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Lothar Kador and William E. Moerner demonstrated optical detection of individual dopant molecules in a solid in 1989. They measured absorption from pentacene in a para-terphenyl host crystal at low temperature. Working in the wings of an inhomogeneously broadened spectral line reduced the number of resonant molecules, while two modulation methods suppressed interfering background signals. This connected careful background control and spectroscopic selection with measurements beyond an ensemble-averaged molecular response.^[ref]

Homepage: www.bimf.uni-bayreuth.de/de/team/Kador-Lothar/

Pentacene is a linearly fused five-ring aromatic hydrocarbon, ${\mathrm{C}}_{22}{\mathrm{H}}_{14}$. Its extended conjugation and crystal packing make it relevant to organic-semiconductor research.

Charge transport in a solid depends on interactions between molecules as well as the electronic structure of an isolated molecule. Pentacene therefore connects molecular orbital ideas with the properties of an ordered molecular material.

In 1989, William E. Moerner and Lothar Kador reported individual pentacene absorbers in para-terphenyl in Optical detection and spectroscopy of single molecules in a solid by W. E. Moerner and L. Kador (pp. 2535–2538). Different local environments spread photon absorption frequencies; tuning into the sparse wings selected few resonant molecules despite many molecules in the illuminated volume.

Their tunable dye laser focused through a $0.85$ numerical aperture lens onto a $100$–$200\mu \mathrm{m}$ crystal immersed in superfluid helium at $1.6\mathrm{K}$. Frequency modulation accompanied either a $5\mathrm{kHz}$ electric field between coated sapphire disks, detected at $10\mathrm{kHz}$, or a $2\mathrm{MHz}$ quartz shear transducer. An avalanche photodiode measured transmitted-light photon absorption, not fluorescence. Double modulation suppressed residual laser amplitude background.^[ref]

Repeated scans gave the predicted modulated line shape. Blank crystals and off-resonance scans gave background; changing modulation frequency expanded and contracted the feature as predicted. Agreement between both perturbation methods supported the individual-absorber inference. The authors acknowledged out-of-focus molecules and the unlikely alternative of multiple absorbers with exactly coincident frequencies.^[ref]

This is evidence for a selected absorber in a prepared, known chemical substance and host. It does not establish arbitrary unknown molecule identity from one spectral feature, provide an atom-resolved photograph, or count every molecule in the crystal. A measurement sensitive to only a narrow excitation range cannot detect a molecule whose transition lies outside that range. This distinction separates the total particle inventory from the number responding to a particular probe.