Audrey Dell Hammerich

Time‐dependent photodissociation of methyl iodide with five active modes

Advances in the time propagation of multidimensional wave packets are exploited to present the A‐band photodissociation dynamics of methyl iodide for five active vibrational modes on the three relevant excited ab initio potential surfaces. The five modes considered represent all of the experimentally observed dynamical activity. The only modes neglected are the asymmetric C–H stretch and the asymmetric deformation of the methyl group. The kinetic energy operator corresponding to these five degrees of freedom is derived. The fully quantum mechanical calculation was implemented upon grids using 2880 distinct time‐dependent configurations, determined by the multiconfigurational time‐dependent Hartree algorithm, for each electronic state. All of the currently known experimental results regarding the umbrella vibration, symmetric C–H stretching vibration, perpendicular rotation, and parallel rotation of the photodissociated methyl radical fragment are well reproduced. The full wavelength dependence of all of t...

Validity of time-dependent self-consistent-field (TDSCF) approximations for unimolecular dynamics: A test for photodissociation of the Xe-HI cluster

The photodissociation dynamics of a collinear model of the van der Waals cluster Xe–HI is used as a testing ground for time‐dependent self‐consistent field (TDSCF) approximations. In this study, the quantum‐mechanical TDSCF and a combined classical/quantal TDSCF (in which the light atom is treated quantum mechanically, the heavy atoms are treated classically) are compared to numerically exact wave packet calculations. Very good agreement is found between the TDSCF approximations and the exact result over the entire subpicosecond time duration of the process. In particular, all the properties related to the quantal degree of freedom in the combined quantal/classical TDSCF method reproduce almost perfectly the exact results. However, the classical mode in the hybrid approximation is somewhat less well described due to insufficient representation of energy transfer between the modes. The conclusions are very promising as to the applicability of TDSCF methods, in particular the hybrid quantal/classical scheme...

Quantum mechanical reactive scattering by a multiconfigurational time-dependent self-consistent field (MCTDSCF) approach

Abstract The major obstacle to the description of systems containing a large number of degrees of freedom is the exponential increase of computational time and effort with dimensionality. A strategy is presented to overcome this obstacle as well as the shortcoming of the omission of correlations, while still maintaining the simplicity and strengths of a mean-field description, based upon identifying the crucial dynamical correlations and incorporating them with multiconfigurations. The collinear reactive scattering of H + H 2 illustrates the techniques involved and their adaptability, flexibility, and breadth of applicability. MCTDSCF simulations, constructed from time-dependent variational principles, are compared with the numerically exact solution of the Schrodinger equation; agreement is found.

Wavepacket dynamics in five dimensions. Photodissociation of methyl iodide

Abstract To demonstrate the applicability of the multiconfigurational time-dependent Hartree (MCTDH) approach to higher-dimensional problems, the photodissociation dynamics of methyl diodide is investigated. A fully quantum-mechanical description is employed which includes as many as five vibrational degrees of freedom and three electronic states. Convergence of the calculations with respect to the number of configurations is demonstrated. Nonorthogonal bond length and bond angle coordinates are used, following for the more intuitive description desirable for complex polyatomic systems. A pseudospectral implementation of MCTDH, employing a complicated kinetic energy operator corresponding to such coordinates, removes the restriction to simplified models used in earlier studies. Results are compared with previous theoretical descriptions and experimental findings. The absorption spectrum is given and important correlations amongst the different degrees of freedom and their relevance to the photodissociative process are discussed.

Chemistry in strong laser fields: An example from methyl iodide photodissociation

Time‐dependent quantum‐mechanical theories and simulations provide a clear and intuitive description of molecular processes. Due to ensuing simplification of the theory and the generally employed numerical algorithms, the vast majority of these treatments are based upon perturbation theory. Especially in light of the current level of experimental sophistication, with experiments being realized which are influenced by the spectral, temporal, and spatial shape of the laser pulse, it is important to move beyond treatments limited to weak fields or idealized δ‐function wave forms. Various methods to examine the results of high‐field simulations are presented. All of the techniques are shown to have the familiar linear response form in the weak‐field limit. In a time‐dependent framework the difference between the linear and nonlinear response expressions can be seen from expectation values over stationary versus nonstationary states. The high‐field photodissociation of methyl iodide illustrates this approach. ...

Computational Studies of Atmospherically-Relevant Chemical Reactions in Water Clusters and on Liquid Water and Ice Surfaces

