eV Collision Energy Research Articles

Influence of collision energy on the dynamics of the reaction O(1D) + CH4(X1A1) → OH(X 2Π) + CH3(X 2A2″)

We studied the effects of collision energy (ET) on the dynamics of the title reaction using the quasiclassical trajectory method on an analytical triatomic potential energy surface that we had derived for this system. We compared the dependence of the scalar and two-vector properties of the reaction on ET with experimental data and obtained a quite good agreement. The results can be explained in terms of the coexistence of two microscopic reaction mechanisms: insertion and abstraction. The former mechanism is the most important one, although the contribution of the latter increases with ET .

Surface induced dissociation of chromium hexacarbonyl on fluorinated alkanethiolate surface in ion cyclotron resonance mass spectrometer: studies of energetics of the process using recursive internal energy distribution search method

Energy transfer in ion-surface collisional activation is characterized for 0–30 eV collisions of chromium hexacarbonyl molecular cations with a monolayer of fluorinated alkanethiolate self-assembled onto a solid gold surface. This surface was mounted on the back trapping plate of the Infinity® cell of a Bruker BioApex 7T ion cyclotron resonance mass spectrometer on the B-field axis orthogonal to the ion beam direction. Internal energy deposition was deduced from fragmentation spectra using a recursive internal energy distribution search method. The efficiency of energy transfer into the ion slowly increases with incident ion energy to a maximum value of 20% at about 23 eV collision energy. Approximate kinetic energy distributions of the fragments were measured by deducing the dependence of ion abundance on trapping potential. From the kinetic energy dependence on mass we infer that rapid decomposition of the molecular cation occurs after it recoils from the surface. Knowledge of both internal and kinetic energy distributions of collisionally activated ions enabled us to deduce the energy deposited into the self-assembled monolayer as a function of collision energy.

Dynamics of the multifragmentation of Nan+ clusters (4 ≤ n ≤ 9)

The multifragmentation of metallic clusters (Nan+, 4 ≤ n ≤ 9) induced by He impact in the 100 eV collision energy range is investigated theoretically. The study is based on classical dynamics on a single potential energy surface obtained in the diatomic in molecules approach. The parity of the clusters is a key parameter in the multifragmentation, which almost always produces a large ionic fragment with an odd number of atoms and one or more neutral monomer(s) and/or dimer(s). The initial temperature of the cluster has a significant influence on the fragmentation pathways of the odd clusters. These findings are in close agreement with experimental data.

Vibrational and collision energy effects on the reaction of CH3CHO+ with methanol

Integral cross sections and product velocity distributions were measured for reaction of acetaldehyde cation with methanol over a center-of-mass collision energy range from 0.1 to 2.2 eV. Reactivity is dominated by exoergic proton transfer (PT), which is strongly suppressed by collision energy, and mildly suppressed by CH3CHO+ vibrational excitation. PT is complex-mediated at low energies, switching to a direct stripping mechanism at high energies. Of the two possible PT channels, it appears that transfer of the aldehyde proton dominates. Hydrogen abstraction (HA) is a minor channel at low collision energies, also complex-mediated. Abstraction is observed from both hydroxyl and methyl sites on methanol, and the two channels have different, and counterintuitive collision energy dependence. Despite being exoergic, with no barriers, the HA channel shows apparent threshold behavior, attributed to competition with the dominant PT channel. The competition indicates that different intermediate complexes must interconvert efficiently, at least for low collision energies. At low energies, HA is strongly enhanced by collision energy, while vibrational excitation has no effect. Finally, there is a minor product channel corresponding to methyl elimination (ME) from a complex. Despite a relatively complicated reaction coordinate, the ME channel shows substantial recoil energy release and an asymmetric velocity distribution. A series of ab initio and RRKM calculations were performed to help interpret the results.

State-to-state dynamics of the H+c-C6H12→H2(v′,j′)+c-C6H11 reaction

The rovibrational state distributions for the H2 product of the H+c-C6H12→H2+c-C6H11 abstraction reaction have been measured at 1.6 eV collision energy under single-collision conditions. The state distributions are compared to those for the H2 product of the reaction of H atoms with CD3(CH2)4CD3 and n-C6H14 that we have measured previously. Abstraction of secondary H atoms from the cyclic C6 alkane produces H2(v′=0,j′) and H2(v′=1,j′) rotational state distributions that are significantly colder than those from the reaction of the linear C6 alkane. These results are interpreted within a simple phenomenological local reaction model that we have developed for these atom+polyatom reactions. The substantial differences between the state distributions for the reactions of the cyclic and linear alkanes indicate that reactions of cyclohexane are not representative of reactions at all secondary C–H.

