= Single-molecule absorption in a molecular crystal
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 $100$–$200\,\mathrm{\mu m}$ 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.<Optical detection and spectroscopy of single molecules in a solid by W. E. Moerner and L. Kador>[pp. 2535–2537: sample, lens, dye laser, FM/Stark and FM/ultrasound apparatus and detection]{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>.<Optical detection and spectroscopy of single molecules in a solid by W. E. Moerner and L. Kador>[Fig. 1(d–g), Fig. 2(b–g), and pp. 2537–2538: blanks, modulation-frequency changes, background and coincident-absorber qualification]{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.
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