Robert Parson

Recombination and relaxation of molecular ions in size‐selected clusters: Monte Carlo and molecular dynamics simulations of I−2 (CO2)n

The equilibrium structures and the recombination dynamics of I−2 molecular ions embedded in clusters of 3–17 CO2 molecules are studied by Monte Carlo and molecular dynamics simulations. The potential model incorporates, in a self‐consistent manner, a description of the I−2 electronic structure that depends on both the I−2 bond length and the solvent degrees of freedom. The influence of the solvent upon the I−2 electronic structure is treated by means of a single effective solvent coordinate, in a manner reminiscent of the theory of electron transfer reactions. This causes the electronic charge to localize on a single I atom when the I–I bond is long or when the solvent cage has become highly asymmetric. The primary focus is the I−2 vibrational relaxation that follows recombination. Simulations of I−2(CO2)16 and I−2(CO2)9 yield vibrational relaxation times of less than 3 ps, even faster than the experimentally observed absorption recovery time of 10–40 ps. It is suggested that the latter time scale is dete...

Ab initio calculations of the ground and excited states of I2− and ICl−

Abstract We performed all-electron ab initio calculations of the first six states of I 2 − and ICl − using a multi-reference configuration interaction method. Spin-orbit coupling is included via an empirical one-electron operator and has a large effect on the dissociation energy. The ground state dissociation energies were in error by 20–30%, probably due to deficiencies in the one electron basis sets. The electronic wavefunctions at the equilibrium geometry were used to calculate the electronic absorption spectrum from the ground state, and good agreement was found with the experimental data.

Charge flow and solvent dynamics in the photodissosiation of cluster ions: a nonadiabatic molecular dynamics study of I2−·Arn

Abstract Experimental studies of photodissociation in I 2 − ·Ar n clusters have shown a rapid onset of caging for n > 10 and bimodal photofragment distributions in both dissociation and recombination channels. We simulate and interpret these results using a Hamiltonian that accounts for the strong perturbation of the solute electronic structure by the solvent. The high-mass products in the recombination channel are identified with excited state recombination. The two classes of dissociation products are identified with ejection of either a neutral I atom or an I − ion from the cluster, with the latter mechanism driven by the negative polarizability of the excited electronic state.

Solvation of electronically excited I2

The interaction potentials between the six lowest electronic states of I−2 and an arbitrary discrete charge distribution are calculated approximately using a one‐electron model. The model potentials are much easier to calculate than ab initio potentials, with the cost of a single energy point scaling linearly with the number of solvent molecules, enabling relatively large systems to be studied. Application of the model to simulation of electronically excited I−2 in liquids and CO2 clusters is discussed. In a preliminary application, solvent effects are approximated by a uniform electric field. If electronically excited (2Πg,1/2) I−2 undergoes dissociation in the presence of a strong electric field, the negative charge localizes so as to minimize the total potential energy. However, in a weak field the negative charge localizes in the opposite direction, maximizing the potential energy. Based on a study of the field‐dependent potential surfaces, a solvent‐transfer mechanism is proposed for the electronic r...

Ultrafast reaction dynamics in cluster ions: Simulation of the transient photoelectron spectrum of I2−Arn photodissociation

Combining an effective Hamiltonian model of electronic structure with nonadiabatic molecular dynamics simulations, we calculate the recently measured transient photoelectron spectrum of I2− dissociated inside a cluster of argon atoms. We find good agreement between calculated and experimental spectra. The transient spectral shifts reflect the dynamics of both the I2− and argon degrees of freedom, revealing pathways and time scales for dissociation, recombination, and vibrational relaxation.

Modeling structure and dynamics of solvated molecular ions: Photodissociation and recombination in I2−(CO2)n

Abstract We describe a method for simulating the reaction dynamics of a molecular ion embedded in a cluster of polarizable solvent molecules. Potential energy surfaces for ground and excited states are calculated from an effective Hamiltonian that takes into account the strong perturbation of the solute electronic structure by the solvent. The parameters of the model Hamiltonian are obtained from a combination of ab initio calculations and spectroscopic data; intermolecular electrostatic and polarization interactions are treated by the distributed multipole analysis of Stone and co-workers, while short range interactions are modeled with empirical pair potentials. Analytical expressions for the derivatives of the effective Hamiltonian allow for efficient computation of forces and nonadiabatic couplings. The intramolecular degrees of freedom of the solvent molecules are held fixed during the simulations using the method of constraints, and electronic transitions are treated using Tullys surface hopping algorithm. The method is applied to the photodissociation and recombination of I2−(CO2)n. Electronic relaxation in this system is found to occur on multiple time scales, ranging from 2 ps to many tens of ps. The relationship of these results with the experimental measurements of Lineberger and co-workers is discussed.

