Semiclassical WKB wavefunctions
Away from turning points, a slowly varying one-dimensional potential admits leading stationary WKB waves ψ≈[A e^(i∫k dx)+B e^(−i∫k dx)]/√k in an allowed region and ψ≈[C e^(∫κ dx)+D e^(−∫κ dx)]/√κ in a forbidden region, with k=√[2m(E−V)]/ħ and κ=√[2m(V−E)]/ħ.
Key knowledge
Away from turning points, a slowly varying one-dimensional potential admits leading stationary WKB waves ψ≈[A e^(i∫k dx)+B e^(−i∫k dx)]/√k in an allowed region and ψ≈[C e^(∫κ dx)+D e^(−∫κ dx)]/√κ in a forbidden region, with k=√[2m(E−V)]/ħ and κ=√[2m(V−E)]/ħ. The local amplitude varies with the inverse square root of the local wavenumber.
Values and equations
k=√[2m(E−V)]/ħ; κ=√[2m(V−E)]/ħ; ψ_allowed≈(A e^(i∫k dx)+B e^(−i∫k dx))/√k
- k,κ
- Allowed/forbidden local wavenumber m⁻¹
- x
- Position m
- m
- Particle mass kg
- E,V
- Energy and potential energy J
- ħ
- Reduced Planck constant J s
- ψ
- Normalized 1D wavefunction when normalization is imposed m⁻¹ᐟ²
- A,B,C,D
- WKB coefficients; m⁻¹ for this normalized 1D convention
- i
- Imaginary unit
Conditions and scope
- Real smooth potential, nonrelativistic constant mass m>0 and separately positive k or κ. The asymptotic expansion must be controlled: the local condition |k′|≪k² (or |κ′|≪κ²) is necessary in the usual slowly varying regime, together with control of higher-order terms.
- The formula fails at E=V and does not supply boundary conditions, normalization, a universal error bound or a solution at sharp steps.
Sources · 1
References
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Evidence and review
Quantum Physics III: chapter 3, semiclassical approximation: §3.2, printed pages 55–58, equations 3.2.15–3.2.22 and 3.2.25–3.2.32: amplitudes, real/imaginary momentum and local slow-variation condition. Uses source wavenumbers k,κ; excludes the missing ħ in its text p=iκ near 3.2.21.
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