WKB Computations on Morse Potential

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The semiclassical Wentzel–Kramers–Brillouin (WKB) method applied to one-dimensional problems with bound states often reduces to the Sommerfeld–Wilson quantization conditions, the cyclic phase-space integrals . It turns out that this formula gives the exact bound-state energies for the Morse oscillator with
. The requisite integral can be reduced to
, in which
and
are the classical turning points
. The integral can be done "by hand", using the transformation
followed by a contour integration in the complex plane, but Mathematica can evaluate the integral explicitly, needing only the additional fact that
The result reads
, which can be solved for
to give
,
, in units with
. The highest bound state is given by
, where
represents the floor, which for positive numbers is simply the integer part. The values of
,
, and
(expressed in atomic units) used in this Demonstration are for illustrative purposes only and are not necessarily representative of any actual diatomic molecule.
Contributed by: S. M. Blinder (January 2011)
Open content licensed under CC BY-NC-SA
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A particle in a one-dimensional potential can be described by the Schrödinger equation: .
The semiclassical or WKB method is based on the ansatz . In the limit as
,
in the exponential satisfies the Hamilton–Jacobi equation for the action function
, with one solution
. It is then shown in most graduate-level texts on quantum mechanics (e.g. Schiff, Merzbacher, etc.) that this usually leads to the Sommerfeld–Wilson quantum conditions on periodic orbits
,
. For one-dimensional problems, the cyclic integral can be replaced by
, where
,
are the classical turning points of the motion and
.
As an étude consider the linear harmonic oscillator, with and
. The quantum condition reads
, which can be solved to give
. This is one of a small number of cases in which the WKB method gives the exact quantum-mechanical energies.
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