Final Rotational State Distributions Research Articles

Molecular Simulation for Atmospheric Reactions: Non-Equilibrium Dynamics, Roaming, and Glycolaldehyde Formation following Photoinduced Decomposition of syn-Acetaldehyde Oxide.

The decomposition dynamics of vibrationally excited syn-CH3CHOO to form vinoxy + hydroxyl (CH2CHO + OH) radicals or to recombine to form glycolaldehyde (CH2OHCHO) are characterized using statistically significant numbers of molecular dynamics simulations using a full-dimensional neural-network-based potential energy surface at the CASPT2 level of theory. The computed final OH-translational and rotational state distributions agree well with experiments and probe the still unknown O-O bond strength DeOO for which best values from 22 to 25 kcal/mol are found. OH-elimination rates are consistent with experiments and do not vary appreciably with DeOO due to the non-equilibrium nature of the process. In addition to the OH-elimination pathway, OH roaming is observed following O-O scission, which leads to glycolaldehyde formation on the picosecond time scale. Together with recent work involving the methyl-ethyl-substituted Criegee intermediate, we conclude that OH roaming is a general pathway to be included in molecular-level modeling of atmospheric processes. This work demonstrates that atomistic simulations with machine-learned energy functions provide a viable route for exploring the chemistry and reaction dynamics of atmospheric reactions.

Rotationally Inelastic Scattering Dynamics of NO from Ag(111): Influence of Interaction Potentials

State-to-state molecular scattering from surfaces represents a sensitive probe of the molecule–surface interaction potential. For NO scattering from Ag(111), a benchmark system for studying gas–surface energy transfer, abundant experimental data have provided us a great testing ground to validate the accuracy of first-principles description of molecule–surface interactions. Here, we use an accurate neural network fitting method to construct two high-dimensional potential energy surfaces (PESs) based on density functional theory (DFT) single points using PBE and revPBE functionals, respectively, for describing the rotationally inelastic scattering of NO from Ag(111). Quasi-classical trajectory calculations based on the two PESs have been compared with a diversity of experimental data and the influence of scattering angle, initial molecular orientation, and incidence energy on the rotational state distribution is discussed. It is found in general that revPBE-based results agree better than PBE-based ones with the measured scattering angle and final rotational state distributions, capturing reasonably the rotational rainbow feature at a small incidence energy and the steric effect. But neither PES reproduces all experimental data quantitatively. The different performance of the two PESs can be attributed to their difference in the attractiveness, anisotropy, and corrugation in the entrance channel. Our results suggest the need of further improvement of the accuracy of DFT with commonly used density functionals toward a chemically accurate description of molecule–surface interactions.

Final rotational state distributions from NO(vi = 11) in collisions with Au(111): the magnitude of vibrational energy transfer depends on orientation in molecule-surface collisions.

When NO molecules collide at a Au(111) surface, their interaction is controlled by several factors; especially important are the molecules' orientation with respect to the surface (N-first vs. O-first) and their distance of closest approach. In fact, the former may control the latter as N-first orientations are attractive and O-first orientations are repulsive. In this work, we employ electric fields to control the molecules' incidence orientation in combination with rotational rainbow scattering detection. Specifically, we report final rotational state distributions of oriented NO(vi = 11) molecules scattered from Au(111) for final vibrational states between vf = 4 and 11. For O-first collisions, the interaction potential is highly repulsive preventing the close approach and scattering results in high-J rainbows. By contrast, these rainbows are not seen for the more intimate collisions possible for attractive N-first orientations. In this way, we reveal the influence of orientation and the distance of closest approach on vibrational relaxation of NO(vi = 11) in collisions with a Au(111) surface. We also elucidate the influence of steering forces which cause the O-first oriented molecules to rotate to an N-first orientation during their approach to the surface. The experiments show that when NO collides at the surface with the N-atom first, on average more than half of the initial vibrational energy is lost; whereas O-first oriented collisions lose much less vibrational energy. These observations qualitatively confirm theoretical predictions of electronically non-adiabatic NO interactions at Au(111).

