The internuclear potential for a diatomic molecule is illustrated by a solid line in the following figure. The curve rises steeply to the left of the minimum, indicating the difficulty in pushing the two nuclei closer together. The curve to the right side of the equilibrium position rises initially but eventually levels off. The potential energy at large seperations is essentially the bond energy. The dashed line shows the potential

U(x) = 0.5 K ( x - x₀ )²

here, x₀ is the equilibrium bond length associated with Hooke's law. Here, K is the spring constant, which gives a quantitative measure of the curvature of the potential energy surface. Although the harmonic-oscillator potential may appear to be a terrible approximation on the experimental curve, it is a good approximation in the region of the minimum.

An analytic expression that is a good approximation to an internuclear potential energy curve is a Morse potential

U(x) = D (1 - e^{-b ( x - x0)} )²

where D and b are the parameters that depend upon the molecule. The parameter D is the dissociation energy of the molecule measured from the minimum of U(x) and b is a measure of the curvature of U(x) at its minimum.

In this experiment, one of the atoms in a diatomic molecule is optically trapped at a fixed position. The other atom is pulled away from this fixed atom using optical forces. The potential energies and corresponding internuclear forces are measured as a function of internuclear separation.