Nonreactive Collisions Research Articles

Collision and reaction cells in atomic mass spectrometry: development, status, and applicationsThe opinions expressed in the following article are entirely those of the authors and do not necessarily represent the views of the Royal Society of Chemistry, the Editor or the Editorial Board of JAAS.

The development and utilization of collision and reaction cells in atomic mass spectrometry is reviewed. These devices have been used for decades in fundamental studies of ion–molecule chemistry and have only recently been applied in the GD-MS and ICP-MS fields. Such cells are used to promote reactive and non-reactive collisions, with resultant benefits in interference reduction, isobar separation, and thermalization/focusing of ions in ICP-MS. Novel ion–molecule chemistry schemes, using a variety of reaction gas reagents selected on the basis of thermodynamic and kinetic principles and data, are now designed and empirically evaluated with relative ease. These chemical resolution techniques can avert interferences requiring mass spectral resolutions of >600000 (m/Δm). Purely physical ion beam processes, including collisional dampening and collisional dissociation, are also employed to provide improved sensitivity, resolution and spectral simplicity. Collision and reaction cell techniques are now firmly entrenched in current-day ICP-MS technology, enabling unprecedented flexibility and freedom from many spectral interferences. A significant body of applications has now been reported in the literature. Collision/reaction cell techniques are found to be most useful for specialized or difficult analytical needs and situations, and are employed in both single- and multi-element determination modes.

Governing Equations for Weakly Ionized Plasma Flowfields of Aerospace Vehicles

A generalized set of conservative equations for simulating the flowfield in a hypersonic weakly ionized gas flows is presented. Additional numerical and physical complexities associated with the plasma state are identified and discussed, including the influence of external and space charge fields and the nonequilibrium coupling of reactive and nonreactive collisions. A restricted set of equations is then employed to simulate and analyze the flowfield of an air plasma generated by associative ionization. With use of a seven-species air model, details of the plasma flowfields are presented and compared with available experiments. Refined estimates of dissociation products, which are the precursors to the ionization reaction, are obtained. Specific attention is given to limiting forms of the electron diffusion coefficient and the influence of vibration-dissociation coupling. Details of the chemical reactions in the flowfield influencing the elastic and inelastic collisional energy transfer are reported to highlight the more important species and reaction mechanisms.

Competition between C−C and C−H Activation in Reactions of Neutral Yttrium Atoms with Cyclopropane and Propene

Branching ratios between C−C and C−H bond activation were measured for reactions of ground-state Y (a2D, s2d1) atoms with two C3H6 isomers (cyclopropane and propene) in crossed molecular beams. For both isomers, C−C bond activation led to formation of YCH2 + C2H4, whereas C−H activation led to YC3H4 + H2 and YH2 + C3H4. The angular and velocity distributions for all three product channels and for nonreactive collisions were measured at several collision energies (Ecoll). For Y + cyclopropane, the branching ratio for YCH2 + C2H4 increased relative to YC3H4 + H2 with increasing Ecoll, this C−C activation channel becoming dominant at Ecoll ≥ 19 kcal/mol. For the propene reaction, φYCH2/φYC3H4 also increased with Ecoll, reaching 0.75:1.00 at Ecoll = 28.8 kcal/mol. For both C3H6 reactants, formation of YH2 + C3H4 was observed as a minor channel at the highest collision energies. Experimental results and Rice−Ramsperger−Kassel−Marcus (RRKM) modeling indicate that for propene reactions all three channels involve initial formation of π-association complexes that undergo insertion into one of the sp3-hybridized β-C−H bonds in the methyl substituent. Decay of the yttrium allyl hydride intermediate by β-H migration leads to the two C−H activation products, YC3H4 + H2 and YH2 + C3H4. We propose that the YCH2 + C2H4 channel involves reverse β-H migration forming the same strongly bound metallacyclobutane intermediate formed in the Y + cyclopropane reaction.