Non-reactive scattering of N2 from the W(110) surface studied with different exchange–correlation functionals

The non-reactive scattering of N(2) from the W(110) surface is studied with six dimensional (6D) classical dynamics and two distinct potential energy surfaces (PES). Here, we use the PESs calculated with density functional theory and two different exchange-correlation functionals, the PW91 [J. E. Perdew et al., Phys. Rev. B, 1992, 46, 6671] and the RPBE [B. Hammer et al., Phys. Rev. B, 1999, 59, 7413]. By analyzing the final rotational state and angular distributions, we extract information on the characteristics of the two PESs in the 6D configurational space. Comparison of the theoretical results with the available experimental data provides detailed information on the validity of each functional. In general, the PW91 PES is more corrugated than the RPBE one in all the configurational space, meaning that there is a stronger dependence of the potential energy on the molecular orientation and position over the surface unit cell. Furthermore, we find that the larger corrugation and the less repulsive character exhibited by the PW91 PES seems to be realistic at distances above the chemisorption well. In contrast, the less corrugated RPBE PES performs better in the region below the chemisorption well.

Narrow Subthreshold Quantum Mechanical Resonances in the Li + HF → H + LiF Reaction

State-to-state, state-specific, and cumulative reaction probabilities are presented for the bimolecular scattering process Li + HF → H + LiF in the ground electronic state. Calculations were performed for zero total angular momentum at total energies from 0.26 to 0.50 eV (relative to HF at its classical equilibrium bond distance and infinitely far from Li). The energy dependence of the state-to-state, initial-state-selected, and cumulative reaction probabilities for LiFH in the low-energy regime displays a pronounced resonance structure due to quasibound states associated with a Li...FH van der Waals well in the entrance valley of the potential energy surface. The lifetimes of the long-lived resonances are obtained by fitting the calculated eigenphase sum to the multichannel Breit-Wigner formula. The final rotational state distributions of the LiF product fragment resulting from decay of the resonance state complexes are presented for two resonances. Quantum numbers are assigned to the resonances using bound-state and quasibound-state calculations in the Li...FH van der Waals well, and possible decay mechanisms are discussed. The lifetimes show a systematic dependence on the translational vibrational quantum number.

Vibrational predissociation of NeBr2 (X, v=1) using an ab initio potential energy surface

Quantum mechanical calculations on the vibrational predissociation dynamics of NeBr2 are performed using an ab initio (coupled cluster using single and double excitations with a noniterative perturbation treatment of triple excitations) potential energy surface. Energy positions, lifetimes, and final rotational state distributions are determined for vibrational predissociation from the two lowest linear (n=0) and T-shaped (n=1) van der Waals levels of NeBr2 (X, v=1). Comparison with the experimental assumption as regards the energy transfer to rotation provides information about the type of isomer involved in the experimental vibrational predissociation process, suggesting that it was the linear one.

Vibrational predissociation of van der Waals molecules: An internal collision, angular momentum model

We describe an “internal collision” model of vibrational predissociation (vpd) in triatomic van der Waals (vdW) molecules based on the angular momentum (AM) model of collision-induced vibration–rotation transfer. The probability of vpd is related to the probability of disposing the vibrational energy into rotational and orbital AM. In T-shaped species, two internal collision configurations are likely to dominate namely, the turning points of excursions by the weakly bound species relative to the diatomic. These two geometries result in a bimodal distribution of final rotational states. Velocity-AM diagrams demonstrate why halogen and hydride vdW molecules have very different properties and illustrate the physics of quantitative calculations that reproduce experimental distributions in a wide range of vdW molecules. We introduce an analogy between a metastable dissociative state and the optical resonator and define a quality factor (Qjl) that relates vpd lifetime to stored energy and to ease of generating rotational and orbital AM by dissociation. Data on vdW molecules of OH are analyzed using the Qjl concept and the accelerated dissociation on forming the vdW complex with an efficient energy acceptor is likened to the formation of a particularly low-Q molecular resonator.

