Book excerpt: In spite of detection intensity constraints necessary to avoid power broadening, the optical absorption spectrum of single molecules of pentacene in p-terphenyl crystals can be measured by (1) using laser FM spectroscopy combined with Stark and/or ultrasonic double modulation (to remove residual amplitude modulation) and (2) recording spectra far out in the wings of the inhomogeneous line to reduce the number of molecules in resonance to one.