eV Collision Energy Research Articles

Dissociative recombination and dissociative excitation of 4HeH+: Absolute cross sections and mechanisms.

Absolute cross sections have been determined for the dissociative recombination and dissociative excitation of $^{4}\mathrm{HeH}^{+}$ for electron energies below 40 eV. The dissociative recombination cross section is in semiquantitative agreement with recent theoretical results by Sarpal, Tennyson, and Morgan [J. Phys. B 27, 5943 (1994)] and Guberman [Phys. Rev. A 49, R4277 (1994); in XIXth International Conference on the Physics of Electronic and Atomic Collisions, Whistler, Canada, AIP Conf. Proc. No. 360, edited by L. J. Dube, J. B. A. Mitchell, J. W. McConkey, and C. E. Brion (AIP, New York, 1995), p. 307]. The calculated resonant structure below a collision energy of 1 eV was not fully reproduced by the experiment. The quantum states of the dissociative recombination products at 0 eV collision energy have been determined; ground-state helium and excited hydrogen atoms (n=2) are dominantly formed, in agreement with recent predictions by Guberman. The dissociative excitation has an onset around 10 eV and follows the shape of the dissociative recombination cross section, illustrating that both processes start with the formation of doubly excited neutral states that lie in the ionization continuum as well as in the dissociation continuum. The dissociative excitation cross section is in quite good agreement with recent calculations by Orel and Kulander. \textcopyright{} 1996 The American Physical Society.

State multipoles and Stokes parameters for the 5 1,3 D 2 excitation of cadmium

Relativistic distorted-wave calculations have been performed for the excitation of the 51,3D2 state from the ground state of cadmium by polarised and un-polarised electrons. A full set of state multipoles of experi-mental interest have been obtained for 20 and 40 eV collision energies. From these results we have calculated the Stokes parameters of the light emitted in the decay to the relevant P states.

The fate of Li and Be in stars and in the laboratory

We connect the observed under-abundances of Li and Be in dwarfs, with recent results on nuclear cross sections at low energies: for collisions of protons with atomic or molecular targets, the measured cross sections seem too high with respect to extrapolations for bare nuclei. Phenomenologically, these anomalous nuclear interactions can be described in terms of an effective screening potential $U_{lab}$ in the range of few hundred eV: in the presence of the electron cloud, nuclei become more transparent to each other as if the effective collision energy is aumented by $U_{lab}$. This implies that fusion cross sections are enlarged and at the same time elastic cross sections are lowered. If something similar occurs in stellar plasma, the nuclear burning temperatures are lowered, whereas diffusion processes are enhanced. We find that the observed Li and Be abundances in the Hyades and in the Sun can be reproduced for effective screening potentials of the plasma in the range of 600-700 eV, close to that found by experiments in the lab.

Surface-induced dissociation of benzene on a PFPE liquid insulator in a time-of-flight mass spectrometer

A perfluorinated polyether (PFPE) surface is used to enhance the fragment ion yield for surface-induced dissociation (SID) in a tandem time-of-flight mass spectrometer. Selection of the parent ion by separation of M + from (M — H) + is obtained with unit resolution by pulsing the voltage on the collision surface. The benzene radical cation is used as a model compound to characterise the instrument. Fragment ions are collected with a mass resolution of 100. The distribution of recoil energies of the SID fragments leaving the collision surface is not sufficient to clarify the limited mass resolution. We estimate an average conversion efficiency of kinetic into internal energy of 30±7% between 20 and 40 eV collision energy by analysing the relative intensities of the SID fragments as a function of collision energy.

A comparison of state-to-state rotational total cross sections using two- and three-dimensional close coupled approaches

State-to-state rotational cross sections have been calculated for the XeCO 2 scattering at 0.01 and 0.03 eV collision energies using the two- and three-dimensional close coupling methods. As will be pointed out, these results are in fair agreement only after the last multiple collision maximum in the two-dimensional dynamics. The full two-dimensional quantum total cross sections, at 0.2 eV of the collision energy, are also presented for the collision process under study.