Molecules as products; the good, the bad and the eccentric

A. J. McCaffery and R. J. Marsh, PhysChemComm, 2001, 4, 112 DOI: 10.1039/B108077F

Photodissociation of cyclic HF complexes: Pentamer through heptamer

Photofragment angular and final rotational state distributions have been measured, corresponding to the vibrational predissociation of HF polymers [(HF)N N=5–7]. The present experiments confirm the theoretical prediction that, for clusters in this size range, the only open dissociation channel corresponds to the “evaporation” of a single HF monomer. The pump–probe experiments reported here indicate that the resulting HF monomer is produced with only modest rotational excitation. Rotational temperatures of 180 and 140 K have been determined for the evaporated HF monomer following dissociation of the HF pentamer and hexamer, respectively. Energy conservation in these photodissociation studies provides upper limits for the dissociation energies (D0), namely, 2941 cm−1 and 2854 cm−1 for the pentamer and hexamer, respectively.

Quantum reaction dynamics of O(3P)+HCl on a new ab initio potential energy surface

Quantum reaction dynamics of O(3P)+HCl↔OH+Cl is studied by using a new ab initio potential energy surface calculated by Ramachandran et al. [J. Chem. Phys. 111, 3862 (1999)]. The hyperspherical elliptic coordinate approach is applied with an emphasis on elucidating reaction dynamics for J (total angular momentum quantum number)=0. In terms of the previously established concept that reactive transitions are nothing but vibrationally nonadiabatic transitions at important avoided crossings, clear interpretations are given for the following dynamical features: (i) reactivity depending on potential energy surface topography, (ii) final rotational state distributions for specified initial rovibrational states, and (iii) resonance structures appearing in some reactions. Thermal rate constants are approximately estimated from the present J=0 results by using the J-shift approximation. The present results are compared with our previous ones based on the different potential energy surface calculated by Koizumi–Schatz–Gordon (KSG). The calculated adiabatic potential energy curves of the present new surface have deep wells in the OH+Cl channel in contrast to the KSG potential energy surface. Consequently, the new surface leads to quite different dynamics from those on the KSG surface. Comparisons with the results obtained by quasiclassical trajectory calculations are also made.

Hybrid quantum/classical simulation and kinetic study of the vibrational predissociation of Cl2⋯Nen (n=2, 3)

A hybrid quantum/classical method is applied to the vibrational predissociation of van der Waals clusters containing a diatomic molecule and several rare gas atoms, Cl2⋯Nen (n=2, 3). The vibrational degree of freedom of the diatomic is treated quantum mechanically while all the other degrees of freedom are treated classically. A kinetic mechanism is proposed in order to interpret the dynamics in terms of the following elementary steps; vibrational predissociation (VP), intramolecular vibrational redistribution (IVR), and evaporative cooling (EC). The resulting lifetimes are in very good agreement with the experimental linewidth measurements of Janda and co-workers, and with the quantum mechanical reduced-dimension results of Le Quéré and Gray on Cl2⋯Ne2. The final rotational state distributions agree very well with the experimental results and exhibit a quasistatistical behavior. The final vibrational distributions reproduce the main experimental features.

Nonexponential decays resulting from crises in predissociation of a highly excited van der Waals complex

Scattering in a highly excited van der Waals complex HeI 2 is investigated in quantum mechanics. Particular attention is paid to crisis, where homoclinic tangency of the stable and unstable manifolds leads to a transition in chaos. Crisis in classical chaos gives rise, in the corresponding quantum system, to multiple decay paths with distinct decay times. The resulting effects are nonexponential decays in the predissociation processes and a transition in the distribution of final rotational states. Implications of these results for control of reaction processes are briefly discussed.

