Concise Physical Chemistry

c21 JWBS043-Rogers September 13, 2010 11:30 Printer Name: Yet to Come

QUANTUM YIELDS 345

Many systems follow Einstein’s law, which argues strongly in favor of the photon

concept of light and the proposed mechanism for energy transfer. Many systems,

however, deviate widely from Einstein’s law, so the true nature of photochemical

reactions must be more complicated than we have pictured so far.

21.2 QUANTUM YIELDS

There are systems for which the number of reacting molecules is smaller than the

number of incident photons and there are those for which it is larger, sometimes much

larger. A system can be characterized by itsquantum yield, defined as

=

number of molecules reacting

number of photons absorbed

A system that obeys Einstein’s law has=1.

When a quantum yield is less than 1, as in the first entry in Table 21.1, we suppose

that there are energy dissipating processes going on. In a complicated molecule, the

quantum of incident energy may be split up among several degrees of freedom and

dissipated eventually as heat. In some molecules, a part of the incoming quantum of

energy is reemitted as light. The system might emit part of the excitation energy as

light and the remainder as heat.

If the energy emitted is less than the energy taken in, the wavelength of the emitted

radiation is the longer of the two. This is the common observation influorescence;

beaming ultraviolet light (shortλ, high energy) on a fluorescent material produces

visible light (longerλ, lower energy). If there is a time lag between absorption and

emission, the phenomenon is calledphosphorescence(Fig. 21.1). Explanation of a

high quantum yield is not quite so simple and involves a new concept, thechain reac-

tion. One postulates a reaction mechanism for aphotoinducedchain reaction in which

1. an energetic species is produced, which


2. brings about a chemical reaction in which it is replicated, and it


3. Passes its energy on to produce another product molecule and another energetic
   species. By this mechanism, each incident photon can be responsible for many
   product molecules, so a high quantum yield results. Explosions are possible.
   The production of HCl from the elements H 2 and Cl 2 is an example of a

TABLE 21.1 Some Experimental Quantum Yieldsa.

Reaction Quantum Yield

(CH 3 ) 2 C O→CH 3 C• O+•CH 3 0.17

CH 3 COOH→CH 4 +CO 2 1

H 2 +Cl 2 →2HCl ∼ 105

aThe symbol•indicates a free radical.