John Vieceli

Molecular dynamics study of the vibrational relaxation of OClO in bulk liquids

The vibrational relaxation of OClO in bulk water, acetonitrile, and ethanol is studied using classical and semiclassical molecular dynamics computer simulations. Nonequilibrium classical trajectory calculations provide insight into the early stages of vibrational energy relaxation of highly excited states. Equilibrium force autocorrelation functions are used to determine the relaxation rate for the v=1→v=0 transition. Good agreement with experiments is found. The calculations suggest that the hydrogen bonding in water, as reflected by the high density of librational modes, is the reason for the fast relaxation in this liquid compared with that in ethanol and acetonitrile.

Molecular dynamics study of the photodissociation and photoisomerization of ICN in water

The photodissociation and photoisomerization of ICN in water is studied using molecular dynamics simulations. A water–ICN potential energy function that takes into account the different ground and excited state charges and their shift as a function of the reaction coordinate is developed. The calculations include nonadiabatic transitions between the different electronic states and allow for a complete description of the photodissociation leading to ground-state and excited-state iodine and to recombination producing ICN and INC. The calculated UV absorption spectrum, the cage escape probability, the quantum yield of ICN and INC, and the subsequent vibrational relaxation rate of ICN and INC are in reasonable agreement with recent experiments. The trajectories provide a detailed microscopic picture of the early events. For example, it is shown that most recombination events on the ground state involve nonadiabatic transitions before the molecule has a chance to completely dissociate on the excited state, an...

Vibrational relaxation at water surfaces

The vibrational relaxation of several diatomic molecules at the surface of liquid water is studied using classical molecular-dynamics computer simulations and compared with the same process in the bulk liquids. Both nonequilibrium classical trajectory calculations and equilibrium force autocorrleation functions are used to elucidate the factors that influence vibrational energy relaxation at the liquid surface region. We find that in general vibrational relaxation rates at interfaces are slower than in the bulk due to reduced friction. However, the degree of the slowing-down effect depends on the contribution of electrostatic forces and is correlated with the structure of the first solvation shell.

Photodissociation of ICN at the liquid/vapor interface of chloroform

The photodissociation of ICN initially adsorbed at the liquid/vapor interface of chloroform is studied using classical molecular dynamics computer simulations. The photodissociation and subsequent geminate recombination on the ground state of ICN is compared with the same reaction in the bulk liquid. We find that the probability for cage escape at the interface is significantly enhanced due to the possibility that one or both of the photodissociation fragments desorb into the gas phase. The desorption probability is sensitive to the initial location and orientation of the ICN. An examination of the energy disposal into these fragments provides additional information about the competition between geminate recombination and cage escape at the interface.

Molecular dynamics study of the photodissociation of OClO in bulk liquids

The electronic spectra and the photodissociation dynamics of OClO on the excited state in bulk water, acetonitrile, and ethanol are computed using classical molecular dynamics computer simulations. The trajectories are run on an ab initio potential energy surface of Peterson [J. Chem. Phys. 109, 8864 (1998)], which is fit to a global three-dimensional analytical surface. The calculated cage escape probability in these liquids seems to correlate with the vibrational relaxation rate of the parent molecule and is in reasonable agreement with experiments in water and acetonitrile, but somewhat overestimates the experimental probability in the case of ethanol.

Vibrational relaxation of ICN bulk and surface chloroform

The vibrational relaxation of ICN at the surface of chloroform is studied using classical molecular dynamics computer simulations and is compared with the same process in the bulk liquid. Non-equilibrium classical trajectory calculations and equilibrium force correlation functions are used to investigate the factors that control the vibrational energy relaxation rate. We find that the relaxation rate depends on the initial excitation energy and is a factor of three slower at the interface than in the bulk. In addition, the frequency dependent friction at the interface is found to be orientation dependent, reflecting strong inhomogeneity.

Static and dynamic electronic spectroscopy at the interface between water and chemically modified self-assembled monolayers

Molecular dynamics computer simulations are used to study the static and dynamic electronic spectroscopy of a chromophore located at the interface between water and self-assembled organic monolayers terminated by either a methyl group or chlorine atom. The roughness and polarity of the monolayer surfaces are varied to determine the dependence of the spectroscopy on the surface composition. Equilibrium trajectories are used to calculate the static electronic spectrum relative to the gas phase and non-equilibrium trajectories are used to monitor the dynamic solvent response immediately following an electronic transition. Relative to bulk water, the interfaces with methyl-terminated monolayers are less polar, while interfaces with chlorine-terminated monolayers are more polar. This is understood in terms of the contribution from each component of the system to the solvation energy. The dynamic solvent response at each interface studied is slower than bulk water. The rate of water relaxation is correlated with the polarization of interfacial water molecules due to the monolayer.