Total and partial decay widths in vibrational predissociation of the HeI 2 van der Waals complex for lower initial vibrational excitations

We presented a calculation of the total and partial decay widths of vibrational . predissociation VP of the HeI molecule for low initial vibrational excitations from the 2 . lowest van der Waals vdW state with total angular momentum J s 0. A time-dependent golden rule wave-packet method was employed in our numerical calculations for the decay widths. The computed total decay widths, lifetimes, and rates of VP are in fairly good agreement with those extrapolated from the experimental data available. Predicted total decay widths as a function of initial vibrational levels exhibit a highly nonlinear behavior. These results demonstrate that a quantum mechanical decay mode for low vibrational excitation remains as well. The total propagation time needed in the time- dependent golden rule wave-packet calculations is much shorter than is the lifetime of the predissociation of HeI . It is shown that the final-state interaction between the 2 fragments is important for determining the final rotational-state distribution partial . decay width . We find that the major peak position in the final rotational-state distribution shifts to lower rotational energy levels with increase of the initial vibrational quantum number, which is evidently different from that for higher vibrational levels. This fact can be clearly explained by the dependence of the amount of kinetic energy released to the product degrees of freedom on the initial vibrational state. Q 1997 John Wiley & Sons, Inc.

Quantum dynamics of O(3P)+HCl→OH+Cl Effects of reagent rotational excitation

Accurate quantum dynamics of the chemical reaction O( 3 P)+HCl(j i , v i =0)→OH(j f , v f =0)+Cl is studied using the hyperspherical coordinate approach and the Koizumi–Schatz–Gordon (KSG) potential-energy surface. The effects of reagent rotational excitation on the dynamics are investigated. The study considers total energies up to 0.7 eV and reagent rotational states, j i =0–10. The total reaction cross-section shows a strong dependence on j i . Initially it decreases with increasing j i , up to j i ≈4, and then increases with further increase in j i . This overall trend is enhanced on increasing the total energy. The final rotational state (j f ) distributions are also investigated. Interestingly, the distribution shows a peak at high j f (≈10) irrespective of j i and the total energy. The qualitative features are quite similar to the experimental results. Finally, the state-resolved rate constant and the thermal rate constant are evaluated. Because of the enhancement of the reaction by the reagent rotational excitation, the state-resolved rate constants for 8j i ⩽10 are two or three orders of magnitude larger than those for j i 5. The thermal rate constant obtained is thus much larger than the experimental data implying that the barrier height of the KSG surface may be too low.

Velocity Dependence of Quasi-Resonant Vibrotational Transfer in Li2*−Rare Gas Collisions

We report measurements of velocity-dependent cross sections for vibrotationally inelastic scattering in the system Li2 + X → Li2 + X, with vi = 9, ji = 42, and X = Xe, Ar, and Ne. The measurements range over a factor of 30 in energy. Quantum levels were chosen to elucidate the quasi-resonant vibrotational transfer process studied previously. A reduction in collision velocity results in both an increase in total vibrationally inelastic cross section and an enhancement of the quasi-resonant effect, with final rotational state distributions as narrow as 2.5ℏ (fwhm) observed. The largest cross section for Δv < 0 is given by the formula Δjpeak = −4Δv and possesses a roughly 1/vrel velocity dependence. For Δv = +1, energy thresholds shift the peak cross section at low velocity to the nearly energy resonant value of Δjpeak = −6. The similarity of the final state distributions for different target gases observed in previously measured rate constants does not hold in the velocity-dependent data; this results in part from the appearance of low-velocity dynamical thresholds for exoergic cross sections in the Ar and Ne systems. We compare the experimental results for X = Ne with cross sections calculated from quasi-classical trajectories on an ab initio potential energy surface; for jf ≥ 46, agreement is quantitative, while for jf < 46, the calculation overestimates the cross sections.

