is described for measuring the appearance energy of a metastable peak. The method involves the comparison of the metastable peak intensity with that of a ‘standard’ metastable peak, over a small range of ionizing electron energies above their onsets. Six fragmentation processes whose reaction energetics are well established were selected to provide suitable calibrant metastable peaks. Results are also presented for fragmentations having large kinetic shifts and for some primary and secondary ion decomposition of current interest.     

Metastable cluster intermediates in the condensation of charged macromolecule solutions

The authors examine the possibility of a two-step nucleation to the bulk condensation transition that proceeds via a metastable liquid cluster intermediate having some preferred size. The metastable intermediate is stabilized by electrostatic repulsion, which becomes screened by small mobile ions at sufficiently large cluster sizes, thus allowing the eventual condensation to a bulk phase. Our calculation employs a capillary model for the cluster and the electrostatic interactions are treated using the Poisson-Boltzmann approach. Condensation via this metastable intermediate may be a very general phenomenon which applies not only to solutions of charged particles (e.g., proteins, colloidal particles, and polyelectrolytes) but to any system involving short-range attraction and long-range repulsion undergoing macrophase separation in which a metastable microphase separation is also possible.     

Modeling contact angle hysteresis of a liquid droplet sitting on a cosine wave-like pattern surface

A liquid droplet sitting on a hydrophobic surface with a cosine wave-like square-array pattern in the Wenzel state is simulated by using the Surface Evolver to determine the contact angle. For a fixed drop volume, multiple metastable states are obtained at two different surface roughnesses. Unusual and non-circular shape of the three-phase contact line of a liquid droplet sitting on the model surface is observed due to corrugation and distortion of the contact line by structure of the roughness. The contact angle varies along the contact line for each metastable state. The maximum and minimum contact angles among the multiple metastable states at a fixed viewing angle correspond to the advancing and the receding contact angles, respectively. It is interesting to observe that the advancing/receding contact angles (and contact angle hysteresis) are a function of viewing angle. In addition, the receding (or advancing) contact angles at different viewing angles are determined at different metastable states. The contact angle of minimum energy among the multiple metastable states is defined as the most stable (equilibrium) contact angle. The Wenzel model is not able to describe the contact angle along the three-phase contact line. The contact angle hysteresis at different drop volumes is determined. The number of the metastable states increases with increasing drop volume. Drop volume effect on the contact angles is also discussed.     

Metastable states in the system of hard spheres

Approximate equations of the theory of liquids were applied to analyze the special features of the structural behavior of the system of hard spheres in the region of densities at which crystals are stable. Correlation functions for the liquid metastable state were obtained. The correlation functions of crystal-like metastable states that can exist under the same conditions as liquid metastable states are also considered. The reliability of the results is substantiated by a thermodynamic comparison with the data obtained using analytic equations of state.     

A multiple replica approach to simulate reactive trajectories

A method to generate reactive trajectories, namely equilibrium trajectories leaving a metastable state and ending in another one is proposed. The algorithm is based on simulating in parallel many copies of the system, and selecting the replicas which have reached the highest values along a chosen one-dimensional reaction coordinate. This reaction coordinate does not need to precisely describe all the metastabilities of the system for the method to give reliable results. An extension of the algorithm to compute transition times from one metastable state to another one is also presented. We demonstrate the interest of the method on two simple cases: A one-dimensional two-well potential and a two-dimensional potential exhibiting two channels to pass from one metastable state to another one.     

Metastable dissociation dynamics of molecular cluster ions

Metastable uni-cluster dissociation for several hydrogen-bonded and van der Waals cluster ions are observed via resonance-enhanced two-photon ionization reflectron time-of-flight (TOF) mass spectrometry. All of the cluster ions studied show evaporation of a single molecule from the respective parent cluster ions as dominant metastable decay processes. Furthermore, the averaged metastable evaporation rate constants (k _evap) of these cluster ions in a fixed time domain of 0.2–50 µs are obtained by analyzing the relative intensity of metastable ion peaks due to evaporation in the acceleration and the field-free drift regions of the TOF mass spectrometer. An intensity anomaly in some of the observed metastable ion peaks, indicative of magic number stability of the cluster ion, is also presented.     

