Single Collision Events Research Articles

A Combined Experimental and Theoretical Study on the Gas‐Phase Synthesis of Toluene under Single Collision Conditions

physical-organic chemistry communities. Toluene portrays the simplest representative of an alkyl-substituted benzene molecule with the methyl group enhancing the reactivity toward radical and electrophile aromatic substitution compared to benzene. It is further considered as a crucial building block to form methyl-substituted polycyclic aromatic hydrocarbons (PAHs) such as 1and 2-methylnaphthalene. In combustion flames, the formation of phenylacetylene 2, styrene 3, and biphenyl 4 (Scheme 1) can be rationalized through phenyl addition. Atomic hydrogen is eliminated upon reaction of the phenyl radical with acetylene (C2H2), [14] ethylene (C2H4), [15] and benzene (C6H6). [16] The analogous reaction of the phenyl radical with methane (CH4) does not lead to toluene, but solely to benzene plus a methyl radical (CH3) through hydrogen abstraction. [17] Consequently, the formation routes of toluene in combustion flames and in the interstellar medium have not been unraveled to date. Here, we show that the reaction of the ethynyl radical (CCH; C2H) with isoprene (2-methyl-1,3-butadiene; C5H8) presents a facile, barrierless route to the toluene molecule in a single collision event in the gas phase through the reaction of two acyclic precursor molecules. This reaction is also of interest to the physical-organic chemistry community since it represents a benchmark system to untangle the formation of a methylsubstituted aromatic molecule through radical substitution reactions involving successive isomerizations through cyclization and hydrogen shifts. Reactive scattering signal from the reaction of [D1]ethynyl radical C2D(X S) with isoprene (C5H8; X A’) was monitored at mass-to-charge ratios (m/z) of 93 (C7H7D ), 92 (C7H6D /C7H8 ), and 91 (C7H5D /C7H7 ). The signal atm/ z= 93 originates from the C7H7D product(s) formed through atomic hydrogen loss, whereas ion counts at m/z= 92 could have two contributions: an atomic deuterium loss connected to the formation of C7H8 products or dissociative ionization of C7H7D products in the electron impact ionizer of the detector. The signal at m/z= 91 may depict two contributions from dissociative electron impact ionization of C7H7D and/or C7H6D product molecules. However, all data could be fit with the product mass combinations of 93 amu (C7H7D) plus 1 amu (H). This finding suggests the existence of a [D1]ethynyl (C2D, 26 amu) versus atomic hydrogen replacement channel and the gas-phase synthesis of a molecule with the molecular formula C7H7D (Figure 1). Considering that the [D1]ethynyl reactant does not have a hydrogen atom, the hydrogen atom is emitted from the isoprene molecule. The laboratory angular distribution (Figure 2) extends at least 408 within the scattering plane and peaks close to the center-of-mass angle at 41.1 1.28. These results suggest indirect scattering dynamics through C7H8D complex(es). Scheme 1. Toluene 1, phenylacetylene 2, styrene 3, and biphenyl 4.

Single Collision Events of Conductive Nanoparticles Driven by Migration

We report that conductive single nanoparticle (NP) collisions can involve a significant component of the mass transport to the electrode of the charged NPs by migration. Previously, collision events of catalytic NPs were described as purely diffusional using random walk theory. However, the charged NP can also be attracted to the electrode by the electric field in solution (i.e., migration) thereby causing an enhancement in the collision frequency. The migration of charged NPs is affected by the supporting electrolyte concentration and the faradaic current flow. A simplified model based on the NP transference number is introduced to explain the migrational flux of the NPs. Experimental collision frequencies and the transference number model also agreed with more rigorous simulation results based on the Poisson and Nernst–Planck equations.

Multiple cosmic collisions and the microwave background power spectrum

Collisions between cosmic bubbles of different vacua are a generic feature of false vacuum eternal inflation scenarios. While previous studies have focused on the consequences of a single collision event in an observer's past, we begin here an investigation of the more general scenario allowing for many "mild" collisions intersecting our past light cone (and one another). We discuss the general features of multiple collision scenarios and consider their impact on the cosmic microwave background (CMB) temperature power spectrum, treating the collisions perturbatively. In a large class of models, one can approximate a multiple collision scenario as a superposition of individual collision events governed by nearly isotropic and scale-invariant distributions, most appearing to take up less than half of the sky. In this case, the shape of the expected CMB temperature spectrum maintains statistical isotropy and typically features a dramatic increase in power in the low multipoles relative to that of the best-fit $\Lambda$CDM model, which is in tension particularly with the observed quadrupole. We argue that this predicted spectrum is largely model-independent and can be used to outline features of the underlying statistical distributions of colliding bubbles consistent with CMB temperature measurements.

