By use of low-temperature (20 K) confocal microscopy, fluorescence spe
ctra of isolated single CdS nanocrystals have been recorded. The narro
west measured line width of the main fluorescence band has been determ
ined to be about 5 meV at the lowest excitation intensities. At higher
intensities at least three additional observations are made: the main
fluorescence band broadens, a line to the blue of the main peak evolv
es, and a peak due to the coupling to a longitudinal optical phonon ap
pears.