Kinetic model for non-sticky collisions in pulsed molecular diffusion tube experiments

A kinetic model for molecular flow is developed to simulate the escape rate profiles that result when molecules are pulsed into the top of a cylindrical tube, experience non-reactive collisions with the tube surface and are detected after exiting at the base. The model includes time-dependent rate coefficients for formation of a pseudo steady-state spatial distribution and for escape. The mean distance traveled down the tube per collision is estimated by applying a cosine directional distribution. Under molecular flow conditions and for non-sticky gas/surface interactions this model allows for the determination of the average number of collisions per molecule before detection. Simulations were carried out on SF 6, Ar, N 2, Ne, He colliding with Pyrex and Teflon-coated surfaces at temperatures ranging from 25 to 64 °C and in tubes with a range of dimensions. Mean collision numbers are in agreement with those calculated from Monte Carlo simulations.

The use of dynamic reaction cell ICP mass spectrometry to facilitate Rb-Sr age determination

Abstract By using a set of granitic whole rock samples, originating from the Regensburg Forest (Germany) and dated (approximately 350 Ma old) previously by means of thermal ionization mass spectrometry (TIMS), the capability of dynamic reaction cell (DRC) inductively coupled plasma mass spectrometry (ICPMS) for Rb-Sr age determination was demonstrated. With DRC-ICPMS, chemical separation of Sr from Rb during the sample pretreatment is no longer required as interference-free determination of the 87 Sr/ 86 Sr isotope ratio can be accomplished by monitoring the signals of the SrF + adduct ions, formed as a result of the selective reaction between the Sr + ions extracted from the ICP and the reaction gas CH 3 F. The mass discrimination was established to depend strongly on the matrix composition. This drawback could be overcome by using the United States Geological Survey reference material G-2 as an external standard. Results obtained by DRC-ICPMS (age and initial 87 Sr/ 86 Sr ratio) showed an excellent agreement with both (a) experimental values obtained by means of quadrupole-based and sector field ICPMS after isolation of Sr via cation exchange chromatography and (b) TIMS literature values. In addition, DRC-ICPMS offers a smaller combined uncertainty on the isotope ratio results as a result of (a) an improved internal isotope ratio precision (<0.1% RSD when also using Ne as an additional non-reactive collision gas) and (b) the fact that, in contrast to quadrupole-based and sector field ICPMS, no correction for the remaining overlap between the signals of 87 Sr + and 87 Rb + is required.

Complex dynamics during the reactive scattering of Si +[formula omitted] and H 2

Reactions of silicon hydrides have recently attracted considerable attention due to their importance in plasma enhanced chemical vapor deposition (PECVD) of silicon for the microelectronics industry. Trajectories of reactive and non-reactive collisions between Si + and H 2 are simulated over a range of collision energies from 0.5 to 9 eV using the semiempirical molecular orbital method PM3, and our results lead to a much deeper understanding of one of the most fundamental reactions for deposition techniques. Vibrational, rotational and translational excitation of the SiH + product are reported as a function of impact kinetic energy. At impact energies below 4 eV, the system passes through configurations in which all three atoms interact strongly for times that can be several orders of magnitude longer than one rotational period. At higher translational energies, the product distributions are consistent with a direct reaction mechanism. All possible energy transfer mechanisms are studied for both low and high impact energies and possible implications for the deposition of silicon are discussed.

Molecular orbital treatment of gas‐phase ion molecule collision rates: Reactive and nonreactive collisions

Abstract I. Introduction 142 II. Reaction Efficiency 144 III. Ion Polar‐Molecule Collisions 145 IV. Momentum‐Transfer Ion‐Molecule Collisions 146 V. Summary 151 Acknowledgments 151

OH( v)+O 3: Does chemical reaction dominate over non-reactive quenching?

We discuss the title atmospheric reaction for vibrational excited states of OH over the range 0⩽ v⩽9. All calculations have employed the quasi-classical trajectory (QCT) method and a realistic double many-body expansion (DMBE) potential energy surface for HO 4 ( 2 A) . For v=9, the calculated thermal rate coefficient is found to agree with measurements of OH quenching reported by others, which has been attributed to both chemical reaction and non-reactive collisions. The temperature dependence of the rate constant is described by a simple model for all studied vibrational states of OH.

