Rotationally Inelastic Collision between Two Polar Molecules

In the work, accurate converged theoretical simulations of the Ne + HD + ( v = 1 ) reaction are presented with the time-dependent wave packet (TDWP) method using the accurate potential energy surface by Lv (J. Chem. Phys. 2010, 132, 014303). In particular, the effect of intramolecular substitution and the translational collision energy on the reactive dynamics is investigated in detail to discern the reaction mechanisms. Total and state-to-state integral cross sections (ICSs) and differential cross sections (DCSs) of the two channels are derived and compared over a wide range of collision energies up to 1.2 eV. The analysis of the ICSs in terms of the total angular momentum contributions shows that the contribution of the J partial wave varies in the two reactive channels at different collision energies. The product state-resolved DCSs reveal that two opposite mechanisms co-exist in the two reactive channels with different ratios at low and high collision energies. Overall, NeH + + D channel appears to form the products via collision complex formation, which survives long enough leading to a forward–backward symmetric total DCS at low collision energies. In comparison, the NeD + + H proceeds through a direct stripping reaction, with predominantly forward scattering in the DCSs.

Charge exchange from doubly charged rare gas cations to simple diatomics proceeds with a large cross section and results in populations of many vibrational and electronic product states. The charge exchange between Xe(2+) and N2, in particular, is known to create N2 (+) in both the A and B electronic states. In this work, we present integral charge exchange cross section measurements of the Xe(2+) + N2 reaction as well as axial recoil velocity distributions of the Xe(+) and N2 (+) product ions for collision energies between 0.3 and 100 eV in the center-of-mass (COM) frame. Total charge-exchange cross sections decrease from 70 Å(2) to about 40 Å(2) with increasing collision energy through this range. Analysis of the axial velocity distributions indicates that a Xe(2+) - N2 complex exists at low collision energies but is absent by 17.6 eV COM. Analysis of the axial velocity distributions reveals evidence for complexes with lifetimes comparable to the rotational period at low collision energies. The velocity distributions are consistent with quasi-resonant single charge transfer at high collision energies.

The scattering of ions by polar molecules has been studied using numerical techniques. Because of the long range potential of interaction the scattering cross sections are very large. The total scattering cross section and rotational energy transfer cross section are of the order of thousands of Å2 for the cases studied. The momentum transfer cross sections are 1.3 to 2.0 times the capture cross sections in contrast to the σm/σc=1.105 for Langevin collisions. However, the scattered intensity I(θ) at impact parameters less than 20Å can be quite anisotropic because of the nature of single reflection collisions.

Excitation, ionization, and electron capture cross sections for collisions of Li 3+ with ground state and excited hydrogen atoms

The rotational excitation of HD by low-energy electron collision is discussed for the first time. In the thermal-energy region, the Born approximation is used to derive relevant cross sections semi-quantitatively. In the eV-region, the adiabatic approximation is used to discuss the problem qualitatively. It is found that except in the neighborhood of the threshold energy, the rotational excitation cross section σ( J → J +2), J being the rotational quantum number of the molecule, does not change much when H 2 is replaced by HD. For HD, transitions with odd Δ J are also allowed. However, the cross sections for these transitions are found to be fairly small. The momentum-transfer and the rotational stopping cross sections are discussed. Excitation of D 2 is also considered for comparison.

A hybrid BEQQ mass spectrometer for studies in gaseous ion chemistry

The reactions of Ar+ with HCl were studied for the two spin-orbit states of Ar+, 2P3/2 and 2P1/2, by the threshold electron-secondary ion coincidence (TESICO) technique. State selected relative reaction cross sections of the two product channels, hydrogen abstraction (HA) to form ArH+, and charge transfer (CT) to form HCl+, were determined at five collision energies in the range of 0.71–3.5 eV. At low collision energies, the cross sections of both channels show no substantial difference between the two spin-orbit states. At high collision energies, the cross sections for the HA channel also show no difference between the two states, whereas those for the CT channel show significant dependence on the spin-orbit state, the cross sections for Ar+(2P1/2) being always larger than those for Ar+(2P3/2). Combining these results with the coincidence time-of-flight (TOF) spectra, a direct reaction mechanism is suggested for both channels. The importance of the nonadiabatic transitions between the surfaces correlating to the Ar+ + HCl and Ar + HCl+(A2Σ) states in leading the reaction is proposed. Proton transfer follows the nonadiabatic transition for the HA channel and radiative or nonradiative transition follows for the CT channel.

