The role of solvent effects in association reactions is studied in atom-cluster collisions. Classical trajectory studies of the systems H+Cl(Ar)(n) (n = 1,12) are used to investigate the influence of size, structure, and internal energy of the `'microsolvation'' on the H+Cl association reaction. The following effects of solvating the chlorine in an Ar, cluster are found. (1) In the H+ClAr system there is a large `'third body'' effect. The single solvent atom stabilizes the newly formed HCl molecule by removing some of its excess energy. The cross section found at low energies is a substantial fraction of the gas-kinetic cross section. The molecule is produced in highly excited vibrational-rotational states. (2) Some production of long-lived HCl...Ar complexes, with lifetimes of 1 ps and larger, is found for the H+ClAr collisions. Weak coupling stemming from the geometry of the cluster is the cause for long life times. These resonance states decay into HCl+Ar. (3) At low collision energy (E = 10 kJ/mol) for H+Cl(Ar)(12), the H+Cl association shows a sharp threshold effect with cluster temperature. For temperatures T greater than or equal to 45 K the cluster is liquidlike, and the reaction probability is high. For T less than or equal to 40 K the cluster is solidlike, and there is no reactivity. This suggests the potential use of reactions as a signature for the meltinglike transition in clusters. (4) At high collision energies (E = 100 kJ/mol) H atoms can penetrate also the solidlike Cl(Ar)(12) cluster. At this energy, the solid-liquid phase change is found not to increase the reaction probability.
The dynamics of P-state F atoms in solid Kr is studied by molecular dynamics simulations in two frameworks: (i) The adiabatic approximation, in which nuclear motion is confined to the lowest adiabatic potential surface of the system; (ii) A method that treats semiclassically non-adiabatic transitions between electronic states in the course of the dynamics. The simulations deal with the spectroscopy of the F atom at different lattice sites, and with orbital reorientation dynamics due to the coupling with lattice vibrations. Also explored is migration of the F atom, following the preparation of an exciplex Kr2+F- which dissociates radiatively in the lattice. Some of the main findings are: (1) p-orbital reorientation dynamics on very short timescales (t less than or similar to 20 fs) is dominated by non-adiabatic mechanisms. Adiabatically, reorientation effects have timescales of the order of 0.25 ps or longer. (2) Lattice vibrations of particular symmetry types are particularly efficient in inducing p-orbital reorientation. (D) Dissociation of a Kr2+F- exciplex can result in migration of the F atom into several lattice sites. The F atom spectroscopy for the different sites is different, and can be experimentally distinguished. (4) The migration probabilities of the F atom calculated adiabatically are much greater than the non-adiabatic ones. The results shed light on the coupling between electron orbital and nuclear dynamics for P-state atoms in solids.
Transitions induced by lattice motions between different electronic states of an F(P-2) atom in solid Kr are studied by molecular dynamics simulations. Hopping between potential surfaces is used in modelling the electronic transitions. Calculations for an initially prepared p-orbital orientation, with the lattice at T=25 K, show (1) the decay of orbital orientation at short times (t less than or equal to 40 fs) is well described by a model of random, uncorrelated surface hopping events; (2) the probability of orbital reorientation events is highly correlated with lattice cage distortions of a particular symmetry type, and weakly correlated with cage radial breathing motions; (3) the rate of electronic transitions is nearly constant in time at thermal conditions.
Collisions of highly anharmonic, weakly bound quantum clusters with atoms and molecules were studied by time-dependent calculations. Collinear collision models were assumed, and systems studied included (H2)2, (D2)2, B-H2, (H2)3, (D2)3 in encounters with Li atoms or H2 molecules. Substantial survival probabilities of the clusters were found, even for collision energies which exceed the cluster binding energy by orders of magnitude. The results are interpreted in terms of the vibrational wave-function of the target cluster, which gives a relatively low weight for the configurations most favorable for collision induced dissociation.
The reaction between an O(3P) atom and a hydrocarbon molecule weakly bound to an argon atom was studied by classical trajectory simulations. The results are compared to those obtained for the reaction of a free hydrocarbon. A simplistic model system was constructed in which the hydrocarbon was represented as a pseudodiatomic molecule. Although simple, the model reproduced correctly the internal energy distribution in the OH produced in the reaction of the free species. It was found that the OH, produced from the reaction of the van der Waals complex, emerges with less internal energy and less translational energy than the OH from the monomeric process. In the case of the complexed reagents, the collision complex lifetime is longer and the oxygen explores portions of the potential energy surface that are not available in the monomeric reaction.
