Potential surfaces and nonadiabatic couplings in ionic systems: A study of the (O2H)+ interactions

The adiabatic potential energy surfaces (PES ) which are most likely to be involved in the elementary mechanism presiding over charge-exchange and direct inelastic collisions between O₂ molecules and collimated beams of protons are discussed. The general behavior of Diatomics-in-molecule (DIM ) model interactions is analyzed in great detail as a function of the molecular vibrational coordinate and of the other internal nuclear coordinates. The general features of the lower two PES are discussed, and the corresponding nonadiabatic coupling terms between these surfaces are also computed and analyzed. These model results turn out to provide very useful indications on the specific dynamical features that are to be considered responsible for the inelastic, vibronic transitions observed in the target molecule during collisional experiments.     

A global potential energy surface and time‐dependent quantum wave packet calculation of Au + H2 reaction

A global potential energy surface (PES) corresponding to the ground state of AuH₂ system has been constructed based on 22 853 ab initio energies calculated by the multireference configuration interaction method with a Davidson correction. The neural network method is used to fit the PES, and the root mean square error is only 1.87 meV. The topographical features of the novel global PES are compared with previous PES which is constructed by Zanchet et al. (Zanchet PES). The global minimum energy reaction paths on the two PESs both have a well and a barrier. Relative to the Au + H₂ reactants, the energy of well is 0.316 eV on the new PES, which is 0.421 eV deeper than Zanchet PES. The calculation of Au(²S) + H₂(X¹Σ_g⁺) → AuH(X¹Σ⁺) + H(²S) dynamical reaction is carried out on new PES, by the time‐dependent quantum wave packet method (TDWP) with second order split operator. The reaction probabilities, integral cross‐sections (ICSs) and differential cross‐sections are obtained from the dynamics calculation. The threshold in the reaction is about 1.46 eV, which is 0.07 eV smaller than Zanchet PES due to the different endothermic energies on the two PESs. At low collision energy (<2.3 eV), the total ICS is larger than the result obtained on Zanchet PES, which can be attributed to the difference of the wells and endothermic energies.     

Proton—molecule collisional interactions. II. Model calculations for vibrational excitation of CO targets

A previously computed potential energy surface (PES) for CO(¹Σ) + H⁺ interaction is here employed to generate vibrational coupling matrix elements within a simplified model. The scattering equations are solved by means of a rigorous, close-coupling (CC) treatment of the vibrational degree of freedom while rotations are dealt with via the infinite order sudden approximation (IOSA). Comparisons are made with other calculations and the energy dependence, up to 50.0 eV of collisions energy, is examined for total, integral and differential cross section vis à vis the experimental findings from proton scattering in the forward direction. The oscillatory behaviour found in the measurements is also present in the computed observables and is here related to the strong, chemical interaction acting between these collisional partners.     

Semiclassical Trajectory Studies of Reactive and Nonreactive Scattering of OH(A 2Σ+) by H2 Based on an Improved Full-Dimensional Ab Initio Diabatic Potential Energy Matrix

We present a new full-dimensional diabatic potential energy matrix (DPEM) for electronically nonadiabatic collisions of OH(A ²Σ⁺) with H₂, and we calculate the probabilities of electronically adiabatic inelastic collisions, nonreactive quenching, and reactive quenching to form H₂O+H. The DPEM was fitted using a many-body expansion with permutationally invariant polynomials in bond-order functions to represent the many-body part. The dynamics calculations were carried out with the fewest-switches with time uncertainty and stochastic decoherence (FSTU/SD) semiclassical trajectory method. We present results both for head-on collisions (impact parameter b equal to zero) and for a full range of impact parameters. The results are compared to experiment and to earlier FSTU/SD and quantum dynamics calculations with a previously published DPEM. The various theoretical results all agree that nonreactive quenching dominates reactive quenching, but there are quantitative differences between the two DPEMs and between the b=0 results and the all-b results, especially for the probability of reactive quenching.     

Energy transfer in classical collinear and perpendicular central collisions of a structureless atom with a morse oscillator

The energy transfer in classical collinear (C_∞) and perpendicular (C_2v) central collisions of an atom with a Morse oscillator is compared. These collision geometries contribute in classical collisions to experimentally observed inelastic backward scattering of alkali ions from H₂ molecules. For both collision geometries the equations of motion reduce to a set of only two coupled differential equations which can be easily solved numerically. The calculations show that the C_2v collisions are much more effective than C_∞ collisions at all but the very lowest energies. The calculated ΔE/E versus E curves for C_2v collisions using a Born-Mayer potential for the atom atom-in-molecule interaction could be fitted to the experimental results for Na⁺-D₂ yielding reasonable potential values.     

