Ultrafast Energy Relaxation in Single Light-Harvesting Complexes

TL;DR

This study uses a novel single-molecule pump-probe spectroscopy technique to observe ultrafast energy relaxation in individual light-harvesting complexes, revealing a distribution of relaxation times and ensemble ergodicity.

Contribution

It introduces a new method to measure ultrafast relaxation in single complexes and links the distribution to pigment energy disorder and ergodic behavior.

Findings

Relaxation time distribution peaks at 95 fs

Distribution shape varies with excitation wavelength

Ensemble is ergodic over seconds

Abstract

Energy relaxation in light-harvesting complexes has been extensively studied by various ultrafast spectroscopic techniques, the fastest processes being in the sub-100 fs range. At the same time much slower dynamics have been observed in individual complexes by single-molecule fluorescence spectroscopy (SMS). In this work we employ a pump-probe type SMS technique to observe the ultrafast energy relaxation in single light-harvesting complexes LH2 of purple bacteria. After excitation at 800 nm, the measured relaxation time distribution of multiple complexes has a peak at 95 fs and is asymmetric, with a tail at slower relaxation times. When tuning the excitation wavelength, the distribution changes in both its shape and position. The observed behaviour agrees with what is to be expected from the LH2 excited states structure. As we show by a Redfield theory calculation of the relaxation times, the distribution shape corresponds to the expected effect of Gaussian disorder of the pigment transition energies. By repeatedly measuring few individual complexes for minutes, we find that complexes sample the relaxation time distribution on a timescale of seconds. Furthermore, by comparing the distribution from three long-lived complexes with the whole ensemble, we demonstrate that the ensemble can be considered ergodic. Our findings thus agree with the commonly used notion of an ensemble of identical LH2 complexes experiencing slow random fluctuations.