An updated version of the CO + CO potential energy surface from [R. Dawes, X. G. Wang and T. Carrington, J. Phys. Chem. A 2013, 117, 7612] is presented, that incorporates an improved treatment of the asymptotic behavior. It is found that this new surface is only slightly different from the other popular PES available for this system in the literature [G. W. M. Vissers, P. E. S. Wormer and A. Van Der Avoird, Phys. Chem. Chem. Phys. 2003, 5, 4767]. The differences are quantified by expanding both surfaces over a set of analytic functions and comparing the behavior of expansion coefficients along the molecule-molecule distance R. It is shown that all expansion coefficients behave similarly, except in the very high energy range at small R where the PES is repulsive. That difference has no effect on low collision-energy dynamics, which is explored via inelastic scattering calculations carried out using the MQCT program which implements the mixed quantum/classical theory for molecular energy exchange processes. The validity of MQCT predictions of state-to-state transition cross sections for CO + CO is also tested by comparison against full-quantum coupled-states calculations. In all cases MQCT gives reliable results, except at very low collision energy where the full-quantum calculations predict strong oscillations of state-to-state transition cross sections due to resonances. For strong transitions with large cross sections, the results of MQCT are reliable, especially at higher collision energy. For weaker transitions, and lower collision energies, the cross sections predicted by MQCT may be up to a factor of 2-3 different from those obtained by full-quantum calculations.

Energy and angular distributions for the hydrogen abstraction reaction O−+CH4→OH−+CH3, exothermic by 0.26 eV, and a prototype ionic pathway for methane oxidation in hydrocarbon flames have been studied in a crossed molecular beam experiment at collision energies of 0.34, 0.44, and 0.64 eV. At the two lower collision energies, two mechanisms contribute to the differential cross section: In the first, low impact parameter rebound collisions form sharply backward-scattered products, while in the second, larger impact parameter collisions produce a broad distribution of forward scattered products. We suggest that the first group of products is formed by collisions with hydrogen atoms oriented essentially along the relative velocity vector and proceeding through a near-collinear O⋯H⋯CH3 geometry, while the second group corresponds to collisions with one of the three off-axis hydrogens. The products are formed on average with 65% of the total available energy in product internal excitation. The product kinetic energy distribution shows structure that correlates with excitation of the ν2 umbrella bending mode of CH3. At the highest collision energy, the product angular distribution shifts entirely to the forward direction, suggesting that the low impact parameter collisions are no longer important in the reactive process. At this energy, the average product internal excitation corresponds to 59% of the total available energy. The data suggest that the majority of product internal excitation resides in the ν2 umbrella bending mode of CH3, with OH in its ground vibrational state.

We have calculated the elastic scattering and rotational excitation cross sections for e−–N2 scattering at 30 and 50 eV using quantum chemical techniques specially designed to be applicable to elastic and inelastic electron scattering by general polyatomic molecules. The angle dependence of the sum of the elastic and rotational excitation differential cross sections is in good agreement with experiment at all scattering angles at both energies, but at 50 eV the difference from experiment exceeds the experimental uncertainty at small scattering angles and near the minimum of the differential cross section. At large scattering angles the rotational excitation cross sections are predicted to exceed the elastic scattering cross sections. The absolute cross sections agree with experiment at some angles but at other angles are as much as 51% (30 eV or 90% (50 eV) higher; this may be due at least in part to the difficulty of putting the experimental results on an absolute scale.

In this paper, we introduce a study of understanding the physical sputtering of target by energetic ions bombardment as a result from cascades of linear collisions. There are two stages of collision cascade: high energy collisions which Thomas – Fermi cross section is applied and low energy collisions that Born – Mayer cross section is characterized. The sputtering may be divided into two parts sputtering potential and sputtering yield. In this paper, we emphasize on sputtering yield which is evaluated under the slowing down of energetic ions in a medium. The resulted yield equation is extended from Boltzmann transport equation. The resulted sputtering yield of Ag, Cu and Pd targets has been measured with different incident ions. There is a variation in the yield with projectile atomic number and a deviation in the maximum of energy is found for both heavy and light projectiles. Also, we also study a formula proposed by Thompson to describe the energy spectrum of atoms sputtered from a target material irradiated by heavy ions and this formula may be expressed in terms of a normalized energy distribution function. A program in matlab is written in order to program the equations and obtain the results.

Energy transfer in molecular collisions is a very fundamental problem in chemical physics, both experimentally and theoretically. Quantitative state-to-state cross sections for energy transfer processes are important for understanding and modelling many kinds of systems, including lasers, shocked gases, planetary atmospheres, and systems containing excited reaction products of any kind. In theory these cross sections can be calculated from first principles using quantum mechanics but in practice this involves severe computational difficulties. The first step is the calculation of molecular interaction potentials, which are a consequence of the electronic structure of the interacting molecules for various geometries.There has been enormous progress in this area in the last few years, 1-3 but we shall not consider it further in this chapter. Instead we shall concentrate on the second step, the dynamical problem that yields the desired inelastic cross sections from the intermolecular potential. Only when both steps have been solved adequately will the ab initio method have reached fruition, but the techniques involved in the two steps are very specialized and to a large extent progress on these steps may occur separately and in parallel. There are two basic approaches to the dynamics problem. First, one can try to develop reliable methods based on simplifying approximations, such as using semiclassical or classical methods or low-order perturbation theory.

