Charles B. Harris

The origin of vibrational dephasing of polyatomic molecules in condensed phases

The vibrational dephasing of polyatomic molecules in condensed phases by intermolecular vibrational energy exchange is treated theoretically. In the exchange model, dephasing arises from random modulation of the vibrational frequency caused by intramolecular anharmonic coupling to low frequency modes which are undergoing intermolecular energy exchange with the bath. The exchange rates are temperature dependent and as a consequence manifest themselves experimentally as a temperature dependent broadening and shift of the Raman spectral line shape. Using a reduced density matrix technique within the constraints of a Markoff approximation, the theory allows the time dependence introduced by the exchange process to be properly accounted for, and allows explicit expressions for the vibrational correlation function and corresponding spectral line shape functions to be derived and related to molecular parameters. Application of the theory can have important consequences experimentally, since an analysis of the te...

Electronic energy transfer to metal surfaces: a test of classical image dipole theory at short distances

Electronic energy transfer from molecules to metal surfaces at distances < 50 A has been measured. A monolayer of luminescent molecules is separated from a single crystal metal surface by a layer of physisorbed Ar. The luminescence quantum yield is measured as a function of the Ar spacer thickness. A cubic distance dependence is observed, in agreement with classical image dipole theory.

Determining Transition State Geometries in Liquids Using 2D-IR

Many properties of chemical reactions are determined by the transition state connecting reactant and product, yet it is difficult to directly obtain any information about these short-lived structures in liquids. We show that two-dimensional infrared (2D-IR) spectroscopy can provide direct information about transition states by tracking the transformation of vibrational modes as a molecule crossed a transition state. We successfully monitored a simple chemical reaction, the fluxional rearrangement of Fe(CO)5, in which the exchange of axial and equatorial CO ligands causes an exchange of vibrational energy between the normal modes of the molecule. This energy transfer provides direct evidence regarding the time scale, transition state, and mechanism of the reaction.

Studies of chemical reactivity in the condensed phase. I. The dynamics of iodine photodissociation and recombination on a picosecond time scale and comparison to theories for chemical reactions in solution

Picosecond transient absorption measurements from 1000–295 nm are used to monitor the recombination dynamics of iodine after photodissociation in a variety of inert solvents. The high time resolution and signal‐to‐noise ratio of these measurements permits the development of a detailed model of this reaction, which should resolve disagreements over the time scales of geminate recombination and vibrational relaxation and over the role of excited electronic state trapping. Most of the atoms which undergo geminate recombination do so in ≤15 ps, in agreement with the predictions of existing molecular dynamics simulations. The subsequent vibrational and electronic energy relaxation of the recombined molecule is relatively slow and accounts for most of the transient absorption dynamics. The relaxing X‐state vibrational population distribution is extracted with an approximate method using calculated spectra of the excited vibrational levels and is compared to recent models. Vibrational relaxation times vary from ...

Phosphorescence-microwave double-resonance (PMDR) spectroscopy

Abstract The effect of saturating the zero-field transitions with microwave radiations on the phosphorescence spectrum is demonstrated at low temperature. The application of this type of double-resonance experiment in determining the most probable intersystem crossing routes, the mechanisms of the phosphorescence radiation of the different vibronic bands, the determination of zero-field splittings and the electronic distributions in the triplet state is indicated.

Torsional dynamics of molecules on barrierless potentials in liquids. I. Temperature and wavelength dependent picosecond studies of triphenyl‐methane dyes

The nonradiative decay dynamics of crystal violet, and other triphenyl‐methane dyes, dissolved in a variety of solvents, are studied as a function of temperature. A linear viscosity dependence of the excited state absorption decay time in n‐alcohol solvents is found at several constant temperatures. The temperature dependence at constant viscosity is anomalously negative over the entire viscosity range (0.6 to 8.0 cP) of these experiments. Various possible mechanisms for the observed behavior are critically discussed. Two color excite‐and‐probe studies reveal ground state bleach recovery times which are independent of excitation wavelength but strongly dependent on probe wavelength. The faster decay on the red side of the ground state absorption is shown to be the result of stimulated emission rather than the influence of a second state.