Solvent-Mediated Electron Hopping: Long-Range Charge Transfer in IBr−(CO2) Photodissociation

CO2 Lends a Hand Solvent plays a complex and multifaceted role in facilitating charge transfer events. One obstacle to understanding its influence is that solvent molecules are in constant motion; just teasing out their arrangement in space at the point in time when an electron hops from one substrate to another is often a great challenge. Sheps et al. (p. 220; published online 4 March) have studied a highly simplified prototype system, in which a single CO2 molecule coordinates, as a solvent might, to an IBr− ion in the gas phase. A combination of ultrafast photoelectron spectroscopy and theoretical simulations was applied that suggests that even this solitary interaction is sufficient to induce electron transfer from iodide to bromine during a dissociation reaction. Energy channeled through CO2-bending vibrations promoted formation of I(CO2) and Br−. The presence of an intervening carbondioxide molecule dramatically changes the electron transfer probability between two halogen atoms. Chemical bond breaking involves coupled electronic and nuclear dynamics that can take place on multiple electronic surfaces. Here we report a time-resolved experimental and theoretical investigation of nonadiabatic dynamics during photodissociation of a complex of iodine monobromide anion with carbon dioxide [IBr–(CO2)] on the second excited (A′) electronic state. Previous experimental work showed that the dissociation of bare IBr– yields only I– + Br products. However, in IBr–(CO2), time-resolved photoelectron spectroscopy reveals that a subset of the dissociating molecules undergoes an electron transfer from iodine to bromine 350 femtoseconds after the initial excitation. Ab initio calculations and molecular dynamics simulations elucidate the mechanism for this charge hop and highlight the crucial role of the carbon dioxide molecule. The charge transfer between two recoiling atoms, assisted by a single solvent-like molecule, provides a notable limiting case of solvent-driven electron transfer over a distance of 7 angstroms.

Structure and dynamics of molecular ions in clusters: I2− in flexible CO2

The structures and dynamics of I2− molecular ions embedded in clusters of flexible solvent molecules are studied using molecular dynamics simulation. The potential model extends the work of Papanikolas et al. [J. Chem. Phys. 102, 2452 (1995)] by taking into account the low-frequency bending vibrations of the solvent molecules. Results are presented for flexible CO2 and for a hypothetical solvent in which the bending force constant of CO2 has been decreased by a factor of 5. The structure and the vibrational relaxation dynamics of I2− in flexible CO2 differ only slightly from what was seen in rigid CO2. In “hyperflexible” CO2, however, the solute becomes strongly polarized even at its equilibrium geometry, and the cluster structures are highly asymmetric, demonstrating that the localizing solvation forces are able to overcome the delocalizing chemical bonding interactions. The pathways for vibrational relaxation are also found to be distinctly different in the flexible and hyperflexible solvent.

A symmetry‐based model for selective rotational energy transfer in collisions of spherical top molecules

Recent state‐resolved experiments have shown that rotational energy transfer in collisions of vibrationally excited spherical top molecules is remarkably selective with respect to the fine structure components of the rovibrational states. It is shown that this selectivity can be explained by means of symmetry arguments and the Harter–Patterson theory of spectral clustering. A new propensity rule, which extends the well‐known symmetric top selection rule to perturbed spherical tops, is derived and shown to account well for the experimental results of Steinfeld and co‐workers on 13CD4 and SiH4.

Fine‐structure selective collisional energy transfer in spherical top molecules: Evidence for a symmetry‐based mechanism from rovibrational eigenfunctions

Recent state‐resolved experiments have shown that rotational energy transfer in collisions of vibrationally excited spherical top molecules is remarkably selective with respect to the fine structure components of the rovibrational states. In a recent paper [J. Chem. Phys. 93, 8731 (1990)], these results were rationalized on the basis of symmetry arguments and the Harter–Patterson theory of spectral clustering. The present paper provides numerical evidence for those assertions. Matrix elements of an atom–spherical top interaction potential are calculated using numerically accurate wave functions from spectroscopic Hamiltonians and using the approximate wave functions given by the Harter–Patterson theory. Agreement between the two calculations is satisfactory and both confirm the propensity rules derived previously, suggesting that the proposed mechanism does in fact operate in these systems.