Thermal Energy Collisions Research Articles

Investigation of fullerene cluster growth mechanisms by carbon atom addition using classical molecular dynamics.

The mechanisms of carbon sticking reactions to C36 and C-C80 fullerenes were investigated with molecular dynamics simulations (MD) using the Second-generation Reactive Empirical Bond Order (SREBO) and Adaptive Intermolecular Reactive Empirical Bond Order (AIREBO) potentials that were specifically optimized for carbon-carbon interactions. Results showed the existence of three possible sticking configurations where the projectile atom can stick either to one, two or three atoms of the target fullerene. They also showed that although the two potentials give similar magnitudes for the sticking cross-sections, they yield fairly different results as far as sticking mechanisms and configurations at thermal collision-energies, i.e., in the range 0.05-0.5eV, are concerned. While AIREBO, that takes into account the long-range Lennard-Jones interaction, essentially results in a surface-sticking configuration with a single atom of the target fullerene, SREBO potential yields both surface- and two neighbors-sticking (2N-sticking) configurations. The fullerene structure is preserved in the last configuration while it can be recovered by a 2000K annealing in the former configuration. Results obtained with SREBO eventually showed larger sticking probabilities for C36 as compared with C80. In spite of this, the sticking cross-sections obtained for C80 are similar to or even larger than those obtained for C36 due to the larger size of C80 that compensates for its smaller sticking probabilities.

Effect of Preparation Processes on Structural Thermal Stability and Collision Energy Dissipation of Common Antirelaxation Coatings on Quartz Substrates.

Paraffin and octadecyltrichlorosilane (OTS) coatings can alleviate collisions between alkali-metal atoms and cell walls and then prolong the atomic spin-polarization lifetime. The surface structure and collision effects of these antirelaxation coatings, as well as the methods to avoid antirelaxation invalidity, have been the focus of researchers. This study investigated the thermolability of coating surface structure and the collision interactions between alkali metal atoms and coatings, considering the influence of various coating preparation factors, where this collision interaction is indirectly analyzed by measuring the collision energy dissipation between an atomic force microscopy (AFM) probe and the atoms on the coating surface. We found that appropriate evaporation time, carbochain length, and postannealing process can enhance the thermostability of the paraffin coating and eliminate its morphological defects. Furthermore, the OTS/water concentration, the soaking time, and the type of solvent have different levels of influence on the cluster formation and the thermostability of the OTS coatings. Moreover, the antirelaxation performance of coatings has been shown to be characterized by counting the energy dissipated when the AFM probe collides with the antirelaxation coating, replacing the conventional light-atom interaction- based method for measuring the relaxation characteristics, but requiring specific coating preparation factors to be maintained.

Electron-ion coincidence study of ionizing thermal energy collisions between He*(21S) and Ar atoms

Ionization in thermal energy collisions between excited He*(21S) and ground state Ar atoms leads to final states involving a free electron and either dissociating He–Ar+(2P3/2,1/2) (PI) or bound HeAr+(2P3/2,1/2) molecular ion states (AI). The higher-lying initially bound HeAr+(2P1/2) levels may be subject to predissociation to unbound He−Ar+(2P3/2) states with release of kinetic energy. By means of coincidence studies of the mass-selected ions and the energy-resolved electrons in conjunction with quantum mechanical calculations of the PI and AI processes we show that essentially all of the initially bound HeAr+(2P1/2) levels are predissociated over the ion detection time of 4 µs.

Giant cross sections for L changing in Rydberg-Rydberg collisions

State changing in thermal-energy collisions between strontium atoms in very-high-$n$, $n\ensuremath{\approx}300$, $n{\phantom{\rule{0.16em}{0ex}}}^{1}{F}_{3}$ Rydberg states is studied. In collisions between Rydberg atoms, the dipole-dipole interaction generates an effective electric field at each atom, which triggers Stark precession and the evolution of their angular momenta, $L$. Such $L$-changing collisions are examined using both quantum and classical theory. The theoretical predictions are experimentally verified by monitoring the damping of quantum beats induced by the sudden application of a DC ``pump'' field. The present work shows that, due to the long-range nature of the interactions, the cross sections, $\ensuremath{\sigma}$, for $L$ changing are considerably larger than the geometric collision cross section. For thermal collisions the cross section $\ensuremath{\sigma}$ for $n{\phantom{\rule{0.28em}{0ex}}}^{1}{F}_{3}$ Rydberg states with $n\ensuremath{\simeq}300$ is $\ensuremath{\approx}8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{2}$. Scaling rules derived in this paper predict that for cold collisions $\ensuremath{\sigma}$ may increase by a further factor of 4.

