Page 43 - Science at the nanoscale

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June 9, 2009
3.1. Basic Quantum Physics and Quantum Confinement
expected to increase with decreasing wavelength as shown in
Fig. 3.1(b). Obviously, classical theory cannot explain what is
observed in the experiment. The motivation to find a mechanism
to account for the observation led to the birth of quantum physics.
3.1.2
Max Planck’s Theory
In order to provide a good explanation of blackbody radiation,
Max Planck proposed the following assumptions:
(a) Atoms of the blackbody radiator behave as harmonic
oscillators. The energy of the oscillators adopt the form
of discrete values of
(3.2)
E = nhv
where n is a positive integer and v is the frequency of
the oscillators. Here h is a new fundamental constant of
−34
nature known as Planck’s constant, h = 6.63 × 10
Joule
sec.
(b) Each atom can absorb or emit radiation energy packet by
going through transition from one state (E = nhv) to an
adjacent energy state (E = [n ± 1] hv). Hence the amount
of energy absorbed or emitted by the atom is equal to hv.
The above condition deviates from classical theory and implies
that the energy of atomic oscillators is quantised. Such quantisa-
tion was subsequently incorporated into other physical quantities
and became a fundamental property of many systems in nature.
Using this simple but revolutionary assumption, together with the 33 ch03
concepts from statistical mechanics, Planck was able to obtain an
expression for the energy density in the blackbody radiation that
agrees surprisingly well with the experimental observations.
3.1.3 Photoelectric Effect
Photoelectric effect refers to the emission of electrons from a
material under the action of light irradiation. The emitted
electrons are known as the photoelectrons. The following are
a summary of the observations made during the photoelectric
experiments.