Translational Energy Effects in the Excited Ca(1D2) + HCl → CaCl(A) + H Reaction. Experiments and a Harpooning Mechanism

By using the time-of-flight technique under crossed-beam conditions, the collision energy dependence of the Ca(1D) + HCl → CaCl(A) + H reaction cross-section has been measured over the 0.2−0.5 eV collision energy range and found to have a broad peak at 0.3 eV. An empirical description of entrance covalent channelsincluding orbital alignment effects due to the open-shell nature of calcium atomsand a harpooning mechanism, involving ionic charge-transfer states, account for the observed translational energy dependence of the reaction cross section.

Molecular dynamics mass spectrometric study of the collision-induced dissociation of CS 2+ ions at low and intermediate collision energies

The dynamics of the collision-induced dissociation of CS 2 + molecular ions to CS and S + fragment ions has been investigated using a crossed-beam tandem hybrid mass spectrometer at collision energies ranging from near threshold to 34.5 eV in the center-of-mass (CM) reference frame. The results show that small impact parameter impulsive collisions are involved in the collisional activation process, resulting in extensive scattering of fragment ions with intensity maxima at non-zero angles. The dissociation of CS 2 + to S + proceeds via a mechanism mainly involving the transfer of the minimum amount of energy required to dissociate the parent ion into S + ( 4S) and CS (X 1 Σ +) at all collision energies. However, the mechanism for the formation of CS + depends strongly on the collision energy. Energy transfer is very efficient for this process and a large fraction of the CM collision energy is converted to internal excitation of the colliding CS 2 + ions up to a maximum of nearly 16 eV (at 34.5 eV collision energy). At the lowest collision energy we have investigated, 6.5 eV, the lowest energy reaction path to form CS + (X 2 Σ +) and S ( 3P) is followed exclusively. However, as the collision energy is increased, several additional dissociation channels contribute towards the total dissociation. These results are rationalized by the efficient electronic excitation of the colliding CS 2 + ions at these collision energies.

Quantum and classical two-dimensional analysis of rainbow structures in the Xe+CO2 rotational excitation at 0.2 eV collision energy and on a repulsive potential.

Rotational rainbows in the collision Xe+${\mathrm{CO}}_{2}$ are analyzed using quantum and classical two-dimensional models. We discuss the effect of the single- and the multiple-collision rotational rainbows on the differential cross sections, both for the fixed transitions and varying scattering angle and for the fixed scattering angle and varying transitions. A comparision is made with the existing centrifugal sudden calculations for this system.

Vibrational mode effects, scattering dynamics, and energy disposal in reaction of C2H+2 with methane

The effects of collision energy and mode-selective vibrational excitation on the reaction of C2H+2 with CH4 and CD4 have been measured, along with the corresponding product velocity distributions. Two distinct reaction mechanisms are active in the energy range below 5 eV. At low energies, a long-lived C3H+6 complex forms efficiently, then decomposes primarily to C3H+5+H and C3H+4+H2. The RRKM lifetime of this complex is estimated to range between ∼10 ns and ∼10 ps over the experimental energy range, and this is sufficient time to allow substantial H-atom scrambling. Complex formation is strongly inhibited by collision energy, weakly inhibited by CC stretching, and enhanced by bending excitation. Competing with the complex-mediated mechanism is a direct H-atom abstraction reaction, producing C2H+3+CH3 with little atom scrambling. This reaction is shown to have a ∼150 meV activation barrier and is strongly enhanced by collision energy, becoming the dominant channel above 0.4 eV. CC stretching provides a weaker enhancement than collision energy, while bending enhances the reaction ∼10 times more efficiently. As collision energies increase, the C2H+3 product is increasingly forward scattered with an increasing fraction of the available energy going into recoil. Energy put into reactant vibration mostly is retained as internal energy of the products. Over the collision energy range from 0.4 to 2.8 eV, the collision time in the direct reaction varies from ≥1.3 ps to ≤70 fs.

Vibrational mode and collision energy effects on a highly constrained reaction: OCS+(ν)+OCS→CS+2+CO2 and S+2+2 CO

We report the effects of collision energy and OCS+ vibrational state (ν1, ν2, and ν3) on the reaction of OCS+ with OCS. Production of CS+2+CO2 is exoergic and the cross section shows no evidence of an activation barrier. Nonetheless, the cross section is only ∼0.1% of the collision cross section, even at low collision energies where formation of an intermediate complex is facile. There appears to be a severe phase-space (steric) bottleneck for this rearrangement reaction. CS+2 production is weakly inhibited by collision energy, and enhanced by all three modes of OCS+ reactant vibrational excitation. Production of S+2 is endoergic and is enhanced by collision energy and by ν2 (bend) and ν3 (CS stretch) excitation. Excitation of ν1 (CO stretch) does not enhance this channel, even though it is the highest energy mode. At high collision energies, S+2 production becomes relatively efficient, suggesting that the reaction mechanism for this channel is direct with no significant bottleneck.