L-changing through very-long-range interactions in high-n, n∼300, Rydberg-Rydberg collisions

SynopsisState-changing in thermal-energy collisions between strontium atoms in high-n, n∼300, n1F3 Rydberg states is studied in an atomic beam and analyzed using classical trajectory Monte Carlo simulations. The L-changing cross section is large, ∼10-4 cm2, much greater than the corresponding “hard-sphere” cross section.

A Quasi Quantum Treatment of the spin orbit state changing and conserving rotationally inelastic NO(X)-He collisions

A Quasi Quantum Treatment (QQT) of the rotational inelastic NO(X)-He collisions in the pure Hund’s case (a) spin orbit state conserving transitions is extended to the mixed Hund’s case (a) spin orbit state conserving and changing transitions. To enable this extension a polar and azimuthal angle dependent intermolecular hard-shell PES, VS(RS,γR,χR)=ES, has been developed. The DCSs and ICSs calculated by QQT are compared with those obtained from QM exact calculations onto a full R-range ab initio PES. The rotationally inelastic scattering of NO(X) from He presents a paradigm for the rotationally inelastic scattering at a thermal collision energy of molecules residing in a Π electronic rotational eigenstate.

Surfactant-Promoted Reactions of Cl2 and Br2 with Br– in Glycerol

Gas-liquid scattering experiments are used to explore reactions of gaseous Cl2 and Br2 with a 0.03 M solution of the surfactant tetrahexylammonium bromide (THABr) dissolved in glycerol. At thermal collision energies, 79 ± 2% of incident Cl2 molecules react with Br(-) to form Cl2Br(-) in the interfacial region. This reaction probability is three times greater than the reactivity of Cl2 with 3 M NaBr-glycerol, even though the interfacial Br(-) concentrations are similar in each solution. We attribute the high 79% uptake to the presence of surface THA(+) ions that stabilize the Cl2Br(-) intermediate as it is formed in the charged, hydrophobic pocket created by the hexyl chains. Cl2Br(-) generates the single exchange product BrCl in a 1% yield close to the surface, while the remaining 99% desorbs as the double exchange product Br2 over >0.1 s after diffusing deeply into the bulk. When NaCl is added to the surfactant solution in a 20:1 Cl(-)/Br(-) ratio, the Cl2 reaction probability drops from 79% to 46 ± 1%, indicating that Cl(-) in the interfacial region only partially blocks reaction with Br(-). In parallel, we observe that gaseous Br2 molecules dissolve in 0.03 M THABr for 10(4) times longer than in 3 M NaBr. We attribute this change to formation of stabilizing interfacial and bulk-phase THA(+)Br3(-) ion pairs, in analogy with the capture of Cl2 and formation of THA(+)Cl2Br(-) pairs. The THA(+) ion appears to be a powerful interfacial catalyst for promoting reaction of Cl2 and Br2 with Br(-) and for ferrying the resultant ions into solution.

Parity-dependent oscillations in collisional polarization transfer: CN(A2Π, v = 4) + Ar

