Abstract
We explore, by theoretical modeling and computer simulations, how nonadiabatic couplings of excited electronic states of a polyatomic chromophore manifest themselves in single-molecule signals on femtosecond timescales. The chromophore is modeled as a system with three electronic states (the ground state and two non-adiabatically coupled excited states) and a Condon-active vibrational mode which, in turn, is coupled to a harmonic oscillator heat bath. For this system, we simulate double-pump single-molecule signals with fluorescence detection for different system-field interaction strengths, from the weak-coupling regime to the strong-coupling regime. While the signals are determined by the coherence of the electronic density matrix in the weak-coupling regime, they are determined by the populations of the electronic density matrix in the strong-coupling regime. As a consequence, the signals in the strong coupling regime allow the monitoring of nonadiabatic electronic population dynamics and are robust with respect to temporal inhomogeneity of the optical gap, while signals in the weak-coupling regime are sensitive to fluctuations of the optical gap and do not contain information on the electronic population dynamics.
Highlights
Starting from the mid 1980s, the monitoring of vibrational wave packets and the making/breaking of chemical bonds with femtosecond time resolution has been explored for molecular ensembles [1].Recently, molecular spectroscopists made the significant step by looking at the dynamics of individual molecules on femtosecond timescales
It is of interest to explore more systematically to what extent information on the dynamics of coupled electronic states in individual chromophores can be extracted from double-pump SM signals with fluorescence detection
We assume that the state |e2 i is optically bright from the ground state |e0 i, while the state |e1 i is optically dark, μ01 = 0
Summary
Starting from the mid 1980s, the monitoring of vibrational wave packets and the making/breaking of chemical bonds with femtosecond time resolution has been explored for molecular ensembles [1]. Femtosecond resolution was brought to the single-molecule (SM) spectroscopy community by van Hulst and coworkers [2,3] Their technique combines fluorescence detection of SMs pioneered by Orrit and Bernard [4] with pulse shaping and phase-locked double-pump excitation, pioneered in femtosecond ensemble molecular spectroscopy by Scherer and co-workers [5]. It is of interest to explore more systematically to what extent information on the dynamics of coupled electronic states in individual chromophores can be extracted from double-pump SM signals with fluorescence detection. We depart from the description of a chromophore as an electronic two-state system and study, by computer simulations of simple models, how nonadiabatic coupling among excited electronic states is manifested on the femtosecond timescale in double-pump SM signals
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