Aromatic excimers: evidence for polynuclear aromatic hydrocarbon condensation in flames

A new assignment for the broad band visible laser induced fluorescence observed in rich hydrocarbon flames is offered. It is postulated that emission from aromatic excimers is observed when exciting aromatic aggregates in flames. Because this emission will only occur when the aromatic molecular units are near one another, this assignment would provide evidence for agglomeration of modestly sized polynuclear aromatic hydrocarbons in flames. Both semi-empirical and density functional theory calculations are performed for several small aromatic dimers (benzene, naphthalene, and pyrene). As observed in several prior experimental studies, stacking of the aromatic rings in parallel, overlapping planes leads to excimer transitions. It is suggested that additional near-infrared optical diagnostics in rich flames may provide further support for this theory. (c) 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved.