Concise Physical Chemistry

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

348 PHOTOCHEMISTRY AND THE THEORY OF CHEMICAL REACTIONS

21.3 BOND DISSOCIATION ENERGIES (BDE)

In photoinduced and many other mechanisms, we are concerned with the stability

and rate of formation of free radicals, for example,

CH 4 →CH• 3 +H•

The enthalpy ofhomolytic cleavageis not easy to measure for these fleeting species,

but they can be calculated as an ordinary dissociation:

AB→A•+B•

Thebond dissociation energyBDE of ethane

CH 3 CH 3 →CH• 3 +CH• 3

to produce methyl radicals is a simple example of the more general case for hydro-

carbons and their radicals:

CH 3 R→CH• 3 +R•

BDE[CH 3 R]=fH 298

[

CH• 3

]

+fH 298 [R•]−fH 298 [CH 3 R]

The radical R may be branched, or it may contain double or triple bonds, or it may

contain two or all three of these structural features (Rogers et al., 2006).

21.4 LASERS

Some atoms and molecules can undergo apopulation inversion, such that an upper

energy state is more highly populated than the ground state. When this happens,

incident radiation is reemitted with its initial energy plus an energy gain obtained as

thegain mediumreturns to its ground state. In commercial lasers, atoms or molecules

comprising the gain medium are intentionally driven into a population inversion by

an optical pump, which emits high-energy radiation or fast electrons.

Alaserconsists of an optical pump that brings about a population inversion in a

gain medium whichlases, bringing about a slight radiative amplification of the light

source. The radiation from this laser gain medium is reflected back and forth between

two mirrors so that amplification is increased on each pass through the medium. One

mirror is intentionally made less reflective than the other so that some of the highly

amplified radiation passes through the less reflective mirror and is emitted as alaser

beam. Einstein developed the theory of lasers long before the first commercial laser

was produced.