Theoretical investigation of rotationally inelastic collisions of the methyl radical with helium

Rotationally inelastic collisions of the CH(3) molecule in its ground X(2)A(2)' electronic state have been investigated. We have determined a potential energy surface (PES) for the interaction of rigid CH(3), frozen at its equilibrium geometry, with a helium atom, using a coupled-cluster method that includes all single and double excitations, as well as perturbative contributions of connected triple excitations [RCCSD(T)]. The anisotropy of the PES is dominated by repulsion of the helium by the hydrogen atoms. The dissociation energy D(e) was computed to equal 27.0 cm(-1). At the global minimum, the helium atom lies in the CH(3) plane between two C-H bonds at an atom-molecule separation R = 6.52 bohr. Cross sections for collision-induced rotational transitions have been determined through quantum scattering calculations for both nuclear spin modifications. Rotationally inelastic collisions can cause a change in the rotational angular momentum n and its body-frame projection k. Because of the anisotropy of the PES due to the hydrogen atoms, there is a strong propensity for Δk = ±3 transitions. Thermal rate constants for state-specific total collisional removal have also been determined.     

Non-exponential correlation function in DPR: A comparison of experiment and the distorted wave born approximation

The recently observed non-exponential decay of the correlation function for the depolarized Rayleigh (DPR) line in N₂ is examined by a partial calculation of the cross section. The calculation, which is based upon a distorted wave Born approximation (DWBA), shows that the presence of inelastic collisions is necessary to account for the experimental results. Moreover, the inclusion of inelastic collisions has the consequence that the diagonal elements of the relaxation matrix are virtually constant rather than dying off as [j(j + 1)]^?1.     

A sensitivity analysis of a diatomics-in-molecules model of the 1A′ states of H2O

A method for computing the sensitivity of diatomics-in-molecules (DIM) potential energy surfaces (PES) to variations in the parameters characterizing the diatomic fragment matrices is applied to the ¹A′ states of H₂O. The analysis, presented explicitly for 2 × 2 and 3 × 3 fragment matrices, identifies those parameters having the largest influence on local features of the PES. Estimates of the parameter alterations necessary to effect specific changes in the PES are easily provided, allowing manipulation of the DIM model so as to obtain a good overall representation of the PES. Local regions of the PES turn out to be sensitive to only a few parameters and are largely unaffected by the rest. This supports the notion, customary in reaction dynamics, that processes may be discussed qualitatively in terms of the local properties of a PES such as the position of a barrier or the type of energy release.     

Calculation of Ionization,Excitation and Electron Capture Cross Sections for Be4++H(2s, 2p) Collisions

A computational study of Be⁴⁺+H(2s, 2p) collisions has been carried out employing the Classical Trajectory Monte Carlo (CTMC) method for the impact energy range from 20 keV/u to 1000 keV/u. The integral n partial cross sections for H(n) excitation and Be³⁺(n) electron capture and, the total ionization and electron capture cross sections are calculated and compared to recent semiclassical results. A general good agreement is observed for the n partial and total electron capture and ionization cross sections. The comparative study of the three inelastic processes show no significant differences between both excited targets.     

Close-coupling elastic and inelastic integral and differential cross sections for Li+ + H2 collisions

The quantum mechanical close-coupling formalism is applied to the study of elastic and rotationally inelastic Li⁺ + H₂ collisions making use of the Kutzelnigg-Staemmler-Hoheisel potential energy surface. Integral and differential cross sections for j = 0 → 0 and j = 0 → 2 are obtained in the collision energy range 0.2 to 0.9 eV and for j = 1 → 1 and j = 1 → 3 at 0.6 eV. A rainbow structure is observed in both the elastic and inelastic angular distributions and a quenching of the fast oscillations is found in the cross sections for j = 1 initially compared to the case j = 0 initially. At 0.6 eV. the calculated quantum mechanical angular distributions are compared to those from a classical trajectory calculation using the same surface and to the experimental ones. The dynamics of rotational excitation in the Li⁺ + H₂ system is contrasted to rotational excitation in systems for which the atom-diatom interaction is predominantly repulsive.     

