title: Two-dimensional spectroscopy can distinguish between decoherence and dephasing of zero-quantum coherences.
abstract: Recent experiments on a variety of photosynthetic antenna systems have revealed that coherences among electronic states persist longer than previously anticipated. In an ensemble measurement, the observed dephasing of a coherent state can occur because of either disorder across the ensemble or decoherence from interactions with the bath. Distinguishing how much such disorder affects the experimentally observed dephasing rate is paramount for understanding the role that quantum coherence may play in energy transfer through these complexes. Here, we show that two-dimensional electronic spectra can distinguish between the limiting cases of homogeneous dephasing (decoherence) and inhomogeneous dephasing by examining how the quantum beat frequency changes within a cross peak. For the antenna complex LH2 isolated from  Rhodobacter sphaeroides , we find that dephasing of the coherence between the B850 and B800 rings arises predominantly from inhomogeneity. In contrast, within the Fenna-Matthews-Olson (FMO) complex from  Chlorobium tepidum , dephasing of the coherence between the first two excitons appears quite homogeneous. Thus, the observed dephasing rate sets an upper bound on decoherence for the LH2 complex while establishing both an upper and lower bound for the FMO complex.
authors.affiliation: The James Franck Institute and Department of Chemistry, The University of Chicago, Chicago, Illinois 60637, USA.
authors.forename: Andrew F
authors.initials: AF
authors.lastname: Fidler
authors.forename: Elad
authors.initials: E
authors.lastname: Harel
authors.forename: Phillip D
authors.initials: PD
authors.lastname: Long
authors.forename: Gregory S
authors.initials: GS
authors.lastname: Engel
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry*; D041883:Chlorobium / Q000502:physiology; D004583:Electrons; D004735:Energy Transfer; D045342:Light-Harvesting Protein Complexes / Q000737:chemistry*; D008956:Models, Chemical; D008958:Models, Molecular; D010788:Photosynthesis; D011789:Quantum Theory; D012242:Rhodobacter sphaeroides / Q000502:physiology; D013057:Spectrum Analysis; D013816:Thermodynamics
chemical_list: C081521:B800-850 light-harvesting complex, bacteria; D001426:Bacterial Proteins; C086320:FMO bacteriochlorophyll protein, Bacteria; D045342:Light-Harvesting Protein Complexes