Gas-phase Collisions Research Articles

On the role of fluctuations, cathode placement, and collisions on the transport of electrons in the near-field of Hall thrusters

The performance of Hall thrusters can be highly sensitive to the position and operational parameters of the external cathode, hinting that the electron transport in the near-field is strongly dependent on the emitted electrons’ initial properties. In addition, the plasma plumes of Hall discharges often exhibit fluctuations which are expected to alter electron trajectories. By implementing recent near-field plasma potential measurements made on a low-power Hall thruster in 3D electron-trajectory simulations, it is shown that electron transport from the external cathode to the thruster channel is strongly sensitive to cathode parameters including position, orientation, and electron emission divergence. Periodic, low-frequency (i.e., 25 kHz) plasma potential fluctuations reduce electron transport to the channel of the thruster by more than 65% compared to the transport achieved with static 3D fields and substantially homogenize the electron density distribution. Additional gas-phase collisions are found to have only marginal effects, even when prescribed to occur at exaggerated rates (reaching 10 MHz). The three-dimensionality of the E and B fields, together with electron-wall collisions, appear to be important drivers of cross-field transport in this region of the discharge, yielding sufficient levels of electron transport to the channel without invoking plasma turbulence.

Comparative NMR Properties of H2and HD in Toluene-d8and in H2/HD@C60

Spin-lattice relaxation times, T(1), have been measured from 200-340 K for the protons in H(2) and HD molecules dissolved in toluene-d(8) and incarcerated in C(60). It is found that HD relaxes more slowly than H(2) in both environments and at all temperatures, as expected from the smaller values of the spin-rotation and dipole-dipole coupling in HD compared to H(2). More detailed analysis using models developed to describe relaxation in both condensed media and the gas phase indicates that transitions among the rotational states of H(2) occur at a rate similar to those of HD in both toluene-d(8) solution and in C(60), in contrast to the situation in gas phase collisions between hydrogen and He or Ar, where the lifetimes of rotational states of HD are markedly shorter than those for H(2). Measurements of the relative (1)H chemical shifts of H(2) and HD, the coupling constant J(HD), and the widths of the HD peaks at various temperatures revealed only small effects with insufficient accuracy to warrant more detailed interpretation.

Vibrational relaxation of NO−(v=1) in icosahedral (Ar)12NO− clusters

Relaxation dynamics of NO(-)(v=1) in icosahedral (Ar)(12)NO(-) clusters are studied using classical dynamics and semiclassical procedures over the temperature range of 100-300 K. The minimum energy of the equilibrium configuration (-9875 cm(-1)) needed in the study is determined by varying the cluster size z in (Ar)(z)NO(-). NO(-)(v=1) is embedded in the cluster, which is filled with low frequency motions: 39 cm(-1) for the argon modes, 77 cm(-1) for the Arc...NO(-) substructure vibration, 109 cm(-1) for the librational frequency of restricted rotation, and 128 cm(-1) for oscillatory local translation. Dynamics calculations show that in the early time period (<20 ps), part of the vibrational energy rapidly transfers to rotation, but most energy transfers to Ar atoms on a long time scale (approximately 1 ns). The long time scale leads to the relaxation rates of 0.403 ns(-1) at 100 K and 0.453 ns(-1) at 300 K. The rates calculated using analytical formulations vary nearly linearly from 0.288 ns(-1) at 100 K to 0.832 ns(-1) at 300 K. Although the temperature dependence is stronger in the latter, both approaches give the rates on a nanosecond time scale. The principal energy transfer pathway is from NO(-) vibration to Ar vibrations via oscillatory local translation, while the NO(-) rotation is in a librational state. The energy transfer probabilities are two orders of magnitude larger than the vibration-to-translation probabilities in the gas phase collision Ar-NO(-)(v=1).

Stability of pure, mixed and nanohydrated clusters of small biomolecules

A large number of studies are devoted to the investigation of the biomolecular ionization and fragmentation dynamics underlying biological radiation damage. Most of these studies have been based on gas-phase collisions with isolated DNA building blocks. The radiobiological significance of these studies is often questioned because of the lack of a chemical environment. We report on experiments, performed in the gas phase, using clusters of these small molecules with the aim to simulate environmental effects. The discussion is focused on the influence of this simple biochemical environment on the cluster stability and the modifications of the fragmentation processes.