Dynamical properties and thermal rate coefficients for the Na + HF reaction using genetic algorithm

The genetic algorithm optimization technique (GAOT) was used to build a new potential energy surface (PES) to the Na + HF → NaF + H reaction. Quasi‐Classical Trajectories and Transition State Theory methods were used to obtain the dynamical properties and thermal rate coefficients (TRCs), respectively, of this new PES. These features were compared with the dynamical properties and TRCs available in the literature. It was found that the GAOT PES agrees very well with other PESs, in which the maximum difference found is smaller than 1.0 Ų for the cross‐sections. These results endow the GAOT approach as a method to build PESs of reactive scattering processes. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Ab initio calculation and quasi‐classical dynamics study of the two lowest potential energy surfaces of the O(1D)+HBr system*

The lowest singlet 1¹A′ and 1¹A″ potential energy surfaces (PES) of the O(¹D)+HBr system have been ab initio computed. The complete active space self‐consistent field (CASSCF) method was used in most of the calculations, considering all the valence orbitals as active. The calculations were complemented with both analytical gradient calculations to characterize the stationary points and multireference configuration interaction (MRCI) calculations at selected nuclear geometries to improve the determination of the barrier heights and of the energetics. Electronic energy values for both PESs were then independently fitted by polynomial expansions in bond order coordinates. On the fitted surfaces quasi‐classical trajectories were separately run. Single‐surface calculations behave qualitatively different for the ground and the excited PES at low collision energies. A satisfactory agreement with existing experimental data was obtained by using the ground PES while calculations performed on the excited 1¹A″ PES worsened the agreement. However, when collision energy is increased, detailed experimental distributions are less well reproduced by calculations on the ground PES. This may imply the participation via nonadiabatic transitions of the 2¹A′ PES at higher energies while the adiabatic ground singlet PES well describes reactive scattering at low collision energy. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001     

Charge transfer during H/H− collisions with Cu(100) and Cu(111) surfaces

We study the H and H⁻ survival probabilities during collisions with Cu(100) and Cu(111) surfaces, at energies ranging from 0.5 to 5 keV and exit angles ranging from 20° to 90°. Calculations are performed with the Wave‐Packet Propagation method adapted to ion‐surface interactions. The projectile survival probability depends on the perpendicular velocity and the copper face being investigated. Projectile's interaction time with the surface and the distance of closest approach are important factors that influence the survival. The H⁻ survival on Cu(100) is much smaller than on Cu(111) but only at low velocities, while becoming higher or comparable to Cu(111) for higher velocities. For very fast collisions, the copper surface behaves like a jellium, and the electron involved in charge transfer does not “feel” the particularities of the surface band structure anymore. While the H survival on Cu(100) seems to not depend on energy and exit angle, the H survival on Cu(111) is both energy and angle dependent, and it is smaller. The study of partial density of states indicates that strong atom‐surface interactions at short distances and the role played by surface states are important factors in determining the neutral fractions obtained after scattering.     

Persistence of MJ following vibrational and rotational energy transfer in molecular iodine excited by a single mode tunable dye laser

Excitation of iodine fluorescence with a single mode tunable dye laser allows extensive observation of features from vibrationally and rotationally transferred states populated through collisions. We have measured the circular polarisation of these features and observe a very high degree of polarisation following both vibrational and rotational energy transfer: thus transitions from states following ΔJ′ = 60 and Δν′ = 1, ΔJ′ = 60 are still highly polarized. This implies strong conservation of M_J throughout energetic inelastic collisions.     

Quantum-chemical interpretation of the difference in the field fragmentation of the molecular ions of dimethyl ether and dimethyl sulfide

The potential energy surfaces (PES) of the positive molecular ions of dimethyl ether and dimethyl sulfide were scanned by the LCAO-MO SCF method in the MINDO/3 valence approximation. On the PES of these radical-cations, apart from the minima corresponding to the equilbrium structures, each has a local minimum which belongs to a cyclic structure. The discovered differences in the stereochemical construction of the cyclic structures of the radical-cations (CH₃)₂O⁺'and (CH₃)₂S⁺' made it possible to explain features of the field fragmentation of their molecular ions. The effect of the external electric field of the ion source on the cyclization and fragmentation stages in the investigated radical-cations was traced.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 25, No. 1, pp. 76–82, January–February, 1989.     