We report the first systematic experimental and theoretical study of the state-to-state transfer of rotational angular momentum orientation in a (2)Π-rare gas system. CN(X(2)Σ(+)) was produced by pulsed 266 nm photolysis of ICN in a thermal bath (296 K) of Ar collider gas. A pulsed circularly polarized tunable dye laser prepared CN(A(2)Π, v = 4) in two fully state-selected initial levels, j = 6.5 F1e and j = 10.5 F2f, with a known laboratory-frame orientation. Both the prepared levels and a range of product levels, j' F1e and j' F2f, were monitored using the circular polarized output of a tunable diode laser via cw frequency-modulated (FM) spectroscopy in stimulated emission on the CN(A-X) (4,2) band. The FM Doppler lineshapes for co-rotating and counter-rotating pump-and-probe geometries reveal the time-dependence of the populations and orientations. Kinetic fitting was used to extract the state-to-state population transfer rate constants and orientation multipole transfer efficiencies (MTEs), which quantify the degree of conservation of initially prepared orientation in the product level. Complementary full quantum scattering (QS) calculations were carried out on recently computed ab initio potential energy surfaces. Collision-energy-dependent tensor cross sections for ranks K = 0 and 1 were computed for transitions from both initial levels to all final levels. These quantities were integrated over the thermal collision energy distribution to yield predictions of the experimentally observed state-to-state population transfer rate constants and MTEs. Excellent agreement between experiment and theory is observed for both measured quantities. Dramatic oscillations in the MTEs are observed, up to and including changes in the sign of the orientation, as a function of even/odd Δj within a particular spin-orbit and e/f manifold. These oscillations, along with those also observed in the state-to-state rate constants, reflect the rotational parity of the final level. In general, parity-conserving collisions conserve rotational orientation, while parity-changing collisions result in large changes in the orientation. The QS calculations show that the dynamics of the collisions leading to these different outcomes are fundamentally different. We propose that the origin of this behavior lies in interferences between collisions that sample the even and odd-λ terms in the angular expansions of the PESs.

Near-Interfacial Halogen Atom Exchange in Collisions of Cl2 with 2.7 M NaBr–Glycerol

Gas-liquid scattering experiments are used to investigate reactions of Cl(2) with a 2.7 M NaBr-glycerol solution at 291 K. Only the single and double halogen exchange products, BrCl and Br(2), are observed to desorb from solution. When Cl(2) molecules strike the surface at thermal collision energies, 76% desorb as Cl(2) before reacting, 1% react to form BrCl, and 23% react to form Br(2). Residence time measurements, modeled by mass-transfer equations for absorption, diffusion, reaction, and evaporation, were used to determine the time and depth scales for Cl(2) escape and BrCl and Br(2) production. This modeling indicates that Cl(2) molecules desorb from the interfacial region in less than 1 μs or are attacked within this time by Br(-) ions and irreversibly captured as Cl(2)Br(-). The products BrCl and Br(2) are created primarily within the top few monolayers of the solution and then evaporate on average 12 and 28 μs after Cl(2) initially reacts with Br(-). Notably, Br(2) is not generated from BrCl via Cl(2)Br(-) → BrCl + Cl(-) and BrCl + Br(-) → Br(2)Cl(-) but from the parallel reaction Cl(2)Br(-) + Br(-) → Br(2)Cl(-)+ Cl(-) that bypasses the BrCl intermediate. Br(2) is then likely released through two pathways, Br(2)Cl(-) → Br(2) + Cl(-) and Br(2)Cl(-) + Br(-) → Br(3)(-) + Cl(-), followed by Br(3)(-) ↔ Br(2) + Br(-). The experiments demonstrate that single and double halogen exchange reactions can occur rapidly and close to the surface even when the products are created by multiple sequential reactions.

Experimental and theoretical study of the chemi-ionization in thermal collisions of Ne Rydberg atoms

The state dependence of associative ionization of the $ns$ and $nd$ Rydberg series of Ne atoms with $5<n<18$ at thermal collision energies is investigated experimentally using a combination of synchrotron radiation and mass spectrometry in a single-beam experiment. The $n$ dependence of the collisional and associative ionization of the Ne Rydberg atoms have been calculated using the dipole resonant mechanism, in particular the associative ionization rates for $5<{n}_{\mathrm{eff}}<20$ have been calculated for cell, single beam, and crossed beam conditions. Refinements of the theory implemented previously have been necessary in order to apply the theory to the collisions of rare gas atoms. The results of the theoretical model for the $n$ dependency of the associative ionization rates of the Ne Rydberg atoms are compared to the experimental data.

Efficient quantum-classical method for computing thermal rate constant of recombination: Application to ozone formation

Efficient method is proposed for computing thermal rate constant of recombination reaction that proceeds according to the energy transfer mechanism, when an energized molecule is formed from reactants first, and is stabilized later by collision with quencher. The mixed quantum-classical theory for the collisional energy transfer and the ro-vibrational energy flow [M. Ivanov and D. Babikov, J. Chem. Phys. 134, 144107 (2011)] is employed to treat the dynamics of molecule + quencher collision. Efficiency is achieved by sampling simultaneously (i) the thermal collision energy, (ii) the impact parameter, and (iii) the incident direction of quencher, as well as (iv) the rotational state of energized molecule. This approach is applied to calculate third-order rate constant of the recombination reaction that forms the (16)O(18)O(16)O isotopomer of ozone. Comparison of the predicted rate vs. experimental result is presented.

