trons that we have already used for photons.13.3.1 Electrons as waves
We started our journey into quantum physics by studying the
random behavior ofmatterin radioactive decay, and then asked how
randomness could be linked to the basic laws of nature governing
light. The probability interpretation of wave-particle duality was
strange and hard to accept, but it provided such a link. It is now
natural to ask whether the same explanation could be applied to
matter. If the fundamental building block of light, the photon, is
a particle as well as a wave, is it possible that the basic units of
matter, such as electrons, are waves as well as particles?
A young French aristocrat studying physics, Louis de Broglie
(pronounced “broylee”), made exactly this suggestion in his 1923
Ph.D. thesis. His idea had seemed so farfetched that there was
serious doubt about whether to grant him the degree. Einstein was
asked for his opinion, and with his strong support, de Broglie got
his degree.
Only two years later, American physicists C.J. Davisson and L.
Germer confirmed de Broglie’s idea by accident. They had been
studying the scattering of electrons from the surface of a sample
of nickel, made of many small crystals. (One can often see such a
crystalline pattern on a brass doorknob that has been polished by
repeated handling.) An accidental explosion occurred, and when
they put their apparatus back together they observed something
entirely different: the scattered electrons were now creating an in-
terference pattern! This dramatic proof of the wave nature of matter
came about because the nickel sample had been melted by the ex-
plosion and then resolidified as a single crystal. The nickel atoms,
now nicely arranged in the regular rows and columns of a crystalline
lattice, were acting as the lines of a diffraction grating. The new
crystal was analogous to the type of ordinary diffraction grating in
which the lines are etched on the surface of a mirror (a reflection
grating) rather than the kind in which the light passes through the
transparent gaps between the lines (a transmission grating).Although we will concentrate on the wave-particle duality of elec-
trons because it is important in chemistry and the physics of atoms,
all the other “particles” of matter you’ve learned about show wave
properties as well. Figure a, for instance, shows a wave interference
pattern of neutrons.
It might seem as though all our work was already done for us,
and there would be nothing new to understand about electrons:
they have the same kind of funny wave-particle duality as photons.
That’s almost true, but not quite. There are some important ways
in which electrons differ significantly from photons:890 Chapter 13 Quantum Physics