Direct femtosecond measurements of single collision dominated geminate recombination times of small molecules in liquids

Abstract We report the femtosecond transient absorption spectra of the CH2I photoproduct of the methylene iodide (CH2I2) photodissociation. The results indicate a disappearance of this photofragment on a ≈ 350 fs time scale in several different solvents. The implication is that in these simple liquids, methylene iodide is undergoing geminate recombination. Recombination yields, but not rates are shown to be solvent dependent. Comparison to the photodissociation dynamics of other small molecules suggests that the kinetics of geminate recombination are dominated by a single collision with the surrounding solvent cage in a large class of molecules and that the process may be universal.

On the Lowest Triplet State of Substituted Benzenes. III. Optically Detected Magnetic Resonance Studies on the 3π π State of 1,2,4,5‐Tetrabromobenzene and 1,2,4,5‐Tetrachlorobenzene

The optically detected magnetic resonance (ODMR) in zero field and the phosphorescence microwave double resonance (PMDR) spectra of 1,2,4,5‐tetrabromobenzene and 1,2,4,5‐tetrachlorobenzene have been obtained monitoring the low temperature (1.3°K) phosphorescence from both trap emission in neat single crystals and emission from a durene host. From the PMDR the major vibronic features of the spectra are assigned. The orbital symmetries in the excited triplet state, the principal and secondary contributions from the individual triplet spin sublevels to the electronic and vibronic origins in phosphorescence, the spin sublevel populations, and lifetimes are determined. In addition the spin Hamiltonian parameters, including the zero‐field splittings and Br and Cl excited state nuclear quadrupole coupling constants, are presented and interpreted. Finally, the relationship between spin sublevel activity in phosphorescence, Frank‐Condon maxima in emission, and molecular distortions are discussed.

Studies of chemical reactivity in the condensed phase. IV. Density dependent molecular dynamics simulations of vibrational relaxation in simple liquids

Molecular dynamics simulations of the photodissociation/recombination process for iodine in liquid xenon at several densities are reported in this paper. These simulations were performed to aid in the understanding and interpretation of recent picosecond experimental investigations on model chemical reaction systems. From these calculations, it was found that geminate recombination occurs primarily within a few picoseconds at all densities considered. This is in agreement with previous molecular dynamics simulations with significantly smaller systems, and with the current interpretation of experimental results. Simulated iodine ground electronic state vibrational relaxation times range from about 1 ns at the lowest density to approximately 250 ps at the highest density reported here. In addition, the functional form of the decay of the average iodine vibrational energy was observed to be nearly independent of density. This result is discussed in terms of simple gas phase isolated binary collision models. ...

Femtosecond studies of electron tunneling at metal-dielectric interfaces

Abstract Femtosecond time-resolved two-photon photoemission spectrocopy (TPPE) has been used to measure the lifetimes of image potential electrons at alkane mono- and bilayers on Ag(111). The n = 1 lifetimes for zero, one, and two layers of n-heptane on Ag(111) are 32 ± 10 fs, 155 ± 20 fs, and 1580 ± 200 fs, respectively. This approximately exponential increase in lifetime is consistent with a tunneling picture in which the adlayer forms a barrier that slows the decay of an image potential electron back into the metal. The existence of the tunneling barrier is consistent with the repulsive electron affinity of the longer chain n-alkanes in the condensed phase. The lifetimes of the higher quantum states indicate that the presence of the monolayer significantly reduces coupling of the image states to the bulk band structure, so that further changes in lifetime are determined by the adlayer barrier and an attempt rate related to the classical oscillation time in the modified image potential well. These results are compared with quantitative predictions of a model by Cole which considers the tunneling barrier presented by the layer and the effect of the layer on the attempt rate. These results are considered in the context of previous TPPE studies of metal-insulator interfaces.