A new diabatic molecular representation for triplet-triplet transitions in He+-He collisions

A “frozen-orbital” diabatic basis has been constructed for an impact parameter treatment of collisions of He⁺ with metastable He(2³S) at 1000 eV laboratory ion energy. Except for the two highest states used, the diabatic states correlate very well with the separated-atom energies, and the Σ-Π rotational couplings deviate little from the proper asymptotic behaviour (≈R^?2). Cross sections and transition probabilities are presented for some elastic and inelastic channels.     

Theoretical investigation of the reactions of La atom and La<Superscript>+</Superscript> cation with carbonyl sulfide

The potential energy surfaces for the La+SCO and La⁺+ SCO reactions have been theoretically investigated by using the DFT (B3LYP/ECP/6-311+G(2d)) level of theory. Both ground and excited state potential energy surfaces (PES) are discussed. The present results show that the reaction mechanism is insertion mechanism both along the C-S and C-O bond activation branches, but the C-S bond activation is much more favorable in energy than the C-O bond activation. The reaction of La atom with SCO was shown to occur preferentially on the ground state (doublet) PES throughout the reaction process, and the experimentally observed species, have been explained according to the mechanisms revealed in this work. While for the reaction between La⁺ cation with SCO, it involves potential energy curve-crossing which dramatically affects reaction mechanism, and the crossing points (CPs) have been localized by the approach suggested by Yoshizawa et al. Due to the intersystem crossing existing in the reaction process of La⁺ with SCO, the products SLa⁺(η²CO) and OLa⁺(η²CS) may not form. This mechanism is different from that of La + SCO system. All our theoretical results not only support the existing conclusions inferred from early experiment, but also complement the pathway and mechanism for this reaction.     

Close-coupling study of rotational energy transfer and differential scattering in H2O collisions with He atoms

Quantum close-coupling scattering calculations of rotational energy transfer (RET) of rotationally excited H(2)O due to collisions with He are presented for collision energies between 10(-6) and 1000 cm(-1) with para-H(2)O initially in levels 1(1,1), 2(0,2), 2(1,1), and 2(2,0) and ortho-H(2)O in levels 1(1,0), 2(1,2), and 2(2,1). Quenching cross sections and rate coefficients for state-to-state RET were computed. Both elastic and inelastic differential cross sections are also calculated and compared with relative experimental results giving generally good agreement in all cases, but less so for inelastic results. Significant differences in the computed collisional parameters, obtained on three different potential energy surfaces (PESs), were found particularly in the ultracold regime. In the thermal regime, the rate coefficients calculated on each of the surfaces are generally in better agreement and comparable, but typically larger, than those obtained in a previous calculation. Unfortunately, a lack of absolute differential or integral inelastic experimental data prevents firm determination of a preferred PES.     

Differentiation of isomeric oxygenated adsorbates using low-energy ion/surface collisions

Reactive scattering of polyatomic ions in the hyperthermal collision energy range (<100 eV) is used to distinguish isomeric oxygenated adsorbates and to quantify their relative amounts when co-adsorbed at a surface. The self-assembled monolayers (SAMs) of interest are constructed from HO-terminated, CH₃O-terminated, and CH₃CH₂O-terminated dialkyl disulfides. Projectile ions used for ion/surface scattering experiments include CF₃⁺, SiCl₃⁺, and the molecular ion of pyridine, C₅H₅N√⁺. Each of these ions exhibits a unique scattered ion profile upon collision with the SAM monolayer surfaces, and so provides different information about the surfaces. Hydrogen atom abstraction by the C₅H₅N√⁺ ion is more prominent at the CH₃CH₂O- and CH₃O-terminated surfaces than the HO-terminated surface, while collisions of SiCl₃⁺ yield reactively scattered products which reflect the chemical composition of these surfaces. For instance, SiCl₂OH⁺ and SiCl₂OCH₃⁺ are scattered from the HO-terminated and CH₃O-terminated surfaces, respectively. Ion/surface collisions involving the CF₃⁺ ion produce chemically sputtered ions from the oxygenated adsorbates, which are valuable for quantitation of those groups. Preferential sputtering of the CH₃O-terminated versus the HO-terminated SAM surface is ascribed to favored thermochemistry and the more accessible CH₃O-terminated adsorbate. Fundamental ion/surface scattering processes, such as inelastic collisions leading to surface-induced dissociation (SID), ion/surface reaction, and chemical sputtering are examined over a range of collision energies for each of the ion/surface types mentioned, and their value in surface analysis is demonstrated.