Far-wing scattering studies on the reaction Li*(2p,3p)+H2→LiH(v″=1,2,J″)+H

We did laser pump–probe far-wing scattering experiments to study the photochemical reactions Li*(2p,3p)+H2→LiH(v″=1,2,J″)+H. We show in this work that the Li(2p)+H2⇒LiH+H reaction takes place when the kinetic energy of the reactants are high enough. We think that the Li(2p)+H2 collisions take place preferentially in bent near-C2v geometry and that the LiH2 2A′ intermediate plays a major role where wide-amplitude internal vibrations eventually result in losing a hydrogen atom from the LiH2 complex. We also have observed the Li(3p)+H2⇒LiH+H reaction. The rotational state distribution of the LiH product for excitation in the red wing is identical to that found in the blue wing region, it does not show any preference to high or low rotational states, and no asymmetry is observed in the reactive to nonreactive branching ratio. We show different possibilities for the reactive and nonreactive collisions and provide a tentative explanation of the reaction mechanism using the highly accurate ab initio potential energy surfaces. We also show that the harpooning model cannot be used to explain the reaction mechanism in the Li*+H2 collision for all the atomic states from Li(2s) to Li(3d), and that no long range electron transfer occurs from the metal atom to the hydrogen molecule.

Dynamics of the OH(v = 1,2,4) + O3 atmospheric reaction

We report a theoretical dynamics study of the title five-atom atmospheric reaction using the quasiclassical trajectory method and a realistic double many-body expansion potential energy surface recently reported for HO4(2A). A comparison with available results on the OH() quenching based on nonreactive collisions is also presented. The results indicate that vibrational excitation strongly enhances reactivity, with removal of vibrationally excited OH occurring predominantly through chemical reaction.

Dynamics of the OH−+D2 isotope exchange reaction: Reactive and nonreactive decay of the collision complex

Reactive and nonreactive collisions between OH− and D2 are investigated over the collision energy range from 0.27 to 0.67 eV by the method of crossed molecular beams. The angular and energy distributions measured for the isotope exchange reaction are quite similar at all collision energies, indicating that the collision dynamics are relatively insensitive to energy over this range. Although the exchange reaction involves the formation of an intermediate complex ion in which bond rearrangement takes place, the OD− products are primarily backward scattered with only low intensity scattering appearing in the forward direction. The forward scattered products do contribute proportionately more intensity to the complete differential cross section as the collision energy increases, suggesting that both direct and collision complex mechanisms are responsible for reaction. The angular distributions are interpreted in terms of the osculating model for chemical reaction occurring in a fraction of the rotational period of the intermediate complex, augmented by a small forward scattered direct component. This model suggests that the complex lifetime is approximately 150 fs at 0.27 eV. When the OD− product is backward scattered, little internal energy is found in either product. A simple impulsive model explains the lack of internal excitation based on the geometry of the transition state of the complex. More of the available energy is deposited into product internal modes when the intermediate complex lives longer and OD− is forward scattered. The collision energy can be redistributed into bending modes of the complex, which impart angular momentum to the fragments when the complex dissociates. The recoil energy distributions for the nonreactively scattered OH− show strong similarities to the reactively scattered OD− distributions and are useful in probing the origin of the product rotational excitation. Structure in the recoil energy distributions corresponding to inelastically scattered OH− indicates the preferred deposit of available energy into specific rotational modes. © 2000 American Institute of Physics.

Reactive Probability of Atomic Oxygen with Material Surfaces in Low Earth Orbit

A theoretical model based on the theory of molecular dynamics for evaluating the reactive probabilities of the atomic oxygen and highly organizedmaterials in low Earth orbit is proposed. The model is constructed to compute the classical trajectory ensembles by solving a system of stochastic differential equations.The reactive probabilities of the atomic oxygen colliding with atoms on the material surface can then be evaluated in accordance with the criteria proposed for reactive and nonreactive collisions. The computed reactive probabilities for atomic oxygen interacting with graphite are presented as a test case and are compared with  ight-test data from STS-46. The model constructed is reliable to some extent as demonstrated in the test case.

