Gas-phase Collisions Research Articles

Molecular dynamics simulation of gold cluster collisions

Molecular dynamics (MD) simulation with an embedded atom method (EAM) potential is used to study gas-phase collisions of two 55-atom gold clusters. The probability of formation of a long-lived collision complex and the structure of this complex, after it is quenched to OK, are determined for head-on collisions in which each collision partner is chosen randomly from an equilibrium thermal bath. Two sets of collisions are studied. In the first the clusters are initially at OK, and in the second they are initially at 300K. All collisions result in cluster aggregation with significant inelastic deformation of the original clusters. The major conversion of potential energy into internal kinetic energy in the collision occurs on the time scale of a single vibrational oscillation. When the clusters are initially at OK, the collision complex is a solid cluster. When the clusters are initially at 300K, the collision complex is a liquid cluster.

The production of heavy multiply charged ions in secondary ion mass spectrometry

In this study it was found that the multiply charged ion emissions (M 2+ and M 3+) of Al, Au, Ag, Cd, Co, Cu, Hg, In, Mo, Sc, Te, Ta, and V, arising from O − bombardment of metal surfaces, exhibited a great variety of energy distributions. This is attributed to the competing surface and gas phase ionization mechanisms. Investigations of the gas phase collision process were carried out by studying the primary ion current density effects on the intensities of Al n+ , Ag n+ , Cd n+ , and In n+ , ( n = 1, 2, 3), t fractionation exhibited by energy deficient Cu 2+ ions, and the image displacement of multiply charged ions with respect to intrinsically formed singly charged ions. These revealed that both doubly and triply charged ions from the heavier elements are formed predominantly from gas phase collisions between the secondary neutral population and the secondary electron, or primary ion population, during single primary ion events. A kinematics study however, discounted the possibility of primary ion interaction, whether surface scattered or not.

Internal Energy Distributions of Product Molecules Studied by Molecular-Beam/Infrared Emission Technique. Partial Oxidation of Butane and Decomposition and Oxidation of CH3OH on a Pt Surface.

Internal energy distributions in the CO and CO2 molecules formed by molecular-beam surface reactions on a polycrystalline Pt foil have been studied by using FT-IR emission spectroscopy. The CO molecules produced by partial oxidation of n-butane or iso-butane at a surface temperature of 1500 K are substantially vibrationally excited (vibrational temperature : Tv=2300 K), but rotationally very cool (rotational temperature : TR=360 K). The low TR value is not due to the rotational relaxation by the gas phase collision. In contrast, the CO molecules produced by CH3OH decomposition are not vibrationally excited at all. This impliee that the mechanism is different between the partial oxidation of butane and the decomposition of CH3OH. On the other hand, the CO2 molecules produced and desorbed by oxidation of CH3OH is both vibrationally and rotationally excited, but the internal energy distributions of the CO2 molecules are different from those formed by the, CO+O2 reaction or the CO+O reaction. These differences in internal energy distributions can provide us with some information on the dynamics (including the structure of the activated complex and the energy transfer processes) of surface catalyzed reactions.

The production of heavy multiply charged ions in secondary ion mass spectrometry

Abstract In this study it was found that the multiply charged ion emissions (M2+ and M3+) of Al, Au, Ag, Cd, Co, Cu, Hg, In, Mo, Sc, Te, Ta, and V, arising from O− bombardment of metal surfaces, exhibited a great variety of energy distributions. This is attributed to the competing surface and gas phase ionization mechanisms. Investigations of the gas phase collision process were carried out by studying the primary ion current density effects on the intensities of Aln+, Agn+, Cdn+, and Inn+, (n = 1, 2, 3), t fractionation exhibited by energy deficient Cu2+ ions, and the image displacement of multiply charged ions with respect to intrinsically formed singly charged ions. These revealed that both doubly and triply charged ions from the heavier elements are formed predominantly from gas phase collisions between the secondary neutral population and the secondary electron, or primary ion population, during single primary ion events. A kinematics study however, discounted the possibility of primary ion interaction, whether surface scattered or not.