Comparison of rate coefficients for rydberg electron and free electron attachment

New experimental results on attachment reactions involving free electrons at sub-meV resolution allow for the first time a conclusive comparison of measured rate coefficients for Rydberg electron attachment with those calculated from the measured free electron cross sections on the basis of the quasi-free electron model for Rydberg electron collisions. Using classical velocity distributions for the highn Rydberg electrons and our measured free electron attachment cross sections, we calculate Rydberg electron attachment rate coefficientsknl for the two cases SF6 and HI for Rydberg binding energies |En| of 0.1–40 meV. We find a significant increase inknl towards lower binding energies, especially for HI, which is due to the deviation of the free electron cross section from the limitings-wave behaviour σ0∼E−1/2. The increase at |En|≲2 meV is in qualitative agreement with our highn Rydberg data (n≳80) if l-mixing due to residual electric fields is taken into account. For low l, Rydberg rate coefficientsknl(|En|) are significantly larger than free electron rate coefficientske(E=|En|), while for circular orbits (l=n−1) they agree. On average, attachment reactions of Rydberg electrons in low l orbits proceed with an effective collision energy substantially smaller than the binding energy |En|.

A mass spectrometric investigation of the gas phase ion—molecule reactions of fullerene cations and dications with various neutral molecules

We present an investigation of the gas phase ion—molecule reactions of fullerene cations and dications with various neutrals utilizing a triple quadrupole mass spectrometer. The neutral reactant gases investigated include Ar, He, N 2, CH 4, NH 3, O 2, D 2O, CH 3OH, (CH 3) 2O, C 3H 6, CH 3CH 2CHCH 2, and (CH 3) 2CH 2. We report associative additions, partial charge transfer, and the formation of various fullerene adducts. The effect of collision energy, collision cell pressure, fullerene structure, and ionization energy on the reported reactions are addressed.

Collision energy effects on the collision‐induced dissociation of multiply charged melittin

The fragmentation of the melittin 4+ ion has been studied over a range of collision energies from 28 eV to 12 keV in a four-sector mass spectrometer. Patterns of fragmentation and energy conversion were found to be analogous to those observed for singly charged peptides. High numbers of collisions were not required for activation at low energies.

An angular study of the He ++H 2→HeH ++H * reaction at 20 eV (c.m.) collision energy

The dependence of the intensity of HeH + particles produced in the title reaction has been measured as a function of scattering angle and product energy. The results exhibit a strong localization in the angle and energy (or velocity) coordinates, which has to be related to crossings of the implied potential energy surfaces.

Comparison of Triple Quadrupole and Double Focusing Mass Spectrometers to Investigate Collision‐Induced Dissociation and Metastable Ions of Glycoconjugates

AbstractGlycoconjugates, such as chromophore‐labeled disaccharides and permethylated glycosphingolipids (GSL) were used for comparison of triple quadrupole and double focusing mass spectrometers in analysis of product ions. A profound effect of collision energy was observed in the product ion spectra of ceramide ions (fragment ions of permethylated GSL): more product ions were observed from a double focusing mass spectrometer. Besides collision energy, the structure of the analyte had a significant effect on the formation of product ions. Despite the fact that masses of protonated molecular ions (MH+) of permethylated GSL are significantly larger than their ceramide fragments, the low‐energy and high‐energy product ion spectra of MH+ are, in general, similar. In a double focusing mass spectrometer of reversed geometry, more metastable ions were observed in the first field free region (FFR) than in the second FFR. The metastable ions observed in the second FFR were similar to those observed in low‐energy collision‐induced dissociation (CID). Although a double focusing mass spectrometer is superior to triple quadrupole instrument for detection of product ions, the poor resolution in either the selection of precursor ion or in the product ion spectra can be a serious problem in analysis of a mixture with similar masses.

