Two-photon induced ultrafast coherence decay of highly excited states in single molecules

Abstract

Coherence is a key aspect of a large variety of processes, ranging from the coherent delocalisation of excitation energy, which is important for energy transfer in supramolecular nanostructures, to coherence between electronic states of a single quantum system, which is essential for quantum optical applications. Coherent control schemes exploit this quantum mechanical property by actively manipulating the outcome of dynamical processes. Moreover, this technique allows measuring dynamical processes under the influence of dephasing. However, going beyond the ensemble averaged situation, i.e. working on the level of single quantum systems, is highly challenging for quantum systems embedded in a solid matrix at elevated temperature. Since interactions between the quantum system and its specific local environment are a priori unknown, this requires a reliable approach to retrieve the relevant parameters governing the ultrafast coherent dynamics. Here, we present measurements of the ultrafast coherence decay of two-photon accessible excited states in single organic molecules embedded in a disordered environment at room temperature. We combine this experimental approach with a quantum dynamics identification procedure, which yields a minimum three-level model to describe the obtained data with very good agreement. In particular, we are able to retrieve the ultrafast (coherent) excited state dynamics in single molecules and demonstrate its sensitivity to the local nanoenvironment from molecule to molecule. This work provides a robust approach to measure and analyse ultrafast quantum dynamics in complex nanosystems.

Hexylation Stabilises Twisted Backbone Configurations in the Prototypical Low‐Bandgap Copolymer PCDTBT

“…Generally, such broad linewidths are characteristic for disordered guest-host systems (disordered PCDTBT chains in an amorphous PMMA matrix), in which many degrees of freedom allow for strong dynamic disorder. Dephasing occurs within some 10 fs at room temperature, as estimated from ultrafast spectroscopy data on conjugated polymers, [57][58][59] and accounts for only about 100 cm À 1 of the Gaussian linewidth. The main contribution of the linewidth stems therefore from unresolved spectral diffusion that is caused by (localised) conformational fluctuations in the local environment of the emitting site.…”

Equation-of-Motion Methods for the Calculation of Femtosecond Time-Resolved 4-Wave-Mixing and N-Wave-Mixing Signals

“…It is worthwhile to consider the two-pulse variant of population-detected spectroscopy, that is phase-locked double-pump population-detected spectroscopy, in some detail. This technique, pioneered in femtosecond ensemble molecular spectroscopy by Scherer and co-workers, was extended to single-molecule spectroscopy by van Hulst and co-workers , and to single-molecule microscopy by Fujiwara and Zhou et al , Double-pump population-detected signals were simulated for a number of systems, from molecules − ,− to antenna complexes. − These signals can be calculated via eq through the numerical solution of driven TDSEs or MEs with a system–field interaction Hamiltonian containing two pump pulses separated by the time delay τ. It is not necessary to perform a phase decomposition of the double-pump population-detected signal, because it can be conveniently decomposed into population and coherence contributions. , If, for example, the pump pulses are temporally well separated and contributions from the states of manifold {II} can be neglected, the DW approximation yields the double-pump population-detected signal in the form S ( τ ) = A ( τ ) + ( B false( τ false) e i φ + B * false( τ false) e − i φ ) e − γ τ Here A (τ) is the contribution which results from the evolution of the chromophore in electronic population, B (τ) is the coherence contribution, φ is the relative phase of the pump pulses, and γ is the electronic dephasing rate.…”

Probing Attosecond Electron Coherence in Molecular Charge Migration by Ultrafast X-Ray Photoelectron Imaging

“…Time-resolved photoelectron emission spectroscopy has been used widely as an efficient technique for investigating nuclear and electronic dynamics in molecular reactions [1][2][3][4][5][6][7][8]. An ultrashort pump laser pulse initiates a coherent transition between ground and excited states in molecules and the time evolution is subsequently monitored after a variable time delay by a second probe pulse.…”