The k-j-j′ vector correlation in inelastic and reactive scattering

Quasi-classical trajectory (QCT) methods are presented which allow characterization of the angular momentum depolarization of the products of inelastic and reactive scattering. The particular emphasis of the theory is on three-vector correlations, and on the connection with the two-vector correlation between the initial and final angular momenta, j and j', which is amenable to experimental measurement. The formal classical theory is presented, and computational results for NO(A) + He are used to illustrate the type of mechanistic information provided by analysis of the two- and three-vector correlations. The classical j-j' two-vector correlation results are compared with quantum mechanical calculations, and are shown to be in good agreement. The data for NO(A) + He support previous conclusions [M. Brouard, H. Chadwick, Y.-P. Chang, R. Cireasa, C. J. Eyles, A. O. L. Via, N. Screen, F. J. Aoiz, and J. Kłos, J. Chem. Phys. 131, 104307 (2009)] that this system is only weakly depolarizing. Furthermore, it is shown that the projection of j along the kinematic apse is nearly conserved for this system under thermal collision energy conditions.

Chemical synthesis in acetonitrile containing discharges. Insights from photoionization experiments with synchrotron radiation

The photoinduced ion chemistry of acetonitrile is investigated by using tuneable VUV synchrotron radiation in an octupolar ion guide with mass spectrometric detection. The appearance energies and yields of primary photoions and secondary ionic products deriving from self-reactions at thermal collision energies are measured. Comparison of the appearance energies of the different product ions provides insights into the reaction mechanisms. Experimental data are interpreted with the help of theoretical calculations (energies and structures at the G3 level of theory) for the various isomers of the most relevant ionic products. We find that the ion C2H2N+ is formed at the threshold with a cyclic structure, but only its linear isomer (lying 0.47eV higher in energy) can react with acetonitrile to form CH2CNCH2+ plus HCN via a cyclic transition state.

Interaction potentials and dynamics for Li + F collisions

We present diabatic potential curves for 1Σ, states of LiF and integral cross sections for elastic scattering of Li(2S) + F(2P) at thermal collision energies. Potential curves were obtained using atomic pseudopotentials and the extended valence-bond method. Pairs of coupled scattering equations were numerically integrated with a recently developed approach. Comparison of cross sections for homopolar (single channel) and homopolar plus ionic (two coupled channels) interactions provides information on the electron jump mechanism during collision.

Time-Dependent Quantum Dynamics Study of the Ne + H2+ (v = 0−9) and D2+ (v = 0−12) Proton Transfer Reactions at Thermal Collision Energies

The Ne + H2+ and D2+ proton transfer reactions were studied (reaction probabilities (P) and cross sections (sigma)) using the time-dependent real wave packet quantum dynamics method at the centrifugal sudden level (CS-RWP method). This was made considering the influence of vibrational excitation of H2+ and D2+ on the dynamics ((v = 0-9, j = 0) and (v = 0-12, j = 0) vibrorotational states, respectively) at thermal collision energies (Ecol = 0.010 to 0.14 eV). The CS-RWP relative cross sections at thermal collision energy (300 K) for Ne + H2+ and Ne + D2+ are in good agreement with the experimental data. The present results suggest the adequacy of the approach used here in describing these and related systems at thermal collision energies and from low to high vibrational excitation of reactants.

