Physical Chemistry , 1st ed.

is usually converted into vibrational energy. Such a process is called an inter-

nal conversion,because the electronic state changes within the molecule itself

and does not include any other participant (like solvent, for example). Then,

from this second electronic state, the molecule can emit a photon in a transi-

tion to the ground state. Because the second electronic excited state has lower

electronic energy than the original electronic state, the photon emitted in the

decay process has a lower energy than the photon absorbed in the excitation

process. The exchange in excited states is usually very fast, on the order of

10  10 to 10^6 seconds. Emission of light of lower energy due to such internal

conversions is called fluorescence.Figure 15.23 shows diagramatically the

processes behind fluorescence.

Fluorescence spectra are useful because twoexcited electronic states are in-

volved: one for the excitation process, and one for the decay process. A knowl-

edge ofbothphotons involved in the overall process is a better identification

tool, and fluorescence spectroscopy is particularly useful in analytical chem-

istry. Since the excitation process must use a photon of higher energy than the

decay process, ultraviolet excitation sources are particularly common in fluo-

rescence spectroscopy. Many large molecules, which have complicated elec-

tronic states, can show fluorescence. Petroleum jelly, teeth, various minerals

like zinc sulfide, and certain dyes fluoresce in the presence of higher-energy

light. Fluorescent paints take advantage of this spectroscopic property. They

absorb the relatively high-energy light and re-emit it as lower-energy photons,

and in doing so appear brighter and, well, fluorescent. Fluorescent lightbulbs

also take advantage of this property by using higher-energy photons emitted

by mercury atoms and converting them into lower-energy visible light. Overall,

the process is more energy-efficient than incandescent lightbulbs, which use

red-hot filaments to generate light. (That is, incandescent lightbulbs are de-

scribed by Planck’s law whereas fluorescent light bulbs are described by quan-

tum mechanics.)

Because fluorescence is a relatively fast process, it ends quickly when the

source of excitation stops: again, on the order of 10^10 to 10^6 seconds. (Such

time intervals are readily measurable with modern equipment, and the mea-

surement of fluorescence processes is common in modern physical chemistry

research.) However, the imposition of reality on molecular systems suggests

that the S0 selection rule is not always followed, and in some cases a for-

mally forbiddeninternal conversion occurs where S0. In most cases a sin-

glet state (2S 

1 1) spontaneously transfers into a manifold defined by a

triplet state (2S 

1 3). Such a conversion is illustrated by Figure 15.24.

These conversions are called intersystem crossings,because electronic states of

differing multiplicity are usually considered different electronic systemsof the

same molecule. After transferring to this new electronic state, the molecule

emits a photon and transfers to a lower electronic state, just like in fluores-

cence. However, because intersystem crossings are formally forbidden by quan-

tum mechanics, they usually take more time to occur. Timescales for photon

emission for these processes are on the order of 10^4 to 10^4 seconds: much

longer than fluorescence processes. This process is called phosphorescence.

Phosphorescence is distinguished from fluorescence in two ways. First, the

electronic states involved require a change in S(usually a forbidden process).

Second, because of the timescale involved, phosphorescence continues even af-

ter the excitation source is turned off. (Strictly speaking, so does fluorescence,

but the timescale implied here is one of human experience. Modern electronics

can detect the decrease in fluorescence after the excitation source is removed,

15.11 Fluorescence and Phosphorescence 549

Figure 15.23 Fluorescence occurs when an

atom or molecule absorbs a photon, vibrationally

relaxes, and then emits a photon to go back to the

ground state. The emitted photon is always lower

in energy than the absorbed photon. Fluorescence

is a relatively fast process.

Singlet manifold

Triplet manifold

Radiationless

transition

T 1

T 2

S 0

S 1

Excitation

Fluorescence