Passive and Active Fragment Ion Mass Defect Labeling: Distinct Proteomics Potential of Iodine-Based Reagents

The exact mass of a peptide differs characteristically from its nominal mass by a value called the mass defect. Limited by possible elemental compositions, the mass defect of peptides has a restricted range, resulting in an unoccupied mass spectral space in every mass-to-charge unit. The method of fragment ion mass defect labeling (FIMDL) places characteristic fragment ions of modified peptides as reporters into unused spectral space where no native peptide fragment ions exist. In this labeling method, peptides are chemically modified in solution and the modified peptides, upon gas-phase collision in a mass spectrometer, generate fragment ions with significantly shifted mass defects. In this work, the efficiency of iodine stable isotope-containing reagents for shifting mass defects of peptide fragment ions was systematically investigated, through derivatization of peptide N-termini with various reagents containing one or more chlorine, bromine, or iodine atoms. The observed efficiency for the iodine atom placing the labeled fragment ions into unoccupied spectral space agreed well with theoretical predictions from averagine-scaling analysis of ion masses. On the basis of the gas-phase stability of different labeling groups and their involvement in collisional dissociation of modified peptides, peptide modifications were classified into three categories: passive, type I active, and type II active. Each modification type has its unique potential in different proteome analyses. Possible proteomics applications of FIMDL are discussed and compared with proteome analyses currently being practiced in the field. Principles obtained from this survey study will provide a guideline in developing novel FIMDL reagents for advanced proteomics analysis.

NH4+ + CH4 Gas Phase Collisions as a Possible Analogue to Protonated Peptide/Surface Induced Dissociation

Results are reported for a direct dynamics simulation of NH(4)(+) + CH(4) gas phase collisions. We interpret the results with protonated peptide/hydrogenated alkanethiolate self-assembled monolayer (H-SAM) surface collisions in mind. Previous theoretical studies of such systems have made use of nonreactive surfaces, and therefore, our goal is to investigate the types and likelihood of peptide/H-SAM reactions. In that vein, the NH(4)(+) + CH(4) reaction represents a simple gas phase system which includes many of the important interactions present in protonated peptide/H-SAM surfaces. Thirty-seven open pathways are seen in the 5-35 eV collision energy range. An energy dependence on the likelihood of forming CN bonds is found. This type of bonding could deposit both the peptide and its molecular fragments on the H-SAM surface. For our gas phase collision system, around 50% of the trajectories result in the formation of CN bonds. For all collision energies in which reactive scattering occurs, CN bond formation is an important reaction pathway.

Gas Phase Conditions for Obtaining Device Quality Amorphous Silicon at Low Temperature and High Deposition Rate

AbstractThe aim of this paper is to find a parameter space for deposition of amorphous silicon films at low substrate temperature by VHF PECVD process for application in solar cell fabrication on cheap plastics. Our studies show that at lower substrate temperature, keeping the pressure constant, the ion energy flux reaching the growth surface decreases, which we partly attribute to increasing gas phase collisions arising from an increase in gas density. The role of hydrogen is two fold: (1) higher hydrogen dilution increases the ion energy and restores it to its required value at low temperatures; (2) a normal to dusty plasma transition occurs at lower hydrogen to silane flow ratio and this transition regime shifts to higher dilution ratios for lower substrate temperatures. Thus the role of high hydrogen dilution at low temperature is to avoid the dusty regime. Thus the role of high hydrogen dilution at low temperature is to avoid the dusty regime. The ion energy flux at low substrate temperature can also be restored to the value obtained at high substrate temperature, without increasing hydrogen dilution, by simply lowering the chamber pressure or increasing the delivered plasma power, though the IEDFs in these cases differ substantially from the IEDF at high temperature conditions. We propose that a low pressure or high power in combination with a modest hydrogen dilution (high enough to avoid dusty regime) will deliver silicon films at low temperature without sacrificing deposition rate.

