um- mechanical effect known
as tunneling comes into play; this effect accounts for chem-
ical reactions that are forbidden by the principles of classi-
cal chemistry. Specifically, entire molecules can \"tunnel\"
(15)through the barriers of repulsive forces from other mole-
cules and chemically react even though these molecules do
not have sufficient energy, according to classical chemistry,
to overcome the repulsive barrier.
The rate of any chemical reaction, regardless of the tem-
(20)perature at which it takes place, usually depends on a very
important characteristic known as its activation energy. Any
molecule can be imagined to reside at the bottom of a so-
called potential well of energy. A chemical reaction corre-
sponds to the transition of a molecule from the bottom of
(25)one potential well to the bottom of another. In classical
chemistry, such a transition can be accomplished only by
going over the potential barrier between the wells, the
height of which remains constant and is called the activa-
請前往ｏｏｐ．ｔｗ閱讀本書最新內容
tion energy of the reaction. In tunneling, the reacting mole-
(30)cules tunnel from the bottom of one to the bottom of another
well without having to rise over the barrier between the
two wells. Recently researchers have developed the concept
of tunneling temperature: the temperature below which
tunneling transitions greatly outnumber Arrhenius transi-
(35)tions, and classical mechanics gives way to its quantum
counterpart.
This tunneling phenomenon at very low temperatures
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