Most calculations of the vibrational scattering of diatom-atom collisions use the breathing sphere approximation (BSA) of orientation averaging the intermolecular potential. The resulting angularly symmetric potential can not cause rotational scattering. We determine the error introduced by the BSA into observables of the vibrational scattering of low-energy homonuclear diatom-atom collisions by comparing two quantum mechanical calculations, one with the BSA and the other with the full angularly asymmetric intermolecular potential. For reasons of economy the rotational scattering of the second calculation is restricted by the use of special incomplete channel sets in the expansion of the scattering wavefunction. Three representative collision systems are studied: H2–Ar, O2–He, and I2–He. From our calculations, we reach two conclusions. First, the BSA can be used to analyze accurately experimental measurements of vibrational scattering. Second, measurements most sensitive to the symmetric part of the intermolecular potential are, in order, elastic cross sections, inelastic cross sections, and inelastic differential cross sections. Elastic differential cross sections are sensitive to the potential only if the collision is ``sticky,'' with scattering over a wide range of angles; I2–He is such a collision. Otherwise the potential sensitivity of elastic differential cross sections is concentrated in the experimentally difficult region of very small angle scattering.

Electrospray mass spectrometry (ESMS) is used to investigate cationic niobium and tantalum alkoxide clusters generated in solution by adding equal molar amounts of B(C6F5)3 to the metal ethoxide, M(OEt)5. The dominant metal ethoxide cations that were observed in the electrospray mass spectrum correspond to M(OEt)4+, M2O(OEt)7+, M2(OEt)9+, and M3O(OEt)12+, where M is niobium or tantalum. In separate experiments heterometallic Nb/Ta ethoxide clusters were produced, and the heterometallic cations present in the electrospray mass spectrum are NbTaO(OEt)7+, NbTa(OEt)9+, Nb2TaO(OEt)12+, and NbTa2O(OEt)12+. Collision-induced dissociation (CID) experiments were completed to provide structural information about the observed metal ethoxide cations. At low collision energy (ECM < 2 eV), the CID fragmentation pattern of Nb3O(OEt)12+ favors loss of Nb2(OEt)9+ and neutral NbO(OEt)3, whereas the fragmentation pattern of Ta3O(OEt)12+ favors loss of Ta2O(OEt)7+ and neutral Ta(OEt)5. In addition, the major CID product of M2O(OEt)7+ and M2(OEt)9+ at low collision energy is M(OEt)4+. During CID experiments of M(OEt)4+, four consecutive ethylene molecules were released leaving M(OH)4+, which at higher collision energies (ECM > 2 eV) liberates two water molecules resulting in MO2+. The CID fragmentation pattern of crotyl alcohol (CH3CHCHCH2OH) substituted and n-butanol (CH3CH2CH2CH2OH) substituted metal alkoxides resembles the fragmentation pattern proposed for M(OEt)4+, in which four consecutive organic molecules are shed leaving M(OH)4+.

Crossed molecular beams have been used to study rotationally and vibrationally inelastic scattering from the (72, K′=0) and the (00, K′=0) levels of 1Au (S1) trans-glyoxal in collision with H2. The center-of-mass collision energy was approximately 80 meV (650 cm−1). Laser excitation was used to select each of the initial vibrational levels with rotational states limited to J′=0–10, K′=0. Dispersed fluorescence was used to monitor quantitatively the final rotational and vibrational levels populated in single collision scattering. Vibrationally inelastic scattering from each initial level has an absolute cross section large enough to allow the process to be observed easily with resolution of accompanying ΔK rotational state changes. Although many final vibrational levels are energetically accessible and in principle observable, only those reached by ±1 quantum changes in the lowest frequency mode, the OHC–CHO torsion ν7, are seen. From the initial level 72, the cross sections for Δυ7=+1 and Δυ7=−1 are nearly identical, mirroring the symmetry of UP and DOWN vibrational transitions recently observed in I*2 inelastic scattering from H2, D2, and He. The total cross section for pure rotationally inelastic scattering from 00 is about nine times larger than that for rovibrationally inelastic scattering, whereas the two cross sections are nearly equivalent for 72. With resolution of ΔK rotational state changes, it is seen that the cross sections for these processes scale approximately exponentially with the amount of energy ΔE transferred between translational motion and the internal degrees of freedom of the glyoxal molecule. For the initial level (00, K′=0), not only do these rotational and rovibrational cross sections obey the same energy scaling law, but where similar ΔE occurs, they are the same size. In contrast, the ΔK rotationally state-resolved cross sections for transitions from (72, K′=0) fall off much more rapidly in the vibrationally elastic (Δυ7=0) channel than in the vibrationally inelastic (Δυ7=±1) channels. Consequently, some rovibrational transitions have cross sections comparable to the pure rotational transitions with the same value of ΔK. These relationships contradict the conventional rule of thumb prescribing large rotational and small vibrational cross sections.