Dissociative Recombination of HCNH + : Absolute Cross‐Sections and Branching Ratios

The dissociative recombination (DR) of HCNH+ has been studied at the heavy ion storage ring CRYRING. The absolute cross-sections have been measured between 0.01 meV and 0.2 eV collision energy. The DR thermal rate coefficients, which can be directly applied to modeling environments in thermal equilibrium, have been found to be 2.8 × 10-7(300/T)0.65 at temperatures T < 1000 K. The DR branching fractions have been measured for different dissociation channels: HCN(HNC)+H (0.67), CN+H2 (0.0), and CN+H+H (0.33) at collision energy of 0 eV. The results show that DR of HCNH+ is an efficient process leading to formation of HCN or HNC isomers, whereas CN production is dominated by three-body fragmentation. The multireference self-consistent calculations in a complete active space have been used as a common background for all studied species and reaction paths. Three-body fragmentation CN+H+H has been considered in one concerted elementary reaction step.

Reaction of acetaldehyde cations with water: The effects of CH3CHO+ vibrational mode and impact parameter on reactivity and product branching

Scattering of mode-selectively excited acetaldehyde cations from D2O was studied in a guided ion beam instrument. The effects of reactant vibrational state and collision energy on reactivity, product branching, and product ion recoil velocity distributions were measured. Ab initio calculations were performed to help understand the reaction coordinate. The dominant reaction is H/D exchange, which occurs in about 40% of low energy collisions, dropping to just a few percent at high energies. H/D exchange is also inhibited by CH3CHO+ vibration, but with a smaller effect than the equivalent amount of collision energy. H/D exchange is mediated by a long-lived complex, and several candidates are identified. The other low energy channel corresponds to methyl elimination from the collision complex. This channel is the most energetically favorable, but is only a few percent efficient, even at low energies, and is negligible at high energies. Methyl elimination is strongly suppressed by both collision energy and vibration, and the vibrational effects are nonmode specific. The most interesting channel is proton transfer (PT), which occurs by a direct mechanism at all collision energies. At low energies, PT occurs only in small impact parameter collisions, while at high energies, PT occurs primarily for large impact parameters, and is suppressed for small impact parameters. PT also shows strongly mode-specific dependence on CH3CHO+ vibrational state.

Collision energy effects on the dynamics of the reaction [formula omitted

A study of the collision energy effects on the dynamics of the title reaction was performed using the quasi-classical trajectories (QCT) method and an analytical triatomic potential energy surface recently derived by our group. Scalar and two-vector properties of the reaction were analysed in terms of the collision energy. The results obtained can be rationalised in terms of the coexistence of reactive trajectories with rebound and non-rebound features, both corresponding to an abstraction reaction mechanism. Future work should account for both the full dimensionality of the system and the possibility of quantum effects.

State-to-state dynamics of the H+CD 3(CH 2) 4CD 3→H 2+CD 3((CH 2) 3CH)CD 3 reaction: Dynamics of abstraction of secondary H in linear alkanes

Quantum state distributions for the H 2 product of the reaction of H atoms with CD 3(CH 2) 4CD 3 at 1.6 eV collision energy have been measured to characterize the dynamics of hydrogen atom abstraction at secondary sites in linear alkanes. The measured distributions are compared with those obtained previously for the H+ n-C 6H 14→H 2+C 6H 13 reaction. The two distributions are quite similar. The small differences seen between them are evaluated within a local reaction model developed for these types of reactions.