Thermal Analysis of Solid-Solid Interactions in Binary Mixtures of Alkylcyclohexanes Using DSC

Differential scanning calorimetry (DSC) was used to construct phase diagrams of binary mixtures of alkylcyclohexanes and to characterize metastable phases formed in the binary mixtures. The experimentally measured liquidus curves were compared to the liquidus curves calculated using ideal solution theory. The measured phase diagrams of pentadecylcyclohexane/nonadecylcyclohexane and octadecylcyclohexane/nonadecylcyclohexane binary mixtures are consistent with theoretical phase diagrams constructed based on the assumption that these mixtures form eutectic systems. It was also observed that a metastable phase formed in some binary mixtures of pentadecylcyclohexane/nonadecylcyclohexane under fast cooling conditions. It is hypothesized that this metastable phase recrystallizes into the eutectic phase upon heating. This revised version was published online in July 2006 with corrections to the Cover Date.     

碳钢在NaNO_2-NaCl溶液中的电流波动

用动电位和恒电位极化法研究了A3碳钢在NaNO2＿NaCl溶液体系中的电流波动特征.A3钢在不含Cl-的0.1mol/LNaNO2溶液中能保持良好的钝态,当加入一定浓度的Cl-后,电流出现了明显的快速上升,再缓慢下降的波动特征,表明此时碳钢表面生成了溶解较快而再钝化相对较慢的亚稳态孔蚀.实验中可观察到因电流波动而产生的累积腐蚀损伤的蚀孔.实验表明,随着Cl-浓度的增加,出现亚稳态孔蚀的初始电位Em降低,相应的电流波动峰值增大,峰频增加;而升高电位,则电流波动的峰值和峰频也都增加,况且再钝化时间延长,由此可见电位的升高促进了亚稳态孔的溶解,并能激活更多的活性点使孔诱发速率增大.     

Spontaneous transformation of water's high-density amorph and a two-stage crystallization to ice VI at 1 GPa: a dielectric study

Dielectric relaxation spectra of a metastable crystal phase formed on implosive and exothermic transformation of pressure-amorphized hexagonal ice have been measured in situ at 0.97 GPa pressure over a range of temperature. The metastable phase showed no relaxation peak at 130 K and 0.97 GPa. When heated at a fixed pressure of 0.97 GPa, it began to transform at approximately 145 K exothermally to a phase whose relaxation rate and equilibrium dielectric permittivity increased. A second, but slower exothermic transformation also occurred at approximately 175 K. After keeping at 213 K, the relaxation rate and equilibrium permittivity reached the known values of these two quantities for ice VI. Thus the metastable phase transformed to ice VI in two stages. It is conjectured that the intermediate phase in this transformation could be ice XII. The rate of transformation is not determined by the reorientational relaxation rate of water molecules in the ices.     

Inelastic neutron scattering in the glassy and metastable phases of methoxy-benzylidene-butyl-aniline

The inelastic neutron scattering (INS) spectra were measured in the glassy and in the metastable phases of methoxy-benzylidene-butyl-aniline. For comparison, the spectrum of benzylidene-aniline was also recorded. Complementary information was provided by Raman scattering measurements which analyzed more accurately the vibrational motions. Strong differences between the INS and Raman spectra were evidenced. The assignment of some lines is discussed to elucidate the respective role of the core and the tail of the molecule in the appearance of the successive metastable phases.     

