LINE-WIDTH STUDY IN FLUORESCENCE EXCITATION-SPECTRA OF SINGLE PENTACENE MOLECULES INTRODUCED AS IMPURITIES IN P-TERPHENYL CRYSTAL

We report results on line widths in the fluorescence excitation spectr a of single pentacene molecules in p-terphenyl crystal flakes for the O-1 and O-2 sites at 1.85 K. A setup for the measurement of fluorescen ce excitation spectra of single impurity molecules with an overall qua ntum efficiency of about 0.1% has been designed. Spectra obtained with a signal-to-noise ratio up to 10:1 were recorded at different laser e xcitation intensities in the range from 1 to 5 mW/cm(2). The line widt hs measured for different pentacene impurity molecules and extrapolate d to zero probing intensity show remarkable variation in the range fro m 5 to 15 MHz. A reason why some lines are narrower than the lifetime- limited one (8 MHz) is discussed.