Number 189 (Story #3), August 9, 1994 by Phillip F. Schewe and Ben Stein LIGHT EMISSION FROM SINGLE MOLECULES is being studied in a number of labs, including AT&T Bell Labs (see Update 186), where individual luminescence centers in quantum wells can be identified, and IBM Almaden, where W.E. Moerner looks at the spectroscopy of single isolated impurity molecules in solids. Unlike the study of single neutral atoms, ions, or electrons in electromagnetic traps, the observations of single molecules constrained on every side by a lattice provides information about the nanoenvironment of the molecule. Moerner uses a small sample (only hundreds of cubic microns in volume), a small impurity concentration (down to 10**-9), and a careful tuning of a micron-sized laser beam, which causes the impurity molecule to fluoresce. The spectroscopy of this fluorescence will change slightly because of changes in the host lattice. One such change is "spectral diffusion," the shift of the molecule's resonance frequency owing to a the presence of a particular phonon (lattice vibration) mode. The ability to control the optical properties of single molecules may lead to extremely high density optical information storage devices and may facilitate the development of a single-molecule light source for microscopy. (Science, 1 July 1994.)