Factors that influence the observed fast fragmentation of peptides in matrix-assisted laser desorption

Fragmentation processes that occur very early during matrix-assisted laser desorption ionization (MALDI) of peptides are examined by utilization of delayed pulsed ion extraction with a linear time-of-flight mass spectrometer. The oxidized B chain of bovine insulin (MW=3495. 95 u), which produces a wide range of fragment ions, is utilized as a probe to examine the effects of several experimental parameters on this process. Experimental evidence suggests that this MALDI process is not prompt fragmentation and involves metastable ion decay that is quite different from that which is observed with postsource decay experiments. This conclusion is based upon the significant differences observed in the fragmentation products produced by the two techniques. This metastable ion decay process also appears to be over within the minimum pulse delay period (320 ns) that is possible with the current pulsed ion extraction hardware. These two observations suggest that either different activation processes are involved in the two techniques or that the much different time frame of the methods influences the observed ion decay pathways. This fast MALDI metastable ion fragmentation also is shown to be influenced by both the MALDI matrix and the laser fluence.     

Water loss from [C5H10O]+˙ isomers: A reassessment

From a detailed analysis of metastable peak shapes it is proposed, contrary to earlier conclusions, that the loss of H₂O from the molecular ions of the C₅H₁₀O isomers cyclopentanol, pentanal and pent-1-en-3-ol yields only [penta-1,3-diene]^+˙ in the metastable time frame. Therefore the composite metastable peak for this process arises from two competing reaction channels yielding a common daughter ion. The observation that the first two isomers also produce [penta-1,4-diene]^+˙ as a daughter ion is attributed to a high energy (ion source) reaction which contributes negligibly to first field free region fragmentations.     

Electron impact induced fragmentations of some 2,2-disubstituted 1,3-dithiolanes

Electron impact induced fragmentations of the title compounds were studied by exact mass measurement and metastable ion analysis. Sulphur atoms within the ring effectively stabilize the positive charge. In most cases the loss of the larger substituent, and not the other, methyl group, gives rise to the base peak in the spectrum. Examination of competing metastable transitions shows that generally this is also the lowest activation energy primary process. In general primary ring cleavage reactions are not important unless there is a heteroatom in the substituent that can assist this cleavage.     

Pre-treatment of multi-walled carbon nanotubes for polyetherimide mixed matrix hollow fiber membranes

A facile and bio-inspired route for the preparation of pure and highly crystalline metastable α-AgVO(3) is presented. Three kinds of proteins (bovine hemoglobin, bovine serum albumin, and lysozyme) were employed as inducer, which had substantial effects on the nucleation and growth of α-AgVO(3). Moreover, the amount of proteins also played a key role over the morphology and crystalline of products. The VO(3)(-)/protein complex acted as a driver to induce the formation of metastable phase, which was confirmed by resonance Rayleigh scattering and UV-vis absorption spectra. The results indicated that tailoring an interaction between protein and inorganic molecules was the key in bio-inspired selective synthesis of metastable phase, which may find applications in the design of other new functional inorganic materials.     

On the use of scans at a constant ratio of B/E for studying decompositions of peptide metal(II)-ion complexes formed by electrospray ionization

The use of sector mass spectrometers to study metastable ion decompositions of peptide metal-ion complexes formed by electrospray ionization is discussed. Products that are formed by charge-separation reactions are characterized by large kinetic energy release distributions. This causes scans at a constant B/E to give incorrect product ion abundances and possibly incorrect mass assignments. Two instrumental methods exist that can be used either to detect the ions or to estimate relative ion abundances: a floated collision cell or mass-analyzed ion kinetic energy spectrometry (MIKES) scans. The floated collision cell, by virtue of an altered B/E scan law, however, discriminates against important metastable ion reactions that occur outside the cell. MIKES scans provide a clearer estimate of product ions that arise by metastable ion charge-separation reactions. Problems with pseudotandem (first field-free region) experiments are also discussed.     

