Journal of Luminescence 94–95 (2001) 805–809 Photon statistics in single-molecule fluorescence at room temperature L. Fleury, J.-M. Segura, G. Zumofen*, B. Hecht, U.P. Wild Physical Chemistry Laboratory, Swiss Federal Institute of Technology, ETH-Z, CH-8092 Z . urich, Switzerland Abstract The fluorescence of single terrylene molecules in a crystalline host is investigated at room temperature by scanning confocal optical microscopy. Photon arrival times are analyzed in terms of inter-photon time distribution, second-order correlation function, and Mandel’s Q-function. Nonclassical photon statistics is observed and a reverse intersystem crossing is detected that accelerates linearly with the applied laser power. Rate equations for the time evolution of the molecular level populations are shown to be appropriate for the analysis of the observations. r 2001 Elsevier Science B.V. All rights reserved. Keywords: Photon statistics; Single-molecule spectroscopy; Scanning confocal optical microscopy 1. Introduction Recent advances in fluorescence microscopy have rendered possible the detection, imaging and spectroscopy of single molecules (SMs) at various conditions [1–6]. Of particular interest here is the nonclassical photon statistics which is a signature for the quantum nature of the object under consideration. In the past, room tempera- ture experiments that reveal the nonclassical character of SM fluorescence, could solely be performed by averaging the effects of a large number of single chromophores [7–9]. The reason for this is the limited amount of fluorescence photons available from a single molecule before photochemical destruction. Recently, it has been shown that for terrylene molecules embedded in a crystal of p-terphenyl the photochemistry is drastically reduced so that a large number of photons could be recorded before photo bleaching [10,11]. Nonclassical photon statistics and an accelerated reverse intersystem crossing were ob- served and studied in terms of an appropriate system of rate equations for the level populations. The investigation of such and similar phenomena is essential for the development of an apparatus that generates single photons on demand [12–15]. In this paper we review our previous study and present more details about the data analysis. 2. Experimental Crystal flakes of p-terphenyl with a thickness of E3 mm doped with a low concentration of terrylene were grown by co-sublimation according to standard procedures [16]. A scanning con- focal optical microscope was employed to image single molecules of terrylene in p-terphenyl [10]. *Corresponding author. Fax: +41-1-632-1021. E-mail address: zumofen@phys.chem.ethz.ch (G. Zumofen). 0022-2313/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII:S0022-2313(01)00367-2