Chemical Reactions in Mixed Molecular Clusters, Studied by Resonant Two-Photon Ionization

After a short overview on the different types of intracluster reactions, observed so far, this survey focuses on nucleophilic ipso-substitution (SN) reactions in clusters consisting of one (1,n)-chlorofluorobenzene cation with one (1:1) of two (1:2) ammonia molecules. The reaction efficiencies are compared with that in bimolecular ion-molecule reactions as studied by Thölmann and Grützmacher. Two types of (SN2)-reactions are observed, 1. abstraction of a chlorine radical, 2. abstraction of HX (X = Cl,F). The latter reaction is only observed in clusters but not in an ion-molecule gas phase collision. For both channels different mechanisms are proposed. The Cl-production strongly depends on the dipole moment of the neutral chromophore. It is rationalized in the framework of the theory of Shaik and Pross on polar reactions by an electron transfer as rate determining step. The HX formation channel, on the other hand is explained by a transition state with a concerted electron and proton transfer. It is strongly promoted by additional solvent molecules, probably due to bifunctional catalysis.

Design considerations for high-flux collisionally opaque molecular beams

Flux distributions and fractional interceptions for high-flux straight-tube sources have been calculated in the collisionally opaque gas flow regime. Previous theoretical work in this regime has been assimilated to provide a useful design guide similar to that provided by Campbell and Valone [J. Vac. Sci. Technol. A 3, 408 (1985)] for transparent regime molecular-beam sources. Several of these large throughput straight-tube sources have been constructed in the laboratory for use in etching studies of semiconductor materials. Reasonable agreement has been found between the available theories and measurements of peak beam fluxes and angular dispersions. While angular dispersion is a function of aspect ratio only (tube diameter-to-length ratio) in the transparent regime theory discussed by Campbell and Valone, additional parameters are required in the opaque regime since gas-phase collisions effect the beam directionality. The dimensionless group η=a/λtip, the ratio of tube diameter to gas mean free path at the doser tip, is suggested as the figure of merit for characterizing angular dispersion in opaque regime tube flow.

Monte Carlo simulation study on the flow conditions of a vacuum chemical epitaxy system

This work presents a numerical simulation, using Monte Carlo method, of the gas phase behavior in a vacuum chemical epitaxy (VCE) system, without hydride cracker. From this simulation, it is shown that the growth rate depends on the gas – mainly hydride —pressure in the growth chamber. This dependence is stronger for pressures above 10 −3 Torr, where the growth rate decreases almost linearly with increasing pressure. The slope of this curve decreases with the sticking coefficient of the organometallic molecules, that is, when growth temperature increases. Experimental results agree well with the predicted results which can, qualitatively, explain the growth rate behavior with the hydride flows used in VCE, and show that gas phase collisions should not be neglected even in chemical beam epitaxy (CBE) systems.

Laser fluorescence study of the Pb+F2, Cl2 reactions: Internal state distribution of the PbCl product and radiative lifetimes of PbF(A,B) and PbCl(A)

The reactions of Pb atoms with molecular reagents F2 and Cl2 have been studied in a molecular beam apparatus by laser fluorescence detection of PbF and PbCl products. The experiments were performed in a beam-gas configuration under single-collision conditions. The PbCl product from Pb+Cl2 was observed in the vibrational levels v≤17 of the ground X1 electronic state. The PbCl product vibrational state distribution was derived; the average vibrational excitation was found to be 21% of the total available energy. For Pb+F2, PbF(X1) product was detected in only the v=0 and 1 vibrational levels, and the rovibrational state distribution could be characterized by a 300 K Boltzmann distribution. By modulating the Pb beam, it was concluded that this observed product was an artifact and did not arise from bimolecular gas-phase collisions. The radiative lifetimes of the PbF(A,B) and PbCl(A) states were also measured and were found to equal 4.9±0.3 μs, <10 ns, and 1.14±0.06 μs, respectively, averaged over the observed vibrational levels.

Symmetric versus asymmetric collisions in ion-induced Auger emission from silicon

We investigate the production of Si 2p Auger electrons during bombardment of Si surfaces by 1.0–5.1 keV Ar + ions. Yields of Auger electrons emitted at ∼ 146° with respect to the ion beam are measured as a function of ion energy. We observe a threshold of ∼ 3.8 keV for formation of a double L vacancy in Si, which is due to ArSi collisions. This value is higher than those of previous studies, which were presumably influenced by contamination of the Ar + beam with multiply charged ions, as follows from our measurements of the variation of the Auger yields with the energy of the electrons in the ion source. We conclude that Auger emission in our energy range is dominated by symmetric collisions involving a fast Si recoil, with excitation in asymmetric, ArSi collisions growing in importance above the ∼ 3.8 keV threshold. The conclusion is also supported by the appearance of additional, Doppler shifted, Auger peaks at high projectile energies. Our threshold value for ArSi collisions is in good agreement with previous studies of gas-phase collisions, and should be used to reevaluate recent computer simulations which fail to predict the observations.

Ablation of high-temperature superconductor studied by resonance ionization mass spectrometry (RIMS)

Laser ablation of bulk high-temperature superconductor (HTS) material provides a useful means of producing high-quality HTS thin films. Mass spectrometric probes of the ablation plume provide a microscopic understanding of the ablation event and plume development as well as providing a process monitor for the thin-film production. Resonance ionization mass spectrometry (RIMS) detection of the ablated neutral species supplies valuable physical information about the ablation event necessary for developing models of the process. Time-of-flight/RIMS detection of Cu, Y, and BaO ablated from YBa2Cu3O7−x indicates the ablation involves post-desorption gas phase collisions, which influence the ablation chemistry and dynamics (e.g., angular and velocity distributions). Approximately equal velocities are observed for all neutral species at constant ablation laser fluence. In addition, BaO exhibits a rotational temperature far less than the directed translational temperature.

Negative charge emission due to excimer laser bombardment of sodium trisilicate glass

We describe measurements of negative charge emission accompanying irradiation of sodium trisilicate glass (Na2O⋅3SiO2) with 248-nm excimer laser light at fluences on the order of 2 J/cm2 per pulse, i.e., at the threshold for ablative etching of the glass surface. The negative charge emission consists of a very prompt photoelectron burst coincident with the laser pulse, followed by a much slower plume of electrons and negative ions traveling with a high density cloud of positive ions, previously identified as primarily Na+. Using combinations of E and B fields in conjunction with time-of-flight methods, the negative ions were successfully separated from the plume and tentatively identified as O−, Si−, NaO−, and perhaps NaSi−. These negative species are probably formed by gas phase collisions in the near-surface region which result in electron attachment.

Spectroscopic investigation of energetic atoms in a DC hydrogen glow discharge

The analysis of the Doppler profile of the Balmer Halpha line in the cathode sheath and negative glow of a hydrogen glow discharge shows an important concentration of anisotropic energetic atoms. This translational energy is regarded as mainly resulting from reactions including ions accelerated by the cathode sheath electric field. The mechanisms involved are gas-phase charge-exchange collisions producing fast atoms moving toward the cathode and simultaneous neutralisation and backscattering of the ions at the cathode surface leading to energetic atoms moving in the other direction. Quantitative analysis shows that most of the emission of the fast atoms results from excitation directly produced by these ion impact reactions.

A sector/ion trap hybrid mass spectrometer of BE/trap configuration

Substitution of a quadrupole ion trap for a quadrupole mass filter as a component of a hybrid mass spectrometer results in a new instrument of BE/trap configuration. Use of a small ion trap (radius 0.5 cm) gives an instrument in which the trap and sectors have similar mass ranges under stand operating conditions. Mass-selective instability scans of the ion trap provide daughter ion spectra for parent ions selected using the sectors and injected into the trap. Instrument performance, including resolution and signal-to-noise characteristics, is comparable in the ion injection mode to the performance in normal ion trap mass spectra recorded with internal ionization. Ions are activated in the course of injection as a result of both surface and gas-phase collisions. Collisions with surfaces are demonstrated by injecting noble gas ions and observing hydrocarbon ions generated by chemical sputtering from surfaces. Stable ions such as the pyrene molecular ion can easily be fragmented and the internal energy deposited is readily controlled through its dependence on the translational energy of the ion beam injected into the trap. The utility of the instrument for characterizing small peptides is illustrated.

Sputtering of excited-state potassium atoms from electron-bombarded KBr crystals

Sputtering of excited-state potassium atoms from electron-bombarded KBr crystals has been investigated at 300 and 443 K by measuring simultaneously the optical emission, Auger, and mass spectra. The beam current dependences of the intensities of the ground-state and excited-state potassium atoms at 443 K are nearly linear and quadratic, respectively, indicating that excited-state potassium atoms are produced by gas-phase collisions of potassium atoms desorbed from the surface. On the other hand, the room-temperature data show complicated dependences, which can be divided into three current regions. In the low-current region (<13 μA), electronic excitation followed by Auger-induced Coulomb repulsion produces the excited-state potassium atoms. In the high-current region (>25 μA), effects due to sample decomposition, nonstoichiometry, and gas-phase collisions play important roles in the production of excited-state potassium atoms. The nonlinear sputtering yield of excited-state potassium atoms in the intermediate-current region may be due to secondary effects including the formation of defects and cluster ions.

