Molecular dynamics study of aqueous solvation dynamics following OClO photoexcitation

Computer study of absorption of oxygen and ozone molecules by water clusters with Cl– and Br–

Infrared absorption and Raman spectra were calculated by using the molecular dynamics method for water clusters with chlorine and bromine ions in a medium of water and either ozone or oxygen molecules. The intensity of IR absorption spectra of clusters with absorbed oxygen increased and that of clusters with absorbed ozone decreased as the number of chlorine ions grew. When Br– were present in the system the inverse behaviour was observed. An increase in the number of ions weakened the intensity of the Raman spectra when either oxygen or ozone was absorbed; for ozone this weakening was more noticeable. A stronger reduction of the integrated intensity of the Raman spectrum with an increase in the number of Br– was observed in the presence of ozone molecules in the system. Cl– ions caused an amplification of the emission power of the IR radiation for both systems in the presence of oxygen and ozone, and Br– strengthened the emission of IR radiation for systems containing ozone, and weakened it for systems with oxygen molecules.

Photoinduced Vibrational Coherence Transfer in Molecular Dimers

At short times that are faster than dephasing, photoinduced evolution of the vibrational subsystem in an electron-phonon molecular structure depends strongly on the electronic evolution. As the electronic population shifts between the donor and acceptor states, in the diabatic description the state with the largest population determines the equilibrium positions and frequencies of the vibrational modes, which oscillate continuously and without loss of coherence. The vibrational coherence transfer between the electronic states detected recently in a number of systems is described theoretically by application of the quantized Hamiltonian dynamics (QHD) formalism [J. Chem. Phys. 2000, 113, 6557] to the coupled electronic and vibrational degrees of freedom of a model heterodimer. The observed coherent modulation of the frequency of the probe signal is represented with simple analytic and numeric QHD models.

Ultrafast vibrationally-induced dephasing of electronic excitations in PbSe quantum dots

Vibrationally induced pure-dephasing of electronic states in PbSe quantum dots (QDs) at room temperature is investigated using two independent theoretical approaches based on the optical response function and semiclassical formalisms. Both approaches predict dephasing times of around 10 fs and reproduce the recently measured homogeneous linewidths of optical absorption well. Because dephasing slows down with increasing cluster size, the dephasing times calculated for the small clusters correspond to the lower end of the experimental data. The dephasing is almost independent of the electronic excitation energy and occurs faster for biexcitons than single excitons. The dephasing time is roughly proportional to the square root of the mass of the lighter atom (Se), suggesting that dephasing should be faster in PbS and slower in PbTe relative to PbSe. Core atoms produce stronger dephasing than surface atoms. In the collective description, pure-dephasing occurs via low-frequency acoustic modes, in support of the elastic QD model of dephasing. Because the electron-phonon coupling in PbSe QDs is relatively weak compared to other semiconductor nanocrystals, fast vibrationally induced dephasing can be expected in semiconductor QDs in general.

Ab initio nonadiabatic molecular dynamics of the ultrafast electron injection across the alizarin-TiO2 interface

The observed 6-fs photoinduced electron transfer (ET) from the alizarin chromophore into the TiO2 surface is investigated by ab initio nonadiabatic (NA) molecular dynamics in real time and at the atomistic level of detail. The system derives from the dye-sensitized semiconductor Grätzel cell and addresses the problems of an organic/inorganic interface that are commonly encountered in photovoltaics, photochemistry, and molecular electronics. In contrast to the typical Grätzel cell systems, where molecular donors are in resonance with a high density of semiconductor acceptor states, TiO2 sensitized with alizarin presents a novel case in which the molecular photoexcited state is at the edge of the conduction band (CB). The high level ab initio analysis of the optical absorption spectrum supports this observation. Thermal fluctuations of atomic coordinates are particularly important both in generating a nonuniform distribution of photoexcited states and in driving the ET process. The NA simulation resolves the controversy regarding the origin of the ultrafast ET by showing that although ultrafast transfer is possible with the NA mechanism, it proceeds mostly adiabatically in the alizarin-TiO2 system. The simulation indicates that the electron is injected into a localized surface state within 8 fs and spreads into the bulk on a 100-fs or longer time scale. The molecular architecture seen in the alizarin-TiO2 system permits efficient electron injection into the edge of the CB by an adiabatic mechanism without the energy loss associated with injection high into the CB by a NA process.

Time resolved infrared absorption studies of geminate recombination and vibrational relaxation in OClO photochemistry

Ultrafast time-resolved infrared absorption studies of aqueous chlorine dioxide (OClO) photochemistry are reported. Following photoexcitation at 401 nm, the evolution in optical density at frequencies between 1000 to 1100 cm(-1) is monitored to investigate vibrational energy deposition and relaxation along the asymmetric-stretch coordinate following the reformation of ground-state OClO via geminate recombination of the primary photofragments. The measured kinetics are compared to two proposed models for the vibrational-relaxation dynamics along the asymmetric-stretch coordinate. This comparison demonstrates that the perturbation model derived from molecular dynamics studies is capable of qualitatively reproducing the observed kinetics, where the collisional model employed in previous UV-pump, visible probe experiments demonstrates poor agreement with experiment. The ability of the perturbation model to reproduce the optical-density evolution observed in these studies demonstrates that for aqueous OClO, frequency dependence of the solvent-solute coupling is important in defining the level-dependent vibrational relaxation rates along the asymmetric-stretch coordinate. The absence of optical-density evolution corresponding to the population of higher vibrational levels (n>8) along the asymmetric-stretch coordinate suggests that following geminate recombination, energy is initially deposited into a local Cl-O stretch, with the relaxation of vibrational energy from this coordinate providing for delayed vibrational excitation of the asymmetric- and symmetric-stretch coordinates relative to geminate recombination, as previously observed.