title: Electron momentum distribution and singlet-singlet annihilation in the organic anthracene molecular crystals using positron 2D-ACAR and fluorescence spectroscopy.
abstract: We present the mapping of electron momentum distribution (EMD) in a single crystal of anthracene by two-dimensional angular correlation of positron annihilation radiation (2D-ACAR). The projected EMD is explained on the basis of the crystallographic features of the material. The EMD spectra provide information about the positron states and their behavior and also about the hindrance of the positronium (Ps) formation in this material. The EMD has exhibited evidence for the absence of free volume defects. The characteristic EMD features regarding the delocalized electronic states are explained. Further, scintillation characteristics such as fluorescence and time-correlated single photon counting have also been studied. The emission peaks are attributed to vibrational bands of fluorescence emission from the singlet excitons and lifetime components are observed to be due to singlet fission and the singlet-singlet excitons annihilation.
authors: Materials Science Center, Department of Nuclear Physics, University of Madras, Guindy Campus, Chennai 600 025, India. sivaji@unom.ac.in sivaji.krishnan@yahoo.com.
authors: Sellaiyan
authors: S
authors: Selvakumar
authors: Krishnan
authors: K
authors: Sivaji
authors: Arjunan
authors: A
authors: Arulchakkaravarthi
authors: Sambasivam
authors: S
authors: Sankar