Do vectors point the way to understanding energy transfer in molecular collisions?

This tutorial review examines the proposition that vector properties reveal more about the underlying potential energy surfaces controlling the inelastic exchange of energy in intermolecular collisions than conventional scalar measurements. Exciting recent experimental progress is summarized in the form of six selected cases studies. The new information that has been extracted is compared with the predictions of complementary theory. Likely future prospects and promising avenues for further progress are discussed. The treatment should appeal to all those with interests in the forces governing intermolecular interactions, especially in gas-phase collisions.

Macroscopic evidences for non-Rice-Ramsperger-Kassel effects in the reaction between H3O+ and D2O: The occurrence of nonstatistical isotopic branching ratio

The dynamics of the isotopic scrambling in the energized and metastable complex D2O-H3O+ has been studied using classical molecular dynamics (MD) trajectories starting from regions of phase space corresponding to an already formed collisional complex. The simulations cover the range of internal energies spanned by gas phase collision experiments. Rate constants for the isotopic exchange and the complex dissociation have been computed; the isotopic branching ratio R=[HD2O+]/[H2DO+] has also been obtained from MD simulations and found to deviate substantially from an equivalent prediction based on a previously proposed kinetic scheme. This finding suggests the possibility that details of the reaction dynamics play a role in defining the isotopic branching ratio. The analysis of trajectory results indicated a relatively long lifetime for the collisional complex and the presence of multiple time scales for the exchange process, with a large fraction of the exchange events being separated only by a single oxygen-oxygen vibration or half of it. The occurrence of these fast consecutive jumps and their different probabilities as a function of the relative direction between first and second jumps suggest the presence of ballistic motion in the complex following each reactive event. This can be explained on the basis of overlapping regions in phase space and it is used to provide an explanation of the difference between kinetic and MD branching ratios.

A thermal extrapolation method for the effective temperatures and internal energies of activated ions

The internal energies of dissociating ions, activated chemically or collisionally, can be estimated using the kinetics of thermal dissociation. The thermal Arrhenius parameters can be combined with the observed dissociation rate of the activated ions using k diss = A thermalexp(− E a,thermal/ RT eff). This Arrhenius-type relation yields the effective temperature, T eff, at which the ions would dissociate thermally at the same rate, or yield the same product distributions, as the activated ions. In turn, T eff is used to calculate the internal energy of the ions and the energy deposited by the activation process. The method yields an energy deposition efficiency of 10% for a chemical ionization proton transfer reaction and 8–26% for the surface collisions of various peptide ions. Internal energies of ions activated by chemical ionization or by gas phase collisions, and of ions produced by desorption methods such as fast atom bombardment, can be also evaluated. Thermal extrapolation is especially useful for ion–molecule reaction products and for biological ions, where other methods to evaluate internal energies are laborious or unavailable.

Insights into Heterogeneous Atmospheric Oxidation Chemistry: Development of a Tailor-Made Synthetic Model for Studying Tropospheric Surface Chemistry

The synthesis, characterization, and oxidation reaction of a tropospherically relevant terpene bound to a glass surface are reported. Vibrational broadband sum frequency generation (SFG) is used to characterize the various terpene-modified glass surfaces and track their interaction with ozone. SFG spectra indicate that, although orientations of the surface-bound terpenes depend on the linker strategies employed, the CC double bond is accessible to gas-phase ozone regardless of the strategy applied. Exposure of the terpene-functionalized surface to ppm levels of ozone at 1 atm and 300 K yields an initial reaction probability of approximately 1 × 10-5 per surface collision, which is significantly higher than the corresponding gas-phase reaction involving 1-methyl-1-cyclohexene (5 × 10-7 from gas-phase collision theory). The interaction of ozone with a saturated octyl silane-functionalized glass surface leads to a slight molecular reorientation, or tilting, of the terminal CH3 groups on a much slower time scale. Our work demonstrates that SFG spectroscopy can be used to determine reaction probabilities of heterogeneous atmospheric reactions and bridges the gap between atmospheric chemistry and surface functionalization.

Development of an ESI-MS screening method for evaluating binding affinity between integrin fragments and RGD-based peptides

Electrospray ionization mass spectrometry (ESI-MS) is used to evaluate binding affinities between integrin fragments and RGD-based peptide ligands. The integrin fragment peptides synthesized and studied were selected based on crystallographic and literature data which highlighted the specific binding pockets for the RGD motif in intact integrin proteins. Relative binding constants were determined by fitting the data obtained from ESI-MS titration experiments to a quadratic equation based on a 1:1 association model. Frontal analysis capillary electrophoresis (FACE) was used as a complementary solution phase technique to measure the absolute binding constants for the same host–guest systems and the results from both techniques were compared. Gas phase collision activated dissociation threshold measurements in an ion trap mass spectrometer also were carried out on the complexes to further validate binding constant measurements. Better correlation was observed between the MS/MS and CE results, than for ESI-MS titration. The ESI-MS titration results showed some correlation; however, it is apparent that there are important methodological shortcomings which need to be addressed. Several factors related to the titration method, ionization efficiencies, enthalpy/entropy contributions during phase transfer, and solvent effects are discussed. This work introduces a first approach to development of a high throughput ESI-MS-based method suitable for screening potential peptide-based drug compounds that target integrins on cancerous cells. It also serves as a cautionary tale to those interested in performing similar types of analyses.

Ion‐Induced Biomolecular Radiation Damage: From Isolated Nucleobases to Nucleobase Clusters

A large number of studies are devoted to the investigation of the biomolecular ionization and fragmentation dynamics underlying biological radiation damage. Most of these studies have been based on gas-phase collisions with isolated DNA building blocks. The radiobiological significance of these studies is often questioned because of the lack of a chemical environment. To clarify this aspect, we studied interactions of keV ions with isolated nucleobases and with nucleobase clusters by means of coincidence time-of-flight spectrometry. Significant changes already show up in the molecular fragmentation patterns of very small clusters.

Low-energy electron collisions with gas-phase uracil

We have studied gas-phase collisions between slow electrons and uracil molecules with a view to understanding the resonance structure of the scattering cross section. Our symmetry-resolved results for elastic scattering, computed in the fixed-nuclei, static-exchange and static-exchange-plus-polarization approximations, provide locations for the expected pi* shape resonances and indicate the possible presence of a low-energy sigma* resonance as well. Electron-impact excitation calculations were carried out for low-lying triplet and singlet excitation channels and yield a very large singlet cross section. We discuss the connection between the resonances found in our elastic cross section and features observed in dissociative attachment.

Modeling of a Plasma Etcher for Charging Free Processing of Nanoscale Structures

Neutral beam etching is proposed as a candidate for reducing plasma-process-induced damage in nanoscale devices. In this paper, neutralization of ion beams due to both gas phase collisions and ion surface interactions based on a PIC (Particle in Cell) simulation of realistic Capacitively Coupled Plasma is presented. It was found that a satisfactory degree of neutralization might be achieved by a combined effect of charge transfer and surface collisions.

Hyperthermal Chemistry in the Gas Phase and on Surfaces: Theoretical Studies

AbstractFor Abstract see ChemInform Abstract in Full Text.

The Surface-Bounded Exosphere of Mercury

Almost thirty years ago, the instruments on the Mariner 10 spacecraft detected traces of hydrogen, helium, and possibly oxygen in the atmosphere of Mercury. There the matter rested until the mid-80’s, when emission lines of sodium (Potter &amp; Morgan 1985) and potassium (Potter &amp; Morgan 1986) were found in the spectrum of Mercury, resulting from resonance scattering of sunlight by metal atoms in the atmosphere. Searches for other metals in the Mercury atmosphere were fruitless until recently, when an emission line attributed to calcium has been observed (Bida, Killen &amp; Morgan 2000). The densities of all these species are so low that gas phase collisions are negligible, so that the Mercury atmosphere is an exosphere bounded at its base by the surface of the planet. The fact that the exosphere is bounded by the Mercury surface means that interactions with the surface must occur. Energy exchange, surface absorption and desorption, and ion neutralization can take place, and these must be accounted for in models of the exosphere. None of these atmospheric species can survive long on Mercury, being lost mainly by photoionization followed by trapping in the solar wind and partly by solar radiation acceleration. Consequently, the species that we observe must be in a steady state, being generated as fast as they are lost to space. The metals must originate from the surface of the regolith, with the supply of fresh surface material maintained by gardening of the surface by meteoroid impact. The metals are released from the surface into the exosphere by photon-stimulated desorption (Madey et al. 1998), particle sputtering (McGrath, Johnson &amp; Lanzerotti 1986), and meteoroid impact vaporization (Morgan, Zook &amp; Potter 1988). Thermal evaporation of species condensed on the cold side may be important for determining their distribution over the planet (Hunten &amp; Sprague 2002). The interaction and relative importance of all these processes has been discussed by Killen &amp; Morgan (1993). Much can be said about these processes, but this review is focused primarily on the current status of observations of sodium, potassium, and calcium.

Using electrospray ionization-mass spectrometry/tandem mass spectrometry and small molecules to study guanidinium–anion interactions

A previous study highlighting the interaction between guanidinium- and phosphonate-functionalized molecules and the development of a screening protocol for noncovalent interactions using ESI-MS and MS/MS methodologies is extended here to incorporate sulfonate- and carboxylate-functionalized binding partners for guanidinium. Multiple high order homomeric and heteromeric adduct ions are observed in the mass spectra when mixtures of complementary analytes are ionized. Comparison of relative binding and ionization efficiencies are made using the solution-phase competition methods and gas-phase collision threshold dissociation ( E 1/2) measurements. Transmission factors are determined to compare the effect of structural variation of the analytes on their relative ionization efficiencies. Results indicate that while phosphonate- and sulfonate-functionalized analytes form more and higher order adduct ion complexes with guanidinium-containing molecules (represented here by free and modified arginines) as a result of the ESI process, when solvent is removed and collisional dissociation is employed, the trend is reversed, and the carboxylate group yields a stronger interaction with guanidinium, relative to the other oxoanions. Ionization differences reflected in the mass spectra are attributed to pH effects present in the condensed phase, whereas differences in stability measured in the gas-phase are attributed to the gas-phase acidities of the oxoanions and their geometric complementarity when forming noncovalent interactions with guanidinium. This work highlights the interaction of guanidinium with oxoanion binding partners using various ESI-MS and MS/MS methods, but also addresses explicitly the advantages and disadvantages of using small molecule analytes for routine analysis of noncovalent interactions.

Better control over the onset of microcrystallinity in fast-growing silicon network

In view of obtaining a Si:H network at the onset of microcrystallinity at a high deposition rate, we have adopted an intelligent approach to find out a tricky plasma condition in radio frequency (rf) plasma-enhanced chemical vapordeposition that provides a better control on growth introducing retarded microcrystallization. The deposition parameter includes a combination of high electrical power applied to the (SiH4+H2)-plasma and high gas pressure in thereaction chamber. High rf power increases the number density of film-forming precursors as well as atomic H density in the plasma, which helps to increase thefilm deposition rate and to promote microcrystallinity, respectively. In addition,high pressure helps not only to increase the film-growth rate by producing a dense plasma but also retards the microcrystallization process by increasing significantlythe gas phase collision frequency and consequently reducing the effective reactivityof atomic H on the surface of a fast-growing Si:H network. A combination of high-power and high-pressure plasma conditions provides a reasonably wide rangeof H2 dilution to work with and better control in producing a Si:H network at theonset of microcrystallinity, while increasing the film-growth rate.

Hyperthermal chemistry in the gas phase and on surfaces: theoretical studies

We review recent theoretical studies aimed at understanding gas/surface and gas-phase collisions at hyperthermal energies. The review is restricted to interactions between neutral species, and particular attention is given to the interactions of hyperthermal ground-state atomic oxygen (O(3P)) with hydrocarbons. Quantum mechanical and molecular dynamics calculations are used to simulate collisions of O(3P) with gas-phase methane, ethane, and propane molecules and with condensed-phase alkanethiolate self-assembled monolayers. The results of such studies are examined in the light of atomic-oxygen degradation of polymeric materials in low Earth orbit (LEO).