John M. Jean

Application of a multilevel Redfield theory to electron transfer in condensed phases

A quantum mechanical theory of photoinduced electron transfer, based on the Redfield theory of relaxation, is developed and applied to the standard two state–one mode system interacting with a thermal bath. Quantum mechanical treatment of the reaction coordinate allows incorporation of both finite vibrational dephasing and energy flow rates into the description of electron transfer dynamics. The field–matter interaction is treated explicitly to properly incorporate the total energy and magnitude of the vibrational coherence present in the initially prepared state. Calculation of the reduced density matrix of the system is carried out in a vibronic basis that diagonalizes the electron exchange coupling so that the method is valid for arbitrarily large coupling strength. For weak electronic coupling, we demonstrate the equivalence between the results from Redfield theory and those obtained from the standard perturbative expression (golden rule) for nonadiabatic electron transfer. We then discuss quantitativ...

Competition between energy and phase relaxation in electronic curve crossing processes

We present results from simulations of vibrational energy and phase relaxation and electronic curve crossing using a multilevel formulation of Redfield theory, which demonstrate the shortcomings of the optical Bloch approximation and the importance of coherence transfer processes in the relaxation dynamics of multilevel systems. Specifically, we show that for a harmonic well, energy relaxation can occur with retention of vibrational phase, and that for sufficiently strong electronic coupling, the product of an electronic curve crossing process can be formed vibrationally coherent even when no coherence is present in the initially excited state.

Fluorescence upconversion study of cis‐stilbene isomerization

The isomerization dynamics of cis‐stilbene in the first excited singlet state were studied by the technique of fluorescence upconversion. Lifetime measurements were made with subpicosecond resolution in 2‐propanol, decanol, n‐hexane, and tetradecane. The cis‐stilbene fluorescence decay curves are single exponential in all solvents except for decanol, where they are adequately described by a double exponential. A weak viscosity dependence of the decay times is observed in both alcohols and alkanes. These results are discussed in terms of the Bagchi, Fleming, Oxtoby theory [J. Chem. Phys. 78, 7375 (1983)] for activationless electronic relaxation in solution, and the limitations of hydrodynamic models of microscopic friction. The fluorescence decay times of α,α’ dideutero cis‐stilbene (cis‐stilbene‐D2) in tetradecane are found to be approximately 20% greater than those of cis‐stilbene. A picosecond component in the fluorescence anisotropy decay measurements made in 2‐propanol suggest that we are directly mea...

Excitation transport and trapping on spectrally disordered lattices

It is widely assumed that the decay of fluorescence in photosynthetic systems can be described as a sum of exponential components and that the amplitude of each component is directly related to the absorption cross-section of the antenna pigments coupled to the fluorescing species. We present exact calculations of excited state decay in two-dimensional regular lattices of different geometries containing multiple spectral forms of antenna pigments. We illustrate by these calculations that there is no simple relation between the decay amplitudes (and resulting time-resolved excitation spectra) and the steady-state absorption spectra. Only in the limit that the electronic excitations reach a rapid equilibrium among all antenna spectral forms does the excitation spectrum depend uniquely on the spectral features of the array. Using the simulations in conjunction with our recent fluorescence studies, we examine excitation transport and trapping dynamics in photosystem I and the limitations imposed by the finite time resolution in single photon counting experiments. In particular, we show that rising components, associated with excitation transfer among different spectral forms, with lifetimes <20 ps would be undetected in a typical photon counting experiment.

Observation of intramolecular vibrational redistribution and vibrational cooling in S1 trans-stilbene and 2-phenylindene in solution

Abstract Picosecond resonance Raman spectra of the S 1 states of trans-stilbene and its torsionally hindered analog 2-phenylindene in solution show that the vibrational cooling times in these two systems are similar. Anti-Stokes scattering from high frequency modes is observed for the first time and provides a direct observation of the IVR process. The data allow us to place a lower limit of 3–5 ps for the IVR time in trans-stilbene. These results provide significant new insight into ultrafast vibrational dynamics in solution and demonstrate the utility of time-resolved Raman spectroscopy as a probe of both intra- and inter-molecular energy transfer.

TIME- AND FREQUENCY-RESOLVED SPONTANEOUS EMISSION AS A PROBE OF COHERENCE EFFECTS IN ULTRAFAST ELECTRON TRANSFER REACTIONS

We present results from quantum dynamical simulations of electron transfer, obtained using a multilevel version Redfield relaxation theory, which illustrate the complex dynamics that result when electron tunneling and vibrational relaxation processes occur on similar time scales. By treating the system‐field interaction quantum mechanically, we examine the extent to which the time‐ and frequency‐resolved emission signal reflects the electron transfer dynamics. The results from these simulations provide new insight into the role of quantum coherences in ultrafast rate processes and time‐resolved spectral measurements.

Vibrational coherence effects on electronic curve crossing

Quantum dynamical simulations of vibrational phase coherence effects on ultrafast electronic curve crossing in condensed phases are presented for both symmetric and barrierless double well potentials. Reaction coordinate probability distributions are constructed from the reduced density matrix of the system, which provide considerable insight into the dynamics of curve crossing. The application of these ideas to retinal isomerization in rhodopsin is presented. Results suggest that coherence transfer processes are important for interpreting recent femtosecond transient absorption results on this system.Quantum dynamical simulations of vibrational phase coherence effects on ultrafast electronic curve crossing in condensed phases are presented for both symmetric and barrierless double well potentials. Reaction coordinate probability distributions are constructed from the reduced density matrix of the system, which provide considerable insight into the dynamics of curve crossing. The application of these ideas to retinal isomerization in rhodopsin is presented. Results suggest that coherence transfer processes are important for interpreting recent femtosecond transient absorption results on this system.

Dynamics of cis-Stilbene Isomerization in Solution

Fluorescence lifetime measurements of cis-stilbene show weak dependence on solvent viscosity and suggest that the reaction coordinate involves substantial ethylenic hydrogen motion.