A theoretical study is presented on the photodissociation dynamics of Cl2 in crystalline xenon at 100 K, and within a range of pressure between 0 and 100 kbar. Temperature/pressure ensemble molecular dynamics simulations were carried out. The potentials used were accurate enough to reproduce the experimental equation of state of solid xenon. The results show that the photodissociation quantum yield varies strongly with pressure, falling from 30% at zero pressure, to 2% at 12.5 kbar, and 0% at higher pressures. These yields are in good agreement with experimental measurements. This behavior is found to be due to the strong effect of pressure on the librational (rotational) amplitudes of the Cl2 molecule and to the sharp dependence of the photodissociation yield on the molecular orientation in the reagent cage.
Evidence for a cage effect in the 193 nm photodissociation of HBr in the Ar-HBr cluster is found. This effect manifests itself as a tail extending toward lower energies in the hydrogen photofragment kinetic energy distribution (KED). This is a consequence of energy transfer in collisions between the light and the heavy atoms. There is good agreement between the experimental and theoretical KEDs.
A hybrid quantum/semiclassical method is proposed and applied to study realistically the dynamics of the three-fragment photodissociation process Ar ... HCl + hv –> Ar + H + Cl. In the method the hydrogen motion is treated by exact quantum mechanics, while the heavy atoms are described by semiclassical Gaussian wave packets. This treatment is expected to reproduce the main quantum features of the dynamics. Part of the wave packet is found to describe resonance events in which the light particle is temporarily trapped inside the Ar ... Cl cage and oscillates periodically between the heavy atoms before it dissociates. Interference between frequency components of the H wave function that populate different resonance levels give rise to interesting quantum effects. Such effects appear in the angular distribution of the hydrogen fragment, which shows some diffraction oscillations, and scattering into classically forbidden regions. Quantum interferences between the resonances are also the cause of a pronounced structure of peaks in the H photofragment kinetic energy distribution (KED). Time-correlation functions of the wave functions involved are computed, and the implications for the absorption spectrum and its relation to the KED of the H atom are discussed. The results demonstrate the power and applicability of quantum/semiclassical time-dependent self-consistent-field (TDSCF) as a tool for studying the dynamics and spectroscopy of realistic molecular systems.
The close coupling wave packet (CCWP) and quasiclassical trajectory methods are used to study rotationally inelastic scattering of N2 from static, corrugated surfaces. The collision energy in these calculations ranges from 10 to 100 meV; 18 711 quantum states are included in the highest energy calculations to ensure convergence. The scattered molecules are analyzed with respect to the polarization of the final angular momentum vector and the amount of energy transferred into rotational motion and translational motion parallel to the surface. Comparisons of quantum and quasiclassical results show that quantum effects are important even with the relatively large mass of N2 and the high scattering energies used and can be seen even after summing over many final quantum states. A test of a factorization relation derived from the coordinate-representation sudden (CRS) approximation gives qualitative agreement with the exact quantum results.
A hybrid quantum/semiclassical simulation method is applied to the photodissociation dynamics of HCI in the Ar-HCI cluster. The method treats the hydrogen quantum mechanically, and the heavy atoms by semiclassical wavepackets. The dynamics of the H atoms if found to show resonances where the H atom rattles between the Ar and Cl atoms before leaving. Particularly interesting is the kinetic energy distribution of the H atom which shows a structure of pronounced peaks, associated with the resonance levels, while the absorption spectrum is structureless. The dynamics of the process is discussed.
The van der Waals cluster molecule ArCO2 is studied computationally by using the vibrational self-consistent field (SCF) approximation, with an approximate but reasonable potential function. Calculations are carried out both for the full six-dimensional motion and for a reduced two-dimensional problem in which the CO2 is held rigid. An interesting dynamical transition is found in the motion of the Ar atom. Its equilibrium geometry is a symmetric T-shape, and for low excitations both the radial and the angular motions in the CO2 plane resemble the states of anharmonic oscillators (smaller intervals with higher excitations). Above the sixth state of the bend in the angle theta, however, the bend spectrum changes to that of a rigid rotor, with spacings of 2Bn(theta) for quantum number n(theta). The one-dimensional effective SCF potentials along the theta coordinate and plots of the wave function both show a dynamical transition, in which, above n(theta) = 6, the motion of the Ar in the CO2 plane is essentially that of a rigid rotor in the theta coordinate. Calculations of the principal moments of inertia support this interpretation.
