Practical Witness for Electronic Coherences

TL;DR

This paper proposes a practical method to distinguish electronic from vibrational coherences in photosynthetic complexes using pump-probe spectroscopy with variable pulse durations, enabling clearer interpretation of quantum effects in biological systems.

Contribution

It introduces an extrapolation technique to identify electronic coherences in ultrafast spectroscopy, providing a feasible experimental approach for many laboratories.

Findings

Method for extrapolating to ultrashort pulses demonstrated

Optimal pulse durations and frequencies determined from absorption spectra

Feasible with pulses of tens of femtoseconds for many systems

Abstract

The origin of the coherences in two-dimensional spectroscopy of photosynthetic complexes remains disputed. Recently it has been shown that in the ultrashort-pulse limit, oscillations in a frequency-integrated pump-probe signal correspond exclusively to electronic coherences, and thus such experiments can be used to form a test for electronic vs. vibrational oscillations in such systems. Here we demonstrate a method for practically implementing such a test, whereby pump-probe signals are taken at several different pulse durations and used to extrapolate to the ultrashort-pulse limit. We present analytic and numerical results determining requirements for pulse durations and the optimal choice of pulse central frequency, which can be determined from an absorption spectrum. Our results suggest that for numerous systems the required experiment could be implemented by many ultrafast spectroscopy laboratories using pulses of tens of femtoseconds in duration. Such experiments could resolve the standing debate over the nature of coherences in photosynthetic complexes.