Energy Transfer and Thermal Accommodation in Ozone Scattering from a Perfluorinated Self-Assembled Monolayer

A modification of the energy transfer model recently proposed by two of us (ref 4) is tested in this work by an extensive comparison with the simulation results for O3 scattering from a perfluorinated self-assembled monolayer (F-SAM) as well as with previous NO + FSAM and Ar + F-SAM scattering results. The model fits very well the trajectory data over a ∼103-fold of incident energies. The percentage of energy transferred to the surface, predicted by the model at high incident energies, decreases with the number of degrees of freedom of the projectile because they compete with the surface degrees of freedom as possible destinations of the incident energy. The distributions of the scattered ozone molecules over translational and rotational states show a low-energy component characterized by a Maxwell–Boltzmann (MB) distribution at the surface temperature that survives at the highest collision energies. The dependence of the fraction of the MB component on the incident energy is an exponential decay function and the rate of decay is similar for the rotational and translational distributions. A non-negligible number of the O3 + F-SAM trajectories that penetrate the surface at high energies have very long residence times (longer than the simulation time), which enables thermal accommodation of the rotational and translational degrees of freedom. A new method to categorize the O3 + F-SAM trajectories, based on the residence time, shows that, at very low incident energies (<10 kcal/mol), thermal accommodation can be achieved in a single collision event.

EUV spectra of Xe xvii–Xe xxi produced in charge-exchange collisions

Charge-state-specific spectra produced from charge exchange collisions between xenon ions and helium target atoms using an ECR (electron cyclotron resonance) source have been recorded in extreme ultraviolet (EUV). At low target gas pressures, the spectra were produced after a single collision event involving electron capture by the projectile ion and in each case intense emission near 11 nm was observed. The spectra of Xe${}^{16+}$ to Xe${}^{20+}$ are compared with atomic structure calculations and the possible mechanisms for electron capture are discussed. There is some evidence that for lower charged ions the spectra in the 10--15 nm region result mainly from transfer excitation or ionization processes rather than direct single electron capture into high $nl$ states.

Scattering of O2 from a graphite surface

Recently an extensive series of measurements has been presented for the angular distributions of oxygen molecules scattered from a graphite surface. Incident translational energies ranged from 291 to 614 meV with surface temperatures from 150 to 500 K. The measurements were taken with a fixed angle of 90° between the source beam and the detector and the angular distributions consisted of a single broad peak with the most probable intensity located at an angle slightly larger than the 45° specular position. Analysis with the hard cubes model for atom–surface scattering indicated that the scattering is primarily a single collision event with a surface having a collective effective mass much larger than a single carbon atom. Limited analysis with a classical diatomic molecular scattering theory was also presented. In this paper a more complete analysis using the classical diatomic molecular scattering theory is presented. The energy and temperature dependence of the observed angular distributions are well described as single collision events with a surface having an effective mass of 1.8 carbon graphite rings. In agreement with the earlier analysis and with other experiments, this suggests a large cooperative response of the carbon atoms in the outermost graphene layer.

The ATLAS Tile Hadronic Calorimeter performance in the LHC Collision Era

The Tile Calorimeter (TileCal), the central section of the hadronic calorimeter of the ATLAS experiment, is a key detector component to detect hadrons, jets and taus and to measure the missing transverse energy. Due to the very good muon signal to noise ratio it assists the spectrometer in the identification and reconstruction of muons. TileCal is built of steel and scintillating tiles coupled to optical fibers and read out by photomultipliers. The calorimeter is equipped with systems that allow to monitor and to calibrate each stage of the readout system exploiting different signal sources: laser light, charge injection and a radioactive source. The performance of the calorimeter has been measured and monitored using calibration data, cosmic muons, LHC single beam and collision events. The results reported here assess the performance of the calibration systems, absolute energy scale, the energy and timing uniformity as well as the calorimeter performance with single hadrons. The results obtained demonstrate a good understanding of the detector and prove that its performance is within the design expectations.