Inelastic low-energy electron collisions with the HBr and DBr molecules: Experiment and theory

Low-energy electron collisions with the HBr and DBr molecules are addressed from the experimental and theoretical points of view. Relative differential cross sections for the excitation of vibrational levels of HBr and DBr up to $v=6$ have been measured as a function of the incident electron energy in the range 0--4 eV. In addition to the shape resonance near 2 eV collision energy, intense and narrow threshold peaks are found for the excitation of the $v=1$ level of HBr and the $v=1$ and $v=2$ levels of DBr. Measurements with high resolution for rotationally cooled molecules have revealed the existence of sharp oscillatory structures in the elastic and $v=\stackrel{\ensuremath{\rightarrow}}{0}1$ cross sections in a narrow range below the dissociative-attachment threshold. The dissociative-attachment cross section has been measured with high resolution of the incident electrons in the range 0.2--1.4 eV. The theoretical analysis is based on an improved nonlocal resonance model, which has been constructed on the basis of existing fixed-nuclei electron-HBr scattering phase shifts and accurate ab initio calculations of the bound part of the ${\mathrm{HBr}}^{\ensuremath{-}}$ potential-energy function. This purely ab initio-based model is used to calculate integral electron-scattering and dissociative-attachment cross sections for HBr and DBr. The theoretical cross sections agree very well with the experimental data. The observed threshold peaks and Wigner cusp structures in the vibrational excitation functions are correctly reproduced. The sharp structures in the $v=\stackrel{\ensuremath{\rightarrow}}{0}0$ and $v=\stackrel{\ensuremath{\rightarrow}}{0}1$ cross sections below the dissociative-attachment threshold, consisting of a superposition of boomerang-type oscillations and quasibound levels of the outer well of the ${\mathrm{HBr}}^{\ensuremath{-}}$ potential-energy function, are quantitatively described by the theory. The high degree of agreement between experiment and theory indicates that the essentials of low-energy electron-HBr collision dynamics are completely understood.

The H+n-C5H12/n-C6H14→H2(v′,j′)+C5H11/C6H13 reactions: State-to-state dynamics and models of energy disposal

The rovibrational state distributions for the H2 product of the H+n-C5H12/n-C6H14→H2+C5H11/C6H13 reactions at 1.6 eV collision energy are reported. The results are compared to measurements made on the kinematically and energetically similar H+RH→H2+R (RH=CH4, C2H6, and C3H8) reactions as well as the atom–diatom reactions H+HX→H2+X(HX=HCl, HBr). For the title reactions, as for all the comparison reactions, the product appears in few of the energetically accessible states. This is interpreted as the result of a kinematic constraint on the product translational energy. Characteristic of the H+RH reactions we have previously studied, the title reactions show increasing rotational excitation of the H2 product with increasing vibrational excitation of it, a correlation that gets stronger as the size of the alkane increases. Trends and variations in the product energy disposal are analyzed and explained by a localized reaction model. This model predicates a truncation of the opacity function due to competing reactive sites in the polyatomic alkane reactant, and a relaxation of the otherwise tight coupling of energy and angular momentum conservation, because the polyatomic alkyl radical product is a sink for angular momentum.

Energy transfer, scattering and dissociation in ion atom collisions: CO2+/Ar

Collision-induced dissociation (CID) and nondissociative scattering of CO2+ ions following collision with a supersonic molecular beam of argon has been studied at low collision energies by crossed-beam tandem mass spectrometry. The center-of-mass (c.m.) velocity contour diagram at 23.8 eV collision energy showed that the scattering of CO2+ ions have two energetically distinct components: elastic collisions at smaller angles in which momentum exchange apparently involves Ar/O repulsive interactions and inelastic collisions at larger angles in which internally excited CO2+ ions recoil from the two-body CO2+/Ar c.m. The most probable energy transfer in the inelastic process is 4.8±0.5 eV, just below the lowest dissociation threshold. The CID processes at the same collision energy leading to fragment ions, CO+ and O+, show similar characteristics. CID occurs via both spectator knock-out and two-body collisions that result into two distinct scattering patterns. The energy transfers for the two pathways for O+ fragment ions are 4.7±0.5 eV for knock-out collisions and 7.6±0.5 eV for the two-body inelastic recoil collision mechanism. It is suggested that CID for O+ via the latter process must involve an electronic state higher than the C state and proceed via curve crossing. Energy transfers for CO+ fragment ions via the two corresponding processes are 5.7±0.5 eV and 7.6±0.5 eV, respectively, clearly suggesting similar mechanisms for energy transfer and dissociation for this CID process also. It is suggested that the bent geometry of the CO2+ ions may be an important factor in promoting two distinct mechanisms.

