Box 18-2 Fluorescence All Around Us

A fluorescent lamp is a glass tube filled with Hg vapor. The inner walls are coated with a phosphor (luminescent substance) consisting of calcium halophosphate (Ca5(P04)3F1 xClJ.) doped with Mn2+ and Sb3+. Hg atoms, promoted to an excited state by 

Emission spectrum of Hg vapor (a) and phosphor (b) in fluorescent lamps. [From S. R. Goode and L A. Metz. Emission Spectroscopy in the Undergraduate Laboratory, J. Chem. Ed. 2003, 80, 1455 J. A. DeLuca. An Introduction to Luminescence In Inorganic Solids, J. Chem. Ed. 1980,57,541.] 

A 13-W fluorescent light that fits in a standard screw-in socket provides the same light as the 60-W bulb it replaces. The expected lifetime of the fluorescent lamp is 10 000 h, whereas that of the incandescent bulb is 750 h. The fluorescent light is more expensive than the incandescent bulb but saves a great deal of electricity and money over its lifetime. 

Most white fabrics also fluoresce. Just for fun, turn on an ultraviolet lamp in a darkened room containing several people (but do not look directly at the lamp). You will discover emission from 

Fluorescence and phosphorescence are relatively rare. Molecules generally decay from the excited state by radiationless transitions. The lifetime of fluorescence is always very short (10-8 to 10-4 s). The lifetime of phosphorescence is much longer (10-4 to 102 s). Therefore, phosphorescence is even rarer than fluorescence, because a molecule in the T] state has a good chance of undergoing intersystem crossing to S0 before phosphorescence can occur. 

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