Ab‐Initio Calculations on the Structures and Electronic States of Dimethylsulfide‐Water Clusters

Ab initio and density functional theory (DFT) calculations have been applied to hydration processes of dimethylsulfide (DMS), which plays an important role in cloud condensation nuclei (CCN) in marine boundary layer. The molecular complexes composed of water and dimethylsulfide, DMS(H2O)n (n=0–3), were examined in the present calculations. The MP4SDQ/6‐311G(2d,p)//B3LYP/6‐311G(d,p) calculations indicated that the binding energy of H2O to DMS is calculated to be 7.7 kcal/mol for n=1. The second water molecule is bound to H2O of DMS(H2O) by a hydrogen bond, and then a 1:2 complex is formed. The binding energy of the second water molecule to DMS(H2O) was calculated to be 10.4 kcal/mol. This energy is larger than that of water dimer (the corresponding binding energy was 6.6 kcal/mol in water dimer). Direct molecular orbital‐molecular dynamics (MO‐MD) calculations were carried out for the hydration reactions DMS(H2O)+H2O→DMS(H2O)2 (1) and H2O+H2O→(H2O)2 (2) at thermal collision energy. The calculations showed that water dimer formation reaction on DMS, reaction (1), is efficiently occurred, whereas half of the trajectories for reaction (2) gave dissociation products (H2O+H2O). The mechanism of hydration of DMS was discussed on the basis of theoretical results.

Cross Sections and Rate Constants for Charge Transfer into Excited States

AbstractSummary: Cross sections and rate constants for charge transfer into excited states in thermal energy collisions are calculated for a gas discharge in He/Cu, Ne/Cu, He/Ag, Ne/Ag, He/Al, Ne/Al, He/Au, He/Hg, He/Cd, and He/Zn mixtures. To our knowledge, the polarizability of the above‐mentioned metal atoms, which determines the ion‐atom interaction and, therefore, the cross section, is calculated for the first time. A cross section for the Ne+Cu impact couple is experimentally obtained.Schematic diagram of the experimental setup.imageSchematic diagram of the experimental setup.

A molecular beam scattering study of weakly bound complexes: the potential energy surfaces for the C 2H 4–Ne, –Ar and –Kr systems

The weakly bound ethylene–Ne, –Ar and –Kr complexes have been investigated by molecular beam scattering experiments. Total integral cross sections have been measured in the thermal collision energy range where quantum interference effects have been measured. The data analysis has been based on an atom-bond representation of the interaction potential energy function, whose parameters have been fitted to the experimental results allowing a full characterization of the structure of the complexes.

Dependence of the differential cross sections for radiative-collisional excitation of metastable atomic states on the polarization of radiation

We calculate the total and differential cross sections for radiative-collisional excitation of the metastable 21 S state of He atoms at collisions with Ne atoms in external radiation fields of various frequencies and polarizations. The calculations are performed for a thermal collision energy of E = 10−3 atomic units and light intensity of I = 1 MW cm−2, which corresponds to a single photon absorption by a quasi-molecule during the collision. Both the differential and total cross sections are shown to depend strongly on the relative orientation of the radiation polarization vector and the initial relative velocity vector of the colliding atoms. We analyze the azimuthal scattering asymmetry related to the orientation of the angular momentum of the absorbed photon.

Experimental and theoretical electron energy spectra due to ionizing collisions of metastable Ar*(3P2), Ar*(3P0) and Kr*(3P0) atoms with ground-state Hg atoms

Using a crossed-beams set-up, we have studied the electron energy spectra due to Penning and associative ionization in thermal energy collisions of state-selected metastable Ar*(4s 3P2, 3P0) and Kr*(5s 3P0) atoms with ground-state Hg atoms at an electron energy resolution of 30 meV. In all three cases the energy spectra exhibit a prominent peak located close to the energy difference between the metastable excitation energies and the Hg ionization energy; they show a sharp drop to lower and an extended tail towards higher electron energy with shapes which differ for the three systems. The energy-integrated ionization cross section for Ar*(3P0) exceeds that for Ar*(3P2) by about 30%. Excitation transfer to autoionizing Hg** states, resulting in a sharp additional structure, is only observed—as a weak channel—in Ar*(3P0) + Hg collisions. For the three investigated systems calculations of the electron energy spectra have been carried out within the local optical potential approximation. As input we use ab initio potential curves for the ionic exit channels as well as for the complex entrance channel potentials. Using these potentials (involving a scaled width for Kr* + Hg), impressive agreement with the experimental electron spectra is observed. In contrast, earlier empirical model potentials for the entrance channel are found to be inadequate. The calculated ionization rate coefficients at T = 300 K amount to (3.72, 5.26) × 10−10 cm3 s−1 for Ar*(3P2, 3P0) + Hg, respectively.