We discuss a generalized dynamic mean-field method combining the advantages of explicit pair correlations and of configuration interaction. The approximate dynamical method, which we call time-dependent self-consistent-field configuration interaction (TDSCF2-CI), is constructed by including N(N-1)/2 TDSCF2 configurations. In each configuration a given pair of N coupled modes is directly (not in the mean-field approach) correlated; the N(N-1)/2 configurations include all such choices of pairs. As such, it has both the usual advantages of TDSCF and improvements due to some inclusion of correlations (exact results for any two-mode problem, improved descriptions of dynamical corrections, and greater accuracy). A three-mode model Hamiltonian is analyzed using five approximate treatments, i.e., the usual TDSCF, the three TDSCF2 forms, and the TDSCF2-CI one. The quantities for comparison with the exact results include the decay P(t) of the initial state, the time dependencies of the energies e (i) of individual modes, and the overlap S (t) of the corresponding approximate wave function with the exact one. We find, indeed, that explicit inclusion of pair correlations improves the description of the quantum dynamics of the system.
A theoretical investigation of the photodissociation dynamics of (HCI)2 at 193 nm is presented. Prior to excitation, the cluster is taken to be in its rotation-vibration ground state. A quantal description of this six-dimensional wave function is computed using diffusion quantum Monte Carlo (DQMC). The photodissociation dynamics are simulated by classical trajectories in which the molecule undergoes vertical excitation to an electronic state that is repulsive along one of the HCI stretch coordinates. The initial conditions for these trajectories are sampled according to the Wigner function which was obtained from the DQMC wave function. In a significant fraction of these trajectories, there is a reactive collision in which the H atom interacts with the H'Cl' molecule to form HCI'. Of the remaining collisions, most are nonreactive, but a small number lead to H-2 formation. The trajectories in which an exchange reaction occurs result typically in formation of HCI' molecules that are rotationally and vibrationally hotter and in H atoms with lower kinetic energies than are found in the nonreactive trajectories. Resonances, in which the H atom undergoes multiple collisions with Cl and H'Cl', are observed in all three classes of trajectories. The above results indicate that this is a rich system for the study of photoinduced chemical reactivity in hydrogen-bonded clusters.
The diffraction of thermal He atoms from mixed Xe + Kr monolayers on Pt(111) was measured, and the results were compared with theoretical studies of these systems. The results shed light on the structural properties of these disordered systems, and on their relation to the He diffraction intensities. Experimentally, the specular (0,0), the (1,0), and the (2,0) Bragg peak intensities were measured for monolayers of different Kr:Xe concentration ratios. The theoretical calculations included Monte Carlo simulations of the mixed disordered monolayers, and quantum calculations in the Sudden approximation of the scattering intensities from the simulated disordered structures. The following main results were obtained: (1) Both experiment and the Monte Carlo simulations suggest that the mixed Xe + Kr monolayers are periodic for all Xe:Kr concentration ratios, the lattice constant varies linearly with the Xe:Kr ratio. The domain size of the 2D crystals, from experiment and theory, is found to be larger than 100 angstrom. (2) The Monte Carlo simulations suggest that the Xe + Kr monolayers form an almost ideal substitutionally disordered lattice. (3) Using a semiempirical Debye-Waller factor, reasonable agreement is found between the theoretical and the measured diffraction intensities, thus supporting the calculated structural model for the disordered surface. (4) The theoretical scattering calculations show that in addition to the diffraction peaks, there are also intensity maxima at non-Bragg positions. These are entirely due to the lattice disorder, and are identified as a recently found new type of Rainbow effect that can furnish important information on disordered surfaces. The results demonstrate the power of He scattering as a tool for exploring substitutionally disordered surfaces.
The photodissociation of HCl in the Ar-HCl cluster is studied theoretically, with the focus on the angular distribution of the H atom photofragments. Excited state resonances, in which the H atom rattles between the heavy atoms, contribute to the process. It is found that for excitation into a resonance state, the measurable angular distribution of H atoms from Ar-HCl clusters oriented in space provides a mapping of the resonance wave function. This predicts the possibility of imaging resonance wave functions in such processes.
A semiclassical time-dependent self-consistent-field (TDSCF) method is developed for dealing with the difficulties caused by the nonpreservation of zero-point energy in classical molecular dynamics simulation. The method is applied to a collinear model of a (Ne)12 cluster at very low temperatures. Classically, this system dissociates rapidly due to its zero-point energy. We show that the system remains stable when treated by the new method. The normal mode dynamics of the anharmonic cluster are calculated and discussed. Interesting results are obtained on the lifetimes of single-mode states and energies due to the coupling between the modes. Some of the single-mode states have subpicosecond lifetimes, while others are stable for at least 60 ps. The results illustrate the power of semiclassical TDSCF as a tool for studying the vibrational dynamics of anharmonic cluster at very low temperatures.