Multiple ionization and fragmentation of polyatomic molecules by fast heavy ions

We investigated molecular fragmentation for CO, C2H2 and C60 molecules induced by impact of various fast heavy ions. Fragment ions produced in electron capture and loss collisions of projectile ions were measured in coincidence with final projectile charge states. As for C60 target, the number distribution of secondary electrons emitted in single collision event were also measured by means of a triple coincident method. The charge state distribution of transiently formed Cr+60 was successfully reproduced by our new model calculation on the basis of statistical energy deposition model developed for polyatomic molecules. Data acquisition using position sensitive detection system allows us to obtain 3D momentum imaging of fragment ions from CO and C2H2 molecules. Typical results relevant to molecular orientation effect and kinetic energy release (KER) are discussed.

Angular Intensity Distribution of a Molecular Oxygen Beam Scattered from a Graphite Surface

The scattering of the oxygen molecule from a graphite surface has been studied using a molecular beam scattering technique. The angular intensity distributions of scattered oxygen molecules were measured at incident energies from 291 to 614 meV with surface temperatures from 150 to 500 K. Every observed distribution has a single peak at a larger final angle than the specular angle of 45° which indicates that the normal component of the translation energy of the oxygen molecule is lost by the collision with the graphite surface. The amount of the energy loss by the collision has been roughly estimated as about 30-41% based on the assumption of the tangential momentum conservation during the collision. The distributions have also been analyzed with two theoretical models, the hard cubes model and the smooth surface model. These results indicate that the scattering is dominated by a single collision event of the particle with a flat surface having a large effective mass. The derived effective mass of the graphite surface for the incoming oxygen is 9-12 times heavier than that of a single carbon atom, suggesting a large cooperative motion of the carbon atoms in the topmost graphene layer.

He/Ar-atom scattering from molecular monolayers:C60/Pt(111) and graphene/Pt(111)

Supersonic He and Ar atomic beam scattering fromC60 and graphene monolayers adsorbed on a Pt(111) surface are demonstrated in order toobtain detailed insight into a gas–molecule collision that has not been studied in detail sofar. The effective masses and phonon spectral densities of the monolayers seen by differentprojectiles are discussed based on classical models such as the hard cube model and therecently developed smooth surface model. Large effective masses are deduced for both themonolayers, suggesting collective effects of surface atoms in the single collision event. Theeffective Debye temperature of graphene was found to be similar to that reported in highlyoriented pyrolytic graphite (HOPG), indicating that the graphene is decoupled wellfrom the Pt substrate. A much smaller Debye–Waller factor was found for theC60 layer, probablyreflecting the strong C60–Pt(111) interaction.

Energy deposition model for I-125 photon radiation in water

In this study, an electron-tracking Monte Carlo algorithm developed by us is combined with established photon transport models in order to simulate all primary and secondary particle interactions in water for incident photon radiation. As input parameters for secondary electron interactions, electron scattering cross sections by water molecules and experimental energy loss spectra are used. With this simulation, the resulting energy deposition can be modelled at the molecular level, yielding detailed information about localization and type of single collision events. The experimental emission spectrum of I-125 seeds, as used for radiotherapy of different tumours, was used for studying the energy deposition in water when irradiating with this radionuclide.

Mechanism of fragmentation and atomization of molecular ions in gasdynamic transport cell

The fragmentation of molecular ions formed upon the electrospraying of a sample and transported through the gasdynamic system of a mass spectrometer equipped with an IESAP (Ion Extraction from Solution at Atmospheric Pressure) source has been experimentally studied. It is established that ion fragmentation in a Kantorowicz-Gray type cell takes place in the immediate vicinity of a skimmer port, apparently, as a result of collisions between ions (accelerated in an electric field) and stagnant gas. Molecular mechanisms of fragmentation are considered and it is concluded that this process can take place in a single ion-molecule collision event.

