Two-Dimensional Electronic Spectroscopy Using Incoherent Light: Theoretical Analysis

Abstract

Electronic energy transfer in photosynthesis occurs over a range of time scales and under a variety of intermolecular coupling conditions. Recent work has shown that electronic coupling between chromophores can lead to coherent oscillations in two-dimensional electronic spectroscopy measurements of pigment-protein complexes measured with femtosecond laser pulses. A persistent issue in the field is to reconcile the results of measurements performed using femtosecond laser pulses with physiological illumination conditions. Noisy-light spectroscopy can begin to address this question. In this work we present the theoretical analysis of incoherent two-dimensional electronic spectroscopy, I(4) 2D ES. Simulations reveal diagonal peaks, cross peaks, and coherent oscillations similar to those observed in femtosecond two-dimensional electronic spectroscopy experiments. The results also expose fundamental differences between the femtosecond-pulse and noisy-light techniques; the differences lead to new challenges and new opportunities.

Photosynthetic light harvesting: excitons and coherence

“…The incoherent 2DES results produce many of the same features observed in the femtosecond experiments-somewhat surprising given the very different nature of the light sources used. Incoherent 2DES was proposed by Turner et al [171] and built on prior experiments in the field of noisy light spectroscopy [172][173][174][175][176] where many of the nonlinear spectroscopies using femtosecond pulses have been reproduced in with incoherent sources capable of obtaining much of the same information.…”

“…First, one of the big questions in the field is whether coherent dynamics are observed in experiments owing to the coherent and ultrafast nature of the excitation. In this direction, two-dimensional spectroscopy with incoherent light as proposed by Turner et al [171] may provide a decisive tool to clarify whether coherent dynamics are only consequence of an 'artificial' excitation process or whether instead the characteristics of the pigment -protein complex allow such effects under solar-like light. Second, conventional spectroscopic techniques deal with ensembles of different complexes, and therefore the averaging over different complexes may obscure certain dynamical features.…”

Coherent multidimensional optical spectra measured using incoherent light

“…1f by scanning s and z from À 80 to þ 80 fs with a step size of 0.8 fs at t ¼ 0. Although almost all femtosecond 2D spectroscopy measurements use frequency-resolved detection, we performed this first measurement using time-integrated detection to be consistent with the I (4) 2D ES theory 40 and nearly all 1D noisy-light measurements, including refs 35-39. We highlight the signal as function of z with s ¼ 0 in Fig.…”

“…In the latter, the interferometric delays correspond only to the delay between identical time points along noisy electric fields. Any arbitrary point on the time profile of the noisy field can conceptually serve as such an 'anchor point', an idea closely related to the 'fingerprint region' concept in the noisy-light literature [35][36][37][38][39][40] . The asterisks in the schematics of the noisy-light fields shown in Fig.…”

“…The 2D spectra contain similar but not identical information as 2D spectra measured using femtosecond pulses. The 2D measurements using incoherent light are extensions of one-dimensional (1D) four-wave mixing spectroscopy measurements using incoherent excitation, a field referred to as noisy-light spectroscopy [35][36][37][38][39][40] . The main feature of noisy-light spectroscopy is that the time resolution is given by the coherence time of the light, and not the temporal envelope.…”