CONSPECTUS: Reactions on water and ice surfaces and in other aqueous media are ubiquitous in the atmosphere, but the microscopic mechanisms of most of these processes are as yet unknown. This Account examines recent progress in atomistic simulations of such reactions and the insights provided into mechanisms and interpretation of experiments. Illustrative examples are discussed. The main computational approaches employed are classical trajectory simulations using interaction potentials derived from quantum chemical methods. This comprises both ab initio molecular dynamics (AIMD) and semiempirical molecular dynamics (SEMD), the latter referring to semiempirical quantum chemical methods. Presented examples are as follows: (i) Reaction of the (NO(+))(NO3(-)) ion pair with a water cluster to produce the atmospherically important HONO and HNO3. The simulations show that a cluster with four water molecules describes the reaction. This provides a hydrogen-bonding network supporting the transition state. The reaction is triggered by thermal structural fluctuations, and ultrafast changes in atomic partial charges play a key role. This is an example where a reaction in a small cluster can provide a model for a corresponding bulk process. The results support the proposed mechanism for production of HONO by hydrolysis of NO2 (N2O4). (ii) The reactions of gaseous HCl with N2O4 and N2O5 on liquid water surfaces. Ionization of HCl at the water/air interface is followed by nucleophilic attack of Cl(-) on N2O4 or N2O5. Both reactions proceed by an SN2 mechanism. The products are ClNO and ClNO2, precursors of atmospheric atomic chlorine. Because this mechanism cannot result from a cluster too small for HCl ionization, an extended water film model was simulated. The results explain ClNO formation experiments. Predicted ClNO2 formation is less efficient. (iii) Ionization of acids at ice surfaces. No ionization is found on ideal crystalline surfaces, but the process is efficient on isolated defects where it involves formation of H3O(+)-acid anion contact ion pairs. This behavior is found in simulations of a model of the ice quasi-liquid layer corresponding to large defect concentrations in crystalline ice. The results are in accord with experiments. (iv) Ionization of acids on wet quartz. A monolayer of water on hydroxylated silica is ordered even at room temperature, but the surface lattice constant differs significantly from that of crystalline ice. The ionization processes of HCl and H2SO4 are of high yield and occur in a few picoseconds. The results are in accord with experimental spectroscopy. (v) Photochemical reactions on water and ice. These simulations require excited state quantum chemical methods. The electronic absorption spectrum of methyl hydroperoxide adsorbed on a large ice cluster is strongly blue-shifted relative to the isolated molecule. The measured and calculated adsorption band low-frequency tails are in agreement. A simple model of photodynamics assumes prompt electronic relaxation of the excited peroxide due to the ice surface. SEMD simulations support this, with the important finding that the photochemistry takes place mainly on the ground state. In conclusion, dynamics simulations using quantum chemical potentials are a useful tool in atmospheric chemistry of water media, capable of comparison with experiment.

NOx reactions on aqueous surfaces with gaseous HCl: Formation of a potential precursor to atmospheric Cl atoms

Chlorine atoms are highly reactive free radicals known to catalyze ozone depletion in the stratosphere and organic oxidation in the troposphere. They are readily produced photolytically upon irradiation of some stable Cl containing species, for instance, nitrosyl chloride, ClNO. We predict the formation of ClNO using ab initio molecular dynamics (AIMD) simulations of an NO2 dimer on the surface of a thin film of water upon which gaseous HCl impinges. The reactant is chloride ion formed when HCl ionizes on the water film. The same mechanism for ClNO production may occur in humid environments when ONONO2 (the asymmetric NO2 dimer examined here) comes in contact with either HCl or sea salt. The film of water serves to (1) stabilize ONONO2 on the film surface so that it is localized and physically accessible for reaction, (2) provide the medium to ionize HCl, and (3) activate ONONO2 making it more susceptible to nucleophilic attack by chloride. This substitution/elimination mechanism is new for NOx chemistry on thin water films and could not be derived from studies on small clusters.

Non-adiabatic reactive routes and the applicability of multiconfiguration time-dependent self-consistent field approximations

A time-dependent non-adiabatic formulation is considered for chemical reactions. Basic algorithms are presented for the photodissociation of CH3I under the influence of strong short laser pulses. Detailed insight into the dynamics of this process is demonstrated. A time-dependent self-consistent field approach is suggested when many degrees of freedom have to be considered simultaneously. The derivation is based on a Liouville space description for which quantum and classical mechanics are treated on equal grounds. A multiconfiguration approach is formulated for explicitly including correlation due to splitting of the probability density and for non-adiabatic motion.

Time Dependent Quantum Mechanical Calculations of the Dissociation Dynamics of the Cluster Hen-I2

A time dependent quantum mechanical framework is used to examine the dis-sociation dynamics of van der Waals clusters, in particular the Hen-I2 system. The time dependent approach exploits the time scale separation between the He motion and the I2 vibration. The formalism used is the Time Dependent Self Consistent Field (TDSCF). In this picture, in which the He degrees of freedom are moving in the average field of the I2 molecule and vice versa, the equations of motion are solved by the Fourier grid method which calculates the operation of the operators constituting the Hamiltonian locally. The result is a very fast convergence with respect to grid size. The TDSCF approximation is tested for the collinear He—I2 system by comparing to an exact time dependent propagation. Good results were obtained for low vibrational excitation of the I2 bond. For higher excitations the TDSCF approximation could not account for the fast dephasing part of the autocorrelation function, nevertheless the long time behavior responsible for the dissociation was represented well. The TDSCF approach was then applied to calculate the dissociation of T-shaped and X-shaped Hen-I2 clusters. The basis of this approximation is the weak interaction between the He atoms, and the extra averaging due to increase in the number of particles. Results show very small dependence of dissociation rate on cluster size in contrast to. an RRKM picture. The symmetry of the He wavefunction to exchange is investigated. A scheme to incorporate part of the correlations responsible for collective motion which are missing in the simple TDSCF approach is presented. This scheme is based on a projection operator approach and the time dependent variational principle. On the basis of symmetry it is predicted that the dissociation rate of a cluster consisting of He3 will be faster than a cluster of He4.