The scope of metastable peak observations

This short review focuses attention upon the present status of metastable ion studies with emphasis upon the relationship between metastable peak shapes, ion structur and fragmentation mechanisms. Some recommendations are made concerning nomenclature and the reporting of observations on Gaussian-type metastable peaks. Experimental methods for recording relative abundances of metastable peaks are critically appraised. The relationship between metastable ion phenomena and isomerization of gaseous ions is reviewed with particular attention drawn to the effect of rate-determining isomerizations. The shapes of Gaussian-type metastable peaks are discussed in some detail and selected examples from recent studies are used to show that such peaks may, by appropriate experiments, be separated into two Gaussian-type components thus revealing new features of the fragmentation reaction. The magnitude and significance of released kinetic energies, T, are considered and it is stated that few conclusions can be drawn from the evaluation of T alone; the importance of accurate thermochemical data as an aid to understanding and interpreting kinetic energy release data is emphasized. Other topics discussed include composite metastable peaks, metastable peaks produced in chemical ionization and field ionization and the partitioning of internal energy of the fragmenting ion into translational degrees of freedom of the products, for reactions with and without a reverse energy barrier.     

Infrared spectroscopy of the solid phases of ammonia

Thin films of solid ammonia (NH(3) and ND(3)) have been characterized using low temperature (25-110 K) Fourier-transform infrared (FTIR) spectroscopy, and the three solid phase (amorphous, metastable, and crystalline) spectra are reported. This work has been motivated by confusion in the literature about the metastable and crystalline phases as a result of an early erroneous report by Staats and Morgan [(J. Chem. Phys. 31, 553 (1959)]. Although the crystalline phase has subsequently been reported correctly, the metastable phase has not been described in the literature in detail. The unique characteristics of the metastable phase, reported here for the first time, include multiple peaks in the nu(2) and nu(3) regions and peak intensities that are dependent on the deposition temperature. This behavior may be the result of (a) preferential molecular orientations in the solid, or (b) exciton splitting due to different crystal shapes in the solid. The amorphous and metastable phases of deuterated ammonia are also reported for the first time.     

Thermal properties of the metastable supersaturated vapor of the Lennard-Jones fluid

p, rho, T data of the supersaturated vapor of the Lennard-Jones fluid are obtained by molecular dynamics simulations. The metastable state points are identified before a phase separation takes place. An estimation of the location of the spinodal is given. The results are compared to two theoretically based equations of state and one empirical equation of state which was parametrized also taking into account metastable state points. The pressure obtained by simulation is found to be lower than that from both theoretically based equations of state, which do not account for the inhomogeneous density distribution of the supersaturated vapor.     

Phase-controlled growth of metastable Fe5Si3 nanowires by a vapor transport method

We report the synthesis of single-crystalline nanowires (NWs) of metastable Fe5Si3 phase via an iodide vapor transport method. Free-standing Fe5Si3 NWs are grown on a sapphire substrate placed on a Si wafer without the use of any catalyst. The typical size of the Fe5Si3 nanowires is 5-15 microm in length and 100-300 nm in diameter. Synthesis of the metastable phase is induced by composition-dependent nucleation from the gas-phase reaction. Depending on the concentration ratio of FeI2(g) to SiI4(g), different phases of iron silicides are formed. The growth of nanowires is facilitated by the initial nucleation of silicide particles on the substrate and further self-seeded growth of the NWs. The present work not only provides a method for the synthesis of metastable Fe5Si3 nanowires but also suggests that the phase controlled synthesis can be further optimized to produce other metal-rich silicide nanostructures for future spintronic devices.     

The effect of Thermal History on the Morphology of Nickel sulphide

The DSC characterisation of the morphology of the metastable a phase of stoichiometric nickel sulphide was carried out using two calorimeters; a TA Instruments 2920 MDSC and a Perkin Elmer DSC-7, and two quenching histories. Based on these quenching histories, significant differences were observed in the heat flow curves, including the observation of a second exothermic peak which is tentatively assigned to be a metastable phase to metastable phase transformation. The kinetic constants for the a to b recrystallisation were determined as a function of degree of conversion using a mechanism free isoconversional model. Variations in the values of the kinetic constants were also ascribed to the quenching histories. Although the differences in morphology observed were ascribed to the processing history, the shift in the position of the a to b recrystallisation peak was partially attributed to the thermal resistances of the instruments used. This revised version was published online in July 2006 with corrections to the Cover Date.