Potential Energy Surfaces for LiH2 and Photochemical Reactions Li*+ H2 ↔ LiH + H

Potential energy surfaces for the Li*+ H2 → LiH + H and the reverse reactions are calculated using ab initio methods. Extensive configuration interactions have been done for a large number of collinear, C2v, and Cs geometrical forms using a large basis set to describe the 2s−3d atomic states of the lithium atom and the neutral and anionic hydrogen molecule. The Li(3s) channel has a small activation barrier and leads to a stable intermediate, 4 2A‘ (LiH2)*, but a diabatic coupling between the 3 and 4 2A‘ surfaces would preferentially lead to a nonreactive inelastic collision producing Li(2p). The Li(2p) channel leads to one entirely repulsive potential surface (3 2A‘) and two attractive potential surfaces (2 2A‘ and 1 2A‘ ‘) resulting in two stable intermediates. The Li(2p) atoms with enough collision energy to overcome the endothermicity of 1624 cm-1 can lead to the reaction producing LiH. Those three excited intermediate complexes are bent (planar) and not linear. The charge transfer from the metal atom to the hydrogen molecule does not occur in the course of the collision but much later, when one hydrogen atom breaks away, implying that the harpooning model does not apply to this case. The reverse reaction LiH + H → Li + H2 can produce (i) the high-energy Li(2s) directly, (ii) the thermal Li(2p), and (iii) the thermal Li(2s) accompanied with a chemiluminescence. The 2 2A‘ intermediate may play an important role in the reverse reaction, too.

Reactive and nonreactive scattering of Na(3S) and (3P) from SF6 and SiF4

Double differential cross sections of sodium in both the 3S ground state and the 3P electronically excited state scattered from SF6 and SiF4 have been measured at center of mass collision energies of 0.6 and 2.0 eV for SF6 and 1.0 eV for SiF4. For the experiments with Na(3P), a two frequency excitation scheme is applied, by which an excitation efficiency (determined by scattering experiments) of 31% is obtained. In the SF6 experiments, one finds a substantial transfer of the collision energy into the vibrational and rotational degrees of freedom of SF6. A transfer of electronic energy of Na(3P) into SF6 is not observed in nonreactive collisions. For the NaF formed in the reaction, an angular distribution originating from a long-lived complex, superimposed by a forward peak caused by a direct reaction mechanism is found. Especially the ground state measurements show an angular-dependent velocity distribution of the product. Compared to forward and backward scattering in sideways collisions, a lower velocity is found. From Na(3S) to Na(3P), the general characteristic does not change. The gain of reactivity with the excitation is decreasing with the collision energy. For the experiments with Na(3P) and SiF4, a quenching channel is found. The energy transferred to SiF4 is in good agreement with the prior distribution, calculated in a simplified RRHO model. The statistical behavior and the shape of the angular distribution of the intensity suggest the interpretation by the formation of a long-lived collisional complex.

Energy dependence of C60–graphite surface collisions

The resilience of C60 fullerene with graphite (0001) surfaces has been investigated by means of molecular dynamics simulations with empirical model potentials. The initial energy of C60 ranges from 30 to 300 eV. It is shown that when the impact energy is above 60 eV, the rebounding energy of C60 is nearly independent of the impact energy. The scattering is highly inelastic and the internal excitation energy of the scattered molecule increases with the incident energy. These results are consistent with experiment. Furthermore, the simulations provide insight into the microscopic aspects of the scattering. The rebounding processes at different energies are found to exhibit similar dynamic behavior and the molecular center-of-mass motion can be regarded as moving in a quadratic harmonic potential. All of these results support a schematic picture to describe nonreactive C60-surface collisions.

Multiquantum vibrational transitions in O 2( v≥25)+O 2( v=0) collisions

The quenching of highly vibrationally excited O 2 in collision with O 2( v=0) is investigated using a semiclassical wave packet method. We find that nonreactive collisions dominate the relaxation of O 2( v) and for v in the range 25 to 30 the reactive channel makes little contribution to the quenching process, in agreement with our earlier investigation. We report vibrational transition cross sections and relaxation rate coefficients for v=25 to 30 of the excited O 2( v) molecule. Our study shows that a multiquantum process involving a 2→1 vibrational transition which has not been included in previous calculations makes an important contribution to the rate coefficients for the relaxation of O 2 vibrational levels v≥27.