Collision-Induced Dissociation of Nitrobenzene Molecular Cations at Low Energies by Crossed-Beam Tandem Mass Spectrometry

The dynamics of the collision-induced dissociation (CID) of the nitrobenzene molecular ion have been investigated in the energy range of 3 eV−50 eV in the center-of-mass (CM) reference frame using a crossed-beam tandem hybrid mass spectrometer−supersonic molecular beam instrument. At these collision energies the dominant CID channel is loss of NO2 from the molecular ion and contribution from competing channels is very small. At 3.1 eV collision energy, fragment ions, C6H5+, are fully backward scattered and as collision energy is increased the most probable scattering angle moves in the forward scattering region but remains above zero up to 50 eV energy. These results suggest that small impact parameter impulsive collisions dominate this CID process in the low collision energy range. The energy transfer in the collisional activation step does not change significantly with increase in collision energy and corresponds to about 0.6 eV above the thermochemical threshold even though the molecular ions were form...

Quasi-classical trajectory study of the dynamics of the H+H 2O reaction: differential cross-sections and product rotational polarization

Quasi-classical trajectory calculations for the H+ H 2 O → OH+ H 2 reaction have been performed on the Ochoa–Clary potential energy surface [J. Phys. Chem. A 102 (1998) 9631] at 1.43 and 2.20 eV collision energies. Differential cross-sections, recoil energy distributions and product rotational polarization (polarization parameters and polarization-dependent differential cross-sections) have been obtained and compared with the existing experimental and theoretical data obtained on different potential energy surfaces. A qualitative agreement between theory and experiment has been found.

Hydride abstraction by NO+ from ethanol: Effects of collision energy and ion rotational state

The effects of NO+ rotational state and collision energy on the reaction NO++C2H5OH→HNO+C2H4OH+ were studied in a guided-ion-beam instrument over the collision energy range from 50 meV to 3.7 eV. Integral cross sections for the reaction are presented. NO+ is prepared in specific rotational levels (N+=0,1 and N+=10) by means of mass-analyzed threshold ionization. Ab initio calculations were used to probe stationary points on the potential energy surface. The reaction is sharply inhibited by collision energy, suggesting a bottleneck for reaction. If rotational energy had a similar effect, ∼50% inhibition from N+=10 excitation would be observed at low collision energy. Instead, rotation is found to have no effect within experimental error. A precursor complex mechanism is proposed to explain the results.

Collision induced fragmentation of small ionic sodium clusters. II. Three-body fragmentation

Multiple fragmentation of small Nan+ clusters (n=3–5) induced by collision with He atoms is investigated in the 200 eV collision energy range. The Nan+→Nan−2++Na+Na channels are studied using a multicoincidence technique allowing for the determination of the velocity vectors of the three fragments. The relative contributions of the two basic mechanisms, namely the electronic excitation and the momentum transfer in elastic binary collisions between the He atom and a Na core are estimated. For Na4+ and Na5+ clusters the momentum transfer mechanism appears as a two-step process: The ejection of a fast Na atom in a binary Na–He collision followed by the evaporation of an additional slow Na atom. However the angular distribution of the slow Na atom is not isotropic showing that a partial memory of the initial binary collision appears in the evaporation process.

Vibrational mode and collision energy effects on proton transfer in phenol cation–methylamine collisions

Mass-analyzed threshold ionization has been used to prepare vibrationally state-selected phenol cations, that were then reacted with methylamine at collision energies ranging from 0.1 to 2 eV. Integral cross sections and product recoil velocity distributions are reported. Ab initio calculations of stationary points on the surface and RRKM (Rice–Ramsperger–Kassel–Marcus) analysis of complex lifetimes are also presented for comparison. The only reaction observed over the entire energy range is exoergic proton transfer (PT). For ground-state reactants, the PT cross section is reduced by increasing collision energy, such that the reaction efficiency declines from ∼71% at low Ecollision to ∼50% at 2 eV. Excitation of either v6a or v12 vibrations inhibits reaction over the entire collision energy range, with the effect being somewhat mode-specific and increasing with increasing Ecollision. At low Ecollision, both vibrational and collision energy inhibit reaction with similar efficiency. Collision energy effects diminish at high Ecollision, while vibration continues to have a strong effect. Product ion velocity distributions are approximately forward–backward symmetric at Ecollision⩽1 eV, but are backward peaked at high energies. Mechanistic implications of these results are discussed.

