title: How two-dimensional brick layer J-aggregates differ from linear ones: Excitonic properties and line broadening mechanisms.
abstract: We study the excitonic coupling and homogeneous spectral line width of brick layer J-aggregate films. We begin by analysing the structural information revealed by the two-exciton states probed in two-dimensional spectra. Our first main result is that the relation between the excitonic couplings and the spectral shift in a two-dimensional structure is different (larger shift for the same nearest neighbour coupling) from that in a one-dimensional structure, which leads to an estimation of dipolar coupling in two-dimensional lattices. We next investigate the mechanisms of homogeneous broadening--population relaxation and pure dephasing--and evaluate their relative importance in linear and two-dimensional aggregates. Our second main result is that pure dephasing dominates the line width in two-dimensional systems up to a crossover temperature, which explains the linear temperature dependence of the homogeneous line width. This is directly related to the decreased density of states at the band edge when compared with linear aggregates, thus reducing the contribution of population relaxation to dephasing. Pump-probe experiments are suggested to directly measure the lifetime of the bright state and can therefore support the proposed model.
authors.affiliation: Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA.
authors.forename: Arend G
authors.initials: AG
authors.lastname: Dijkstra
authors.affiliation: Max Planck Institute for the Structure and Dynamics of Matter, Luruper Chaussee 149 Bldg. 99, 22761 Hamburg, Germany.
authors.forename: Hong-Guang
authors.initials: HG
authors.lastname: Duan
authors.affiliation: Zernike Institute for Advanced Materials, Nijenborgh 4, 9747 AG Groningen, The Netherlands.
authors.forename: Jasper
authors.initials: J
authors.lastname: Knoester
authors.affiliation: Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA.
authors.forename: Keith A
authors.initials: KA
authors.lastname: Nelson
authors.affiliation: Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA.
authors.forename: Jianshu
authors.initials: J
authors.lastname: Cao