A classical path/forced rotor theory of state-to-state rotational energy transfer

The problem of rotational energy transfer (RET) is examined from a point of view intermediate between the current approaches based on empirical models or numerical solution of the coupled scattering equations. A semiclassical perspective is taken in which a classically described collision with an atom exerts a time dependent torque on the rotor and the resulting forced rotor dynamics is addressed quantum mechanically. By treating the anharmonicity in the rotational energy levels as a perturbation, a simple approximate expression is found for the inelastic transition probability. It reveals a marked difference between the distribution of final rotational states incurred from an individual collision trajectory as compared to trajectory averaged measures of RET, such as cross sections. The theory is applied to the scattering of Li2(A 1Σ+u) by Ne, Ar, Xe;Na2(A 1Σ+u) by He and Ne; N+2(X 2Σ+g) by He, and CN(X 2Σ+) by He. Its predictions compare well with those from a fully quantum mechanical description of rigid rotor scattering and with experiment. The insight into the energy transfer dynamics gained from the semiclassical approach is used to examine the assumptions underlying empirical models of rotational energy transfer.

Accurate Quantum Dynamics of the Light Atom Transfer Chemical Reaction O + HCl .fwdarw. OH + Cl

collision energy considered in this study ranges up to -0.7 eV, and the total angular momentum J is required up to -120. The potential energy surface employed is the one derived by Koizumi, Schatz, and Gordon (KSG) based on ab initio data. The effects of the potential energy surface topography on the dynamics are analyzed in terms of the collision energy dependence and the j, dependence. The effects of the nonlinearity of the transition state of the KSG surface is clearly manifested in these dynamics. Not only the accurate integral cross section and the rate constant are evaluated, but also the Q, dependence of the dynamics and final rotational state distribution are analyzed, where C2, is the z-component of J in the initial arrangement channel. The jf distribution at Ecoll 2 0.5 eV shows an interesting resemblance with the feature recently observed experimentally. The energy-shift approximation is extended so as to cover the general triatomic systems which require a large number of J. This extended CCPA (constant centrifugal potential approximation) is shown to work well.

Quantum dynamics of the Mu+H2(HD,D2) and H+MuH(MuD) reactions

Quantum mechanically accurate calculations are carried out for the following reactions involving muonium atom (Mu) using the hyperspherical coordinate approach: Mu+H2→MuH+H, Mu+D2→MuD+D, Mu+HD→MuH(MuD)+D(H), H+MuH→MuH+H, and H+MuD ↔MuH+D. The initial vibrational state is restricted to the ground state (vi=0) and the collision energies considered are up to ∼1.2 eV. The various aspects of the dynamics, such as the isotope effects, the initial rotational state (ji) dependence, and the final rotational state (jf) distribution are analyzed for a wide range of ji and jf. Some of the isotope effects can be interpreted in terms of the variations in reaction barrier and endothermicity. The following two intriguing features are also found: (1) strong enhancement of reaction by initial rotational excitation, and (2) oscillation of integral cross section as a function of collision energy in the case of the Mu-transfer reactions.

Quantum dynamics of the Mu + H 2 and Mu + D 2 reactions

Quantum mechanically accurate calculations are carried out for the Mu + H 2 and Mu + D 2 reactions using the hyperspherical coordinate approach. The results for the rate constant are in good agreement with experiment. Comparisons are also made with other more approximate calculations. The effect of initial rotational excitation and final rotational state distribution are also analyzed.

The Renner-Teller-induced predissociation of HCO(Ã 2A′): Wavepacket calculations using new ab initio potential energy surfaces

We present first results for the Renner-Teller-induced predissociation of HCO(à 2A′) using new ab initio potential energy surfaces for the à (2A′) and the X̃(2A') states. The quantum mechanical dynamics calculations are performed in the time-dependent representation with the CO bond distance being fixed. The linewidths and final rotational state distribution of CO following the decay of various highly excited bending states of HCO (à 2A′) agree with recent experimental data. The dissociation mechanism is discussed in terms of the topology of the X̃ (2A')-state potential energy surface.