Plasma processes under vacuum conditions

In the first part the basics of low pressure plasma processes are briefly reviewed considering excitations by gas phase collisions and electrical potential near surfaces in contact with the plasma (sheat formation). In the second part we discuss the special situation given by the combination of vacuum technology and low pressure plasma technology. The impact of plasma technology on pumps and vacuum vessels is discussed taking into account in particular degassing, impurity situation in the gas phase and gas flow. This is done with a special emphasis on large area industrial machines. In the third part we present a new type of design for a machine for plasma enhanced CVD. This so-called plasma box type reactor is a good example for vacuum technology adapted to a plasma process. The plasma box reactor is designed for large area industrial applications, ensures high thin film uniformity and a uhv-type gas phase purity. The plasma box reactor was tested by deposition of amorphous silicon (a-Si) (for solar cells or thin film transistors), first results of these a-Si depositions are presented.

Conductance considerations in the reactive ion etching of high aspect ratio features

Very simple vacuum conductance arguments indicate that in the reactive ion etching of high aspect ratio features, the conductance is adequate to allow etch products to flow out of the feature without building up a pressure which would allow gas phase collisions to become important. On the other hand, the conductance can be expected to limit the flow of the reactive species to the bottom of the feature where the etching is taking place, thus creating the possibility of an etch rate dependence on the aspect ratio of the etched feature.

Characterization of ground-state neutral and ion transport during laser ablation of Y1Ba2Cu3O7−x using transient optical absorption spectroscopy

Transient optical absorption spectroscopy has been utilized for the first time to study the transport of ground-state Y, Ba, Cu, and Ba+ following excimer laser ablation of Y1Ba2Cu3O7−x pellets. Spectral broadening of the atomic lines monitored in both absorption and emission is reported, indicating the existence of gas phase collisions in the plume of ejected material. Time-of-flight velocity distributions of the nonemitting neutrals and ions determined by the absorption technique are broadened and shifted to lower velocities than the velocity distributions inferred from excited-state fluorescence in the plume. Absorption by ground-state Y+, YO, BaO, and CuO also has been observed with this technique. The absorption technique, and its application as an in situ monitor of neutral and ion transport during deposition of superconducting thin films, is described.

The reactions of radical positive ions of aromatic compounds with alkyl halides

The collision complexes formed in the ion/molecule reactions of the benzene radical positive ions and neutral alkyl iodides were investigated by using tandem sector mass spectrometry (MS/MS), ion trap, and Fourier transform mass spectrometry. The collision complexes are nonisomerizing iodonium radical positive ions, which dissociate to gas-phase arenium ions. The benzene radical positive ion also forms collision complexes with bromoalkanes, although these adducts do not go on to give arenium ions. The benzene radical positive ion is unreactive with chloroalkanes. The scope of the reaction was extended by showing that covalent gas-phase collision complexes form between neutral 2-iodopropane and the radical positive ions of toluene, nitrobenzene, chlorobenzene, furan, thiophene, and pyridine. The results establish that the reaction between the radical positive ion of an aromatic species and neutral iodoalkanes is a method for forming gas-phase arenium ions. It is possible that solution arenium ions may be formed by using this reaction.

Attenuation of Polyatomic Ion Interferences in Inductively Coupled Plasma Mass Spectrometry by Gas-Phase Collisions

A double quadrupole arrangement to remove polyatomic ions by collisions with an added target gas has been evaluated. The ions Ar2+, ArO+, and ArN+ were attenuated by factors of 50–400 to count rates of ≤100 counts s−1. Fifty to seventy percent of the analyte ions were retained. Removal of polyatomic ions and retention of analyte ions were favored by low-energy collisions with Xe or CH4 rather than typical target gases like N2 or Ar. Products of ion-molecule reactions between background ions and target gas in the first quadrupole could be observed.

Collisional cooling of the NO-He system The pressure broadening cross sections between 4·3 and 1·8 K

We report the results of a very low temperature pressure broadening experiment on the NO-He system. A newly developed collisional cooling technique was used to provide an environment for the study of gas phase collisions below 4K. In this temperature regime typical rotational energies are greater than available thermal energies. The helium pressure broadening cross section for the J = 1/2-3/2 transition of NO in the 2Π1/2 electronic state was found to drop by ∼35 per cent from 26 A2 at 4·2K to 17A2 at 1·8K. The 300K cross section is 22A2. The results of this experiment are compared with results from an earlier experiment on the CO-He system in which the cross section rose by ∼30 per cent over this temperature range. The spectroscopic attributes of the collisional cooling technique are also discussed, some of which are: narrow line widths, large absorption coefficients, and the simplification of otherwise complex spectra. In addition, because, hvr > kT, the number of available collisional channels is vastl...