On the formation of deuterium fullerene complexes in collisions of C 60 with D 2

The collision dynamics of C 60 with deuterium is studied using an approximate LCAO-LDA scheme, combined with molecular dynamics (MD), and consideration of the collision statistics. The MD simulations of D 2+C 60 collisions show the encapsulation of deuterium, elastic scattering of D 2, dissociative scattering, and formation of exohedral C 60-D complexes. The formation of deuterium C 60 complexes shows a distinct maximum between 20 and 30 eV collision energy.

Kinetics of the Reactions of F- with CF3Br and CF3I as a Function of Temperature, Kinetic Energy, Internal Temperature, and Pressure

Abstract : The reactions of F- with CF3Br and CF3I have been studied at several temperatures as functions of pressure and ion-neutral average center-of-mass kinetic energy ((KE sub cm)). Rate constants and product branching fractions were measured by using a variable temperature-selected ion flow drift tube apparatus. Both title reactions involve two product pathways: displacement and association, the latter being the major channel under most experimental conditions. The rate constants for displacement are governed by total energy, regardless of the relative contributions by translational and internal energy, indicating that the displacement reactions behave statistically. In contrast, the rate constants for association depend strongly on the internal energy of CF3X (rotations and vibrations) compared with the relatively weak effect of translational collision energy. For CF3Br the rate constants for the association channel increase with increasing pressure while those for displacement are independent of pressure, indicating that the additional reactivity leading to more association at higher pressure appears not to compete with the displacement pathway. This lack of competition may be the result of two different reaction intermediates. The more efficient CF3I reaction is at or near the high-pressure limit for association, showing no pressure dependence in both the association and displacement channels. Therefore, there appears to be no competition between these channels for CF3I as well, but the data are less definitive in this case.

Negative ion chemical ionization and collisionally activated decomposition mass spectra ofO-2,3,4,5,6-pentafluorobenzyloxime derivatives of prostaglandins

Prostaglandin E1, prostaglandin E2, prostaglandin E3, 6-oxo-prostaglandin F1α, 1-carboxy-rioprostil and rioprostil were derivatized to the pentafluorobenzyloxime/trimethylsilyl ether/methyl ester and pentafluorobenzyloxime/trimethylsilyl ether (rioprostil) respectively. In the high-mass region, except for the 6-oxo-PGF1α, derivative, negative ion chemical ionization (methane) mass spectra show an intense [M - HF]−. ion. Collisionally activated decomposition mass spectra (0.2 Pa argon collision gas pressure, 15 eV collision energy) of this parent ion ([P]−.) of the syn and anti oxime isomers are quite different. The syn isomers have intense [P  (C1–C7)]− and [C6F4CH2O]− daughter ions, while the most abundant peaks in the spectra of the anti isomers are [P  OH  TMSOH]−, [P  OH  2TMSOH]− and [(C9–C10)  HF]−. Negative ion chemical ionization mass spectra of 6-oxo-prostaglandin F1α derivatives have intense [M  HF  (CH3)2SiCH2]− ions. Most prominent daughter ions are [P  (C1-C5)]− and [P - 2HF  H  (C1–C5)]−.

Molecular Dynamics Simulations of Hypervelocity Buckminsterfullerene Collisions

ABSTRACTMolecular dynamics simulations of high-energy collisions between various combinations of C60 and C70 fullerenes were performed to calculate the threshold for molecular fusion of these clusters as a function of the center-of-mass collision energy. For collision energies below 90 eV, only non-reacting collisions occurred with no observation of any fusion. However, at higher collision energies molecular fusion of the colliding clusters was observed with the fusion probability approaching 1 by 160 eV collision energy. The non-fusing, rebounding collisions showed deeply inelastic behavior with the loss of translational energy to internal energy varying from 50 to 70 percent.

Surprising positive correlation between rotational and vibrational excitation in the H+ CDCl 3→HD + CCl 3 reaction

The rotational quantum state distributions of the HD product of the H + CDCl 3→HD + CCl 3 reaction at 1.6 eV collision energy have been measured using coherent anti-Stokes Raman scattering. We observe HD products in v′=0, 1 and 2 with the fraction of available energy in HD rotation increasing as the energy in HD vibration increases. Despite the presumed pseudo-atomic nature of the CCl 3 radical in the reaction, the behavior of this reaction is closer to that of the H + alkane hydrogen atom abstraction reactions than that of the H + HX→H 2 + X (XCl, Br, I) reactions.