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 pentacene defects, thus confirming single pentacene detection using absorption techniques.⁴ Using 1-10 μm thick crystals, tightly focused laser beams, and high collection efficiency, background emission noise is significantly reduced below the fluorescence emission rate of single pentacene defects. To observe single molecules, the laser is tuned out into the wings of the pentacene O₁ site inhomogeneous line until the number of defects per homogeneous linewidth is less than 1. 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: class I consist of stable, time-independent defects with lifetime limited homogeneous linewidths below 4 K, and class II have time varying resonance frequencies.

© 1991 Optical Society of America