Interactions of Hyperthermal TiClx+ (x = 0−4) Ions with Graphite Surfaces

The interactions of hyperthermal chlorotitanium ions, TiClx+ (x = 0−4) with HOPG surfaces were studied by low-energy ion scattering of TiClx+ from HOPG and by XPS depth profile analysis of the TiClx+ exposed HOPG surfaces. Positive ion scattering data for 5−500 eV collision energies showed exclusively dissociative scattering of the molecular TiClx+ into TiCl+ and Ti+ fragments. TiCl3+ and TiCl2+ daughter fragments were not detected in the surface-induced dissociation (SID) spectra of the corresponding larger incident TiClx+ ions. The observed survival of >100 eV TiCl3+ and TiCl2+ incident ions in the scattered ion spectra and the calculated trend in the electron affinities of TiClx neutrals implicate the role of bond dissociation in the efficient electron transfer to the departing TiCl3 and TiCl2 fragments during SID. XPS depth profile analysis showed a good correlation between the collision energy and the type of incident TiClx+ with the quality of TiC thin films formed on the surface of HOPG. Higher collision energies resulted in higher carbidic C to Ti ratios and higher carbidization efficiencies whereas a larger number of Cl atoms in the incident TiClx+ ion gave lower stoichiometries and lower carbidization efficiencies in the resulting films.

Electronic structure and dynamics of O(3P)+CO(1Σ+) collisions

The potential energy surfaces of the three lowest electronic triplet states of CO2 which lead to O(3P)+CO(1Σ+), 3A′, 1 3A″, and 2 3A″, have been computed at the completeactive-space-self-consistent-field plus second-order perturbation theory (CASSCF-MP2) level with a modest 631+G(d) basis. Potential energy surfaces are fit with a global functional form. The 3A′ state has a well 0.9 eV deep and the 1 3A″ state has a 0.2 eV well with respect to the O(3P)+CO(1Σ+) dissociation threshold. The 3A′ and 1 3A″ states are both bent at their minima and have a barrier at 0.2 eV and 0.3 eV above threshold, respectively. The 2 3A″ state is mostly repulsive, and has a saddle at C2v geometries. We have run classical trajectory calculations for O(3P)+CO(1Σ+) collisions using these surfaces. Results agree well with available vibrational relaxation and oxygen atom exchange measurements except at low temperature. Comparisons are also made with measured vibrational excitation cross sections and infrared emission spectra of the nascent CO products at 3.4 eV collision energy. These results show a high degree of vibrational and rotational excitation with a nearly statistical population which is evident in a distinct spectral “bandhead” signature. Analysis of the trajectories show that almost all collisions which lead to oxygen atom exchange and/or vibrational energy transfer occur when the O(3P) approaches the CO at OCO angles between 80° and 140°, passes over the barrier and through the wells of the 3A′ and 1 3A″ states, and interacts with the repulsive wall of the carbon end of the CO nearly perpendicular to the CO bond.

Dynamics by crossed-beam digital imaging: collisional energy effects on the Ba(1P1) + N2O→BaO(A′ 1Π, v′ ≤ 3) + N2 differential reaction cross-section

The dynamics of the excited-state Ba(1P1) + N2O→BaO*(A′ 1Π, ′⩽3) + N2 reaction were investigated by crossed-beam digital imaging at 0.065 eV collision energy. To this end, the spatial (angular) distribution of the long-lived BaO*(A′ 1Π, ′⩽3) product was imaged using a CCD camera. The analysis of the product angular distribution suggests a direct mechanism in which the excited-state BaO*(A′ 1Π, ′⩽3) shows a backward scattering with the total energy of the reaction mostly channelled into product internal energy. A comparison between previous results obtained for the same reaction investigated at 0.155 eV (C. Rinaldi et al., Phys. Chem. Chem. Phys., 2000, 2, 723) and the present data suggests little change in the dynamics except for a modest enhanced forward character of the BaO* angular distribution when the reactant collision energy increases from 0.065 up to 0.155 eV. A comparison of the excited-Ba(1P1) and ground-state Ba(1S0) + N2O reaction translational energy disposal with an impulsive and kinematic model prediction for atom–molecule reactions leads to the conclusion that kinematic factors play an essential role in both reactions. However, the ground-state reaction seems to proceed ia a more direct rebound mechanism with a predominance of collinear transition-state configurations.