Electron in a Box
Michael Fowler
8/30/06
Plane Wave Solutions
The best way to gain understanding of Schrödinger’s equation is to solve it for various potentials. The simplest is a one-dimensional “particle in a box” problem. The appropriate potential is V(x) = 0 for x between 0, L and V(x) = infinity otherwise—that is to say, there are infinitely high walls at x = 0 and x = L, and the particle is trapped between them. This turns out to be quite a good approximation for electrons in a long molecule, and the three-dimensional version is a reasonable picture for electrons in metals.
Between x = 0 and x = L we have V = 0, so the wave equation is just
.
A possible plane wave solution is
.
On inserting this into the zero-potential Schrödinger equation above we find E = p2/2m, as we expect.
It is very important to notice that the complex conjugate, proportional to
, is not a solution to the Schrödinger equation! If we
blindly put it into the equation we get
E = -p2/2m,
an unphysical result. However, a wave function proportional to 
gives E = p2/2m,
so this plane wave is a solution to the equation.
Therefore, the two allowed plane-wave solutions to the zero-potential
Schrödinger equation are proportional to 
and respectively.
Note that these two solutions have the same time dependence 
.
To decide on the appropriate solution for our problem of an electron in a box, of course we have to bring in the walls—what they mean is that y = 0 for x < 0 and for x > L because remember |y |2 tells us the probability of finding the particle anywhere, and, since it’s in the box, it’s trapped between the walls, so there’s zero probability of finding it outside.
The condition y = 0 at x = 0 and x = L reminds us of the vibrating string with two fixed ends—the solution of the string wave equation is standing waves of sine form. In fact, taking the difference of the two permitted plane-wave forms above gives a solution of this type:
.
This wave function satisfies the Schrödinger equation between the walls, it vanishes at the x = 0 wall, it will also vanish at x = L provided that the momentum variable satisfies:
Thus the allowed values of p are hn/2L, where n = 1, 2, 3… , and from E = p2/2m the allowed energy levels of the particle are:
.
Note that these energy levels become more and more widely spaced out at high energies, in contrast to the hydrogen atom potential. (As we shall see, the harmonic oscillator potential gives equally spaced energy levels, so by studying how the spacing of energy levels varies with energy, we can learn something about the shape of the potential.)
What about the overall multiplicative constant A in the wave
function? This can be real or complex. To find its value, note that at a fixed
time, say t = 0, the probability of the electron being between x
and x + dx is 
or
.
The total probability of the particle being somewhere between 0, L must be unity—so
.
Hence
.
When A is fixed in this way, by demanding that the total probability of finding the particle somewhere be unity, it is called the normalization constant.
Stationary States
Notice that at a later time the probability distribution for the wave function
is the same, because time only appears as a phase factor in
this time-dependent function, and so does not affect 
. A state with a time-independent probability distribution is
called a stationary state. Such a state must have a definite energy—as
can be seen by constructing a solution to Schrödinger’s equation by adding
states of different energies, for example:
.
(You can check the normalization constant at t = 0). For general x,
the two terms in the bracket rotate in the complex plane at different rates, so
their sum has a time-varying magnitude. That is to say, 
| varies in time, so the particle must be moving around—this
is not a stationary state. Of course, the total probability of
finding the particle somewhere in the box remains unity: the
normalization constant is time-independent.
The Time-Independent Schrödinger Equation: Eigenstates and Eigenvalues
In contrast to the superposition of different energy states in the equation above, a stationary state has a precisely defined energy, and the energy only appears in the overall phase factor,
Putting this wave function into the Schrödinger equation we find
This is the time-independent Schrödinger equation, and its solutions are the spatial wave functions for stationary states, states of definite energy. These are often called eigenstates of the equation.
The values of energy corresponding to these eigenstates are called the eigenvalues.
An Important Point: What, Exactly, Happens at the Wall?
Consider again the wavefunction for the lowest energy state of a particle
confined between walls at x = 0 and x = L. The reader
should sketch the wavefunction from some point to the left of x = 0 over
to the right of x = L. To the left of x = 0, the
wavefunction is exactly zero, then at x = 0 it takes off to the right
(inside the box) as a sine curve. In other words, at the origin the slope of
the wavefunction 
is zero to the left,
nonzero to the right. There is a discontinuity in the slope at the origin:
this means the second derivative of is infinite
at the origin. On examining the time-independent Schrödinger equation
above, we see the equation can only be satisfied at the origin because
the potential becomes infinite there—the wall is an infinite potential. (And,
in fact, since becomes zero on
approaching the origin from inside the box, the limit must be treated
carefully.)
It now becomes obvious that if the box does not have infinite walls, but
merely high ones, describing a confined
particle cannot suddenly go to zero at the walls: the second derivative must
remain finite. For non-infinite walls, and its derivative
must be continuous on entering the wall. This has the important physical
consequence that will be nonzero at least for some distance
into the wall, even if classically the confined particle does not have enough
energy to “climb the wall”. (Which it doesn’t, if it’s confined.) Thus, in
quantum mechanics, there is a non-vanishing probability of finding the particle
in a region which is “classically forbidden” in the sense that it doesn’t have
enough energy to get there.