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Particle in a box is the simplest model of a bound particle. We imagine particle to be confined in a one-dimensional box. A particle will be trapped inside such a box if potential is infinite \(U\) at both ends and zero inside the box as shown in Figure 54.5.1. It turns out that potential at the edge has to be infinite if particle is to be permanently trapped. Otherwise, particle will “tunnel” through a finite barrier using a mechanism called quantum tunneling.

\begin{equation} U = \left\{ \begin{array}{ll} \infty \amp \quad x \lt 0 \\ 0 \amp \quad 0\le x \le a\\ \infty \amp \quad x \gt a. \end{array} \right.\label{eq-particle-in-a-box-potential}\tag{54.5.1} \end{equation}

First thing to note is that since potential is infinite outside the box, wavefunction for the particle will be zero putside the box.

Inside the box, wave function for energy \(E\) will satsisfy

Both standing wave and progressive waves are possible inside the box. Here, we solve for standing waves corresponding to specific values of energy \(E\text{.}\) These solutions are also called stationary states. They have angular frequency \(\omega = E/\hbar\) and wavenumber \(k=\sqrt{2mE}/\hbar\text{.}\) It is better to write Eq. (54.5.4) as

where \(k\) is the wavenumber of the wave related to energy by

We seek a solution of this equation with zero value at the boundaries \(x = 0\) and \(x = a\text{.}\) IT is easy to verify that the solution will be a sine function of \(kx\) with some restrictions on \(k\text{.}\)

It automatically satisfies \(\psi(0) = 0\text{.}\) To satisfy \(\psi(a) = 0\text{,}\) we demand

Since \(A \ne 0\text{,}\) we must have

Now, if \(n=0\) we will have \(k=0\text{,}\) which would give \(\psi(x) = 0\text{.}\) That would mean the particle is not even in the box. Therefore, \(n \ne 0\text{.}\) Additionally, from Eq. (54.5.6) energy \(E\) is related to \(k^2\text{,}\) therefore, both plus and minus values of \(k\) correspond to the same energy. Therefore, we will drop the negative integers. Labeling the allowed values of \(k\) by an integer index \(n\text{,}\) the allowed values of \(k\) will be

Putting these values of \(k_n\) in Eq. (54.5.6) we obtain the allowed values of energy, which will be labeled similarly with an index, \(E_n\) to be:

That is,

We can write this more compactly as

with

The wave function \(\psi\) is also labeled by the same index \(n\text{:}\)

The constant \(A\) can be fixed by the normalization condition, which states that the probability of detecting the particle anywhere be \(1\text{.}\) Since the wave function is zero outside the box, we will have the following integral relation.

Therefore,

Taking \(A^2\) outside the integral, writing the square of the sine function in terms of the double angle of cosine, and then performing the integral immediately gives

Therefore, we will have \(A = \pm\sqrt{2/a}\text{.}\) Since wave function is always used as square we drop the minus sign and use only the positive \(A\) here. This gives us the normalized wave function.

Figure 54.5.2 shows the probability densities corresponding to the three lowest energy states of a particle in a box.

The lowest energy state, called the ground state, has no nodes between the walls; that is, there is no place inside the box where the probability of finding the particle is zero. The ground state is also symmetric about the center of the box. The state with the next higher energy is the first excited state. The wave function of the first excited state has one node between the walls and hence the probability density has a value zero at one point at an inside point of the box. The second excited state has two nodes and each successively higher energy state has one more node.

An important point of the energy levels of the particle in a box is the separation between the states varies as \(n^2\) of the states. Thus, the separation of the energy levels \(n = n_1\) and \(n=n_2\) will be