Chemical dynamics of triacetylene formation and implications to the synthesis of polyynes in Titan's atmosphere

For the last four decades, the role of polyynes such as diacetylene (HCCCCH) and triacetylene (HCCCCCCH) in the chemical evolution of the atmosphere of Saturn's moon Titan has been a subject of vigorous research. These polyacetylenes are thought to serve as an UV radiation shield in planetary environments; thus, acting as prebiotic ozone, and are considered as important constituents of the visible haze layers on Titan. However, the underlying chemical processes that initiate the formation and control the growth of polyynes have been the least understood to date. Here, we present a combined experimental, theoretical, and modeling study on the synthesis of the polyyne triacetylene (HCCCCCCH) via the bimolecular gas phase reaction of the ethynyl radical (CCH) with diacetylene (HCCCCH). This elementary reaction is rapid, has no entrance barrier, and yields the triacetylene molecule via indirect scattering dynamics through complex formation in a single collision event. Photochemical models of Titan's atmosphere imply that triacetylene may serve as a building block to synthesize even more complex polyynes such as tetraacetylene (HCCCCCCCCH).

Identification of 4d–5p transitions in the spectra of Sn XV–Sn XIX recorded from collisions between Sn ions and He

Charge-state-specific extreme ultraviolet (EUV) spectra produced from charge exchange collisions between tin ions in charge states from Sn15+ to Sn18+ and rare gas target atoms using an ECR (electron cyclotron resonance) source have been recorded at Tokyo Metropolitan University. At low target gas pressures, the spectra were produced by single collision and capture events, and in each case they were dominated by intense emission near 13 nm. In the spectra of Sn XV–Sn XIX this emission has been shown to arise from 4s24pm−14d–4s24pm−14f + 4s24pm−24d2 transitions. However, in addition, a distinct group of lines resulting from 4s24pm−14d–4s24pm−15p (2 ⩽ m ⩽ 6) transitions centred near 11.5 nm in Sn XV was seen to shift to shorter wavelength with increasing ionic charge, and the strongest lines were identified by comparison with atomic structure calculations. These identifications are the subject of the present work.

Gas−Surface Chemical Reactions at High Collision Energies?

Most gas-surface chemical reactions occur via reaction of adsorbed species to form a thermal-energy ( approximately kT) product; however, some instances exist where an energetic projectile directly reacts with an adsorbate in a single-collision event to form a hyperthermal product (with a kinetic energy of a few eV). Here we show for the first time that 30-300 eV F(+) bombardment of fluorinated Ag and Si surfaces produces "ultrafast" F(2)(-) products with exit energies of up to 90 eV via a multistep direct-reaction mechanism. Experiments conclusively show that the projectile F atom ends up in the fast molecular product despite the fact that the impact energy is far greater than typical bond energies.

Current Transients in Single Nanoparticle Collision Events

Electrochemical hydrazine oxidation and proton reduction occur at a significantly higher rate at Pt than at Au or C electrodes. Thus, the collision and adhesion of a Pt particle on a less active Au or C electrode leads to a large current amplification by electrocatalysis at single nanoparticles (NPs). At low particle concentrations, the collision of Pt NPs was characterized by current transients composed of individual current profiles that rapidly attained a steady state, signaling single NP collisions. The characteristic steady-state current was used to estimate the particle size. The fluctuation in collision frequency with time indicates that the collision of NPs at the detector electrodes occurs in a statistically random manner, with the average frequency a function of particle concentration and diffusion coefficient. A longer term current decay in single current transients, as opposed to the expected steady-state behavior, was more pronounced for proton reduction than for hydrazine oxidation, revealing microscopic details of the nature of the particle interaction with the detector electrode and the kinetics of electrocatalysis at single NPs. The study of single NP collisions allows one to screen particle size distributions and estimate NP concentrations and diffusion coefficients.

Rapid accretion and differentiation of iron meteorite parent bodies inferred from 182Hf–182W chronometry and thermal modeling

