Abstract

Recent advances in high-efficiency fluorescence excitation spectroscopy of pentacene in p-terphenyl crystals [1-3] have improved the signal to noise ratio for the detection of single molecular defects, thus confirming earlier single molecule detection in solids using absorption techniques[4]. Using 1-10 μm thick samples, tightly focused laser beams, and high collection efficiency, background emission noise is significantly reduced below the fluorescence emission rate of single molecular impurities at low (liquid helium) temperatures. To observe single molecules, the laser is tuned out into the wings of the inhomogeneously broadened 0-0 electronic transition until the number of defects per homogeneous linewidth is less than 1. This new regime of single molecule spectroscopy, in addition to representing optical detection at the ultimate 1/N_A = 1.6 × 10⁻²⁴ mole level, has also opened the door to the observation of a variety of new physical effects. With the improved detection sensitivity, we have observed two classes of pentacene defects present in both the red and blue wings of the inhomogeneous line in p-terphenyl crystals: class I defects are of stable, time-independent impurities with lifetime-limited Lorentzian homogeneous linewidths below 4 K, and class II defects, surprisingly, have time varying resonance frequencies. In recent work on perylene single molecular defects in the polymer poly(ethylene), controllable persistent spectral hole-burning of a single molecule has been observed in addition to spectral diffusion.

© 1992 Optical Society of America