Quasiclassical trajectory studies of H(D)+HF(DF) collisions at 2 eV

This paper presents a quasiclassical trajectory study of nonreactive and reactive (F transfer) collisions involving H+HF, D+HF, H+DF, and D+DF, with emphasis on comparisons of product vibration/rotation distributions with those in an accompanying paper by Lindner, Lundberg, Lovejoy, and Leone. We find that the nonreactive rotational distributions for each isotope and each vibrational state are composed of a large peak at low J plus a long tail that extends to high J. These results are in general agreement with experiment. The low J peak arises from direct collisions of H with the F atom in HF, while the high J tail is due partly to failed reactive (F-exchange) collisions, and partly to collisions of H with the H atom in HF. The calculated reactive rotational distributions are controlled by the kinematic propensities expected for a heavy–light–heavy mass combination and a nearly isotropic potential. This result does not appear to match experiment, although limitations in the data preclude a detailed comparison.

Kinetics of the toluene-iodine complex formation as revealed by Raman spectroscopy

The kinetics of formation of the toluene—iodine charge-transfer complex has been investigated using Raman spectroscopy of the iodine vibrational line. Using low donor concentrations, the non-reactive part of the line profile can be separated out from the total lineshape with the aid of the inverse Fourier transforms of the Raman spectrum. The remaining part arises in the complex formation and dissociation process only, and can be used to extract the complex lifetime. Spectra were recorded at 300 K from a solution of iodine dissolved in dilute (0–3 volume%) mixtures of the donor in n-heptane used as an inert solvent. Using for the equilibrium constant the value 0·500 l mol−1 obtained by Childs using the polyiodide solubility method, the lifetime of the toluene—iodine complex as determined from the residual linewidth of the reactive part of the total line profile is found to be 0·9−0·2 +0·3 ps. The equilibrium constant value of 0·315 l mol−1 obtained by Childs using the spectroscopic method is not consistent with a simple two-state model. The meaning of the concept of the lifetime of the complex is discussed. It is argued that it may describe the influence of several different processes, including both true dissociation of the complex and non-reactive collisions between the iodine and the donor molecules.

Collisional Excitation of H 2 Molecules by H Atoms

view Abstract Citations (25) References (13) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Collisional Excitation of H 2 Molecules by H Atoms Lepp, S. ; Buch, V. ; Dalgarno, A. Abstract Semiclassical trajectory calculations of the rate coefficients for rotational and vibrational energy transfer were carried out for collisions of hydrogen atoms with hydrogen molecules at a temperature of 1000 K, using three potential energy surfaces. For one surface the calculations were carried down to 200 K. The rate coefficients for reactive collisions are similar for all three surfaces. For nonreactive collisions, rate coefficients for pure rotational transitions in which j changes from 0 to 2 and from 1 to 3 change significantly from one surface to another but for higher angular momenta transitions the differences tend to disappear. The validity of the semiclassical calculations is assessed. Publication: The Astrophysical Journal Supplement Series Pub Date: May 1995 DOI: 10.1086/192164 Bibcode: 1995ApJS...98..345L Keywords: ISM: MOLECULES; MOLECULAR PROCESSES full text sources ADS |

Classical trajectory studies of the reaction CH4+H→CH3+H2

Trajectory data are reported for the reaction CH4+H→CH3+H2, designed to provide information that can be used to test approximate quantitative theories for the dynamics of abstraction reactions. A potential function was devised which properly reflects the nuclear permutation symmetry of the process. Microscopic reaction rate coefficients were obtained as functions of fixed rotational and vibrational energy, and of the angular momentum. The data indicated significant uncoupling between the various modes although, at a minimum, the symmetric stretch is directly coupled to the reaction coordinate at the transition state. The data were used to test the assumption that the total angular momentum, J, may be approximated by the orbital angular momentum, L. L is approximately conserved from the reactant to the saddle point configuration in reactive and nonreactive collisions and may be well approximated by J. The angular momentum about the long axis of the reacting system (equivalent to the K quantum number) is not conserved in either reactive or nonreactive trajectories.