New high-precision W isotope measurements are presented for 33 iron meteorites from 8 magmatic groups (IC, IIAB, IID, IIIAB, IIIE, IIIF, I VA and IVB), 2 non-magmatic groups (IAB–IIICD and IIE), and one ungrouped iron (Deep Springs). All magmatic irons have ε182W values that are, within errors, equal to, or less radiogenic than, the Solar System initial of −3.47±0.20. A method was developed to correct the measured ε182W values of magmatic iron meteorites for the presence of cosmogenic effects produced during space exposure to galactic cosmic rays. The corrected data provide new constraints on the timing of metal-silicate differentiation in iron meteorite parent bodies, which must have taken place within a few million years (<2 to 6 My) of condensation of calcium–aluminum-rich inclusions (CAIs). Metal-silicate differentiation ages (from 182Hf–182W systematics) were combined with parent body sizes (from metallographic cooling rates) into a model of planetesimal heating by 26Al-decay, to constrain the accretion timescale of iron meteorite parent bodies. Accretion of iron meteorite parent bodies most likely occurred within 1.5 My of the formation of CAIs. The fast accretion times of iron meteorite parent bodies are consistent with dynamical models indicating that these objects may have originated in the terrestrial planet-forming region, where the accretion rates were high. Our W isotopic data for non-magmatic IAB–IIICD and IIE irons provide new constraints for their formation mechanisms. In particular, they support formation of IAB–IIICD iron meteorites by melting during a single collision event dated at 4–7 My after formation of the Solar System.

Coincidence method for measuring the mass of neutral fragments emitted in a delayed fragmentation process from a singly charged molecule: Fragmentation pathway of adenine

We have studied by mass spectrometry the fragmentation pathways of a molecule of biological interest: adenine ${\mathrm{H}}_{5}{\mathrm{C}}_{5}{\mathrm{N}}_{5}\phantom{\rule{0.3em}{0ex}}(\mathrm{Ad})$. The adenine molecules are ionized and excited in a single collision event mode with slow highly charged ${\mathrm{Ar}}^{8+}$ ions. A method based on the detection in coincidence of neutral and charged fragments has been developed for studying delayed fragmentation channels. The measured spectra show that the well-stated process, i.e., the successive emission of $\mathrm{HCN}$ fragments, is not the only fragmentation pathway. Although in the first fragmentation step the dominant channel is the emission of a neutral ${\mathrm{HCN}}^{0}\phantom{\rule{0.3em}{0ex}}(88%)$, in the second fragmentation step the branching ratios for the delayed emission of an ${\mathrm{HCN}}^{0}$ and an ${\mathrm{H}}_{2}{\mathrm{CNCN}}^{0}$ from the intermediate parent ion ${\mathrm{H}}_{4}{\mathrm{C}}_{4}{\mathrm{N}}_{4}^{+}\phantom{\rule{0.3em}{0ex}}[(\mathrm{Ad}\text{\ensuremath{-}}\mathrm{HCN}{)}^{+}]$ are surprisingly similar, 54% and 46%, respectively. As a consequence, the role of these two latter channels in the reversal process, i.e., the formation of adenine in gas phase, should be considered. Ab initio calculations of molecular energies show that the energy levels of these two fragmentation channels are very close. This is in qualitative agreement with the measured comparable branching ratios.

Observing Single Nanoparticle Collisions by Electrogenerated Chemiluminescence Amplification

We demonstrate a novel method of observing single particle collision events with electrogenerated chemiluminescence (ECL). A single event is characterized by the enhancement of ECL intensity during the collision of an individual platinum nanoparticle (Pt NP) on an indium tin oxide electrode, which catalyzes the oxidation of Ru(bpy)3(2+) and a coreactant, for example, tri- n-propylamine (TPrA), present in the solution. Every collision produces a unique photon spike whose amplitude and frequency can be correlated with the size and concentration of the Pt NPs. A large amplification of ECL intensity can occur by choosing an appropriate measuring electrode and using high concentrations of Ru(bpy)3(2+) and the coreactant.

Simultaneous loss and excitation of projectile electrons in relativistic collisions of U90+(1s2) ions with atoms

We study relativistic collisions between helium-like uranium ions initially in the ground state and atoms in which, in a single collision event, one of the electrons of the ion is emitted and the other is transferred into an excited state of the residual hydrogen-like ion. We consider this two-electron process at not very high impact energies, where the action of the atom on the electrons of the ion can be well approximated as occurring solely due to the interaction with the nucleus of the atom and, hence, the process can be regarded as a four-body problem. Using the independent electron model we show that a very substantial improvement in the calculated cross sections is obtained if, instead of the first-order approximation, the relativistic symmetric eikonal and continuum-distorted-wave-eikonal-initial-state models are employed to describe the single-electron probabilities for the excitation and loss, respectively.