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| | The Bohr-Sommerfeld quantization condition for this problem is |
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| <math> (n - \frac{1}{4})\pi h = \int_{0}^{r_{0}}\sqrt{2m[E-V_{0} ln(r/a)]}dr </math> | | <math>\int_{0}^{x_{0}}\sqrt{2m\left [E-V_{0}\ln\left (\frac{x}{a}\right )\right ]}\,dx=(n - \tfrac{1}{4})\pi\hbar.</math> |
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| <math>( E = V_{0} ln(r_{0}/a) '''defines''' r_{0} )</math> | | Note that <math>E=V_{0}\ln\left (\frac{x_{0}}{a}\right )</math> ''defines'' <math>x_{0}.\!</math> We may then rewrite the integral as |
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| = <math> \sqrt{2m} \int_{0}^{r_{0} } \sqrt{ V_{0} ln(r_{0} / a) - V_{0} ln(r/a) } dr = \sqrt{ 2m V_{0}} \int_{0}^{r_{0}}\sqrt{ln(r_{0} / a)}dr</math>
| | <math>\sqrt{ 2m V_{0}} \int_{0}^{xr_{0}}\sqrt{ln\left (\frac{x_{0}}{x}\right )}\,dx=(n - \tfrac{1}{4})\pi\hbar.</math> |
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| '''Let''' <math> x\equiv ln(r_{0}/a)</math>
| | Let us now make the substitution, <math>\xi=\ln\left (\frac{x_{0}}{x}\right ).</math> We then obtain |
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| '''so''' <math> e^{x} = r_{0}/r </math>'''or''' <math> r = r_{0}e^{-x} \Rightarrow dr = -r_{0}e^{-x}dx
| | <math>\sqrt{2mV_{0}}x_{0}\int_{0}^{\infty}\sqrt{x}e^{-x}\,dx=(n-\tfrac{1}{4})\pi \hbar,</math> |
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| (n-\frac{1}{4})\pi \hbar = \sqrt{2mV_{0}}(-r_{0})\int_{x_{1}}^{x_{2}}\sqrt{x}e^{-e}dx </math> '''. Limits :''' <math> \begin{cases}
| | or, evaluating the integral, |
| & \text{ } r=0 \Rightarrow x_{1}=\infty \\
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| & \text{ } r=r_{0} \Rightarrow x_{2}=0
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| \end{cases}
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| </math>
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| <math> (n-\frac{1}{4})\pi \hbar = \sqrt{2mV_{0}}(r_{0})\int_{0}^{\infty }\sqrt{x}e^{-x}dx=\sqrt{2mV_{0}}r_{0}\Gamma (3/2)=\sqrt{2mV_{0}}r_{0}\frac{\sqrt{\pi }}{2} </math>
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| <math> r_{0}=\sqrt{\frac{2\pi }{mV_{0}}}(n-\frac{1}{4})\Rightarrow E_{n}=V_{0}ln[\frac{\hbar}{a}\sqrt{\frac{2\pi }{mV_{0}}}(n-\frac{1}{4})]=V_{0}ln(n-\frac{1}{4})+V_{0}ln[\frac{\hbar}{a}\sqrt{\frac{2\pi }{mV_{0}}}] </math> | | <math>\tfrac{1}{2}\sqrt{2\pi mV_{0}}x_{0}=(n-\tfrac{1}{4})\pi \hbar.</math> |
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| <math> E_{n+1}-E_{n}=V_{0}ln(n+\frac{3}{4})-V_{0}ln(n-\frac{1}{4})=V_{0}ln(\frac{n+3/4}{n-1/4}) </math> ''',which is indeed independent of m (and a).''' | | Solving for <math>x_0,\!</math> we obtain |
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| Back to [[WKB Approximation]] | | <math>x_{0}=\sqrt{\frac{\pi}{2mV_{0}}}(2n-\tfrac{1}{2})\hbar.</math> |
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| | The energy spectrum is thus |
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| | <math>E_n=V_0\ln\left [\sqrt{\frac{\pi}{2mV_{0}a^2}}(2n-\tfrac{1}{2})\hbar\right ].</math> |
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| | If we now calculate the spacing between two adjacent energy levels, we obtain |
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| | <math>E_{n+1}-E_{n}=V_{0}\ln\left (\frac{n+\tfrac{3}{4}}{n-\tfrac{1}{4}}\right ).</math> |
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| | We see that this spacing is indeed independent of mass (and, in fact, of <math>a\!</math> as well). |
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| | Back to [[WKB Approximation#Problems|WKB Approximation]] |
Latest revision as of 13:37, 18 January 2014
The Bohr-Sommerfeld quantization condition for this problem is
![{\displaystyle \int _{0}^{x_{0}}{\sqrt {2m\left[E-V_{0}\ln \left({\frac {x}{a}}\right)\right]}}\,dx=(n-{\tfrac {1}{4}})\pi \hbar .}](https://wikimedia.org/api/rest_v1/media/math/render/svg/2cf224782adf5aaca89c704abb854fc33b64e0be)
Note that 
defines 
We may then rewrite the integral as
Let us now make the substitution, 
We then obtain
or, evaluating the integral,
Solving for 
we obtain
The energy spectrum is thus
![{\displaystyle E_{n}=V_{0}\ln \left[{\sqrt {\frac {\pi }{2mV_{0}a^{2}}}}(2n-{\tfrac {1}{2}})\hbar \right].}](https://wikimedia.org/api/rest_v1/media/math/render/svg/e1408d7722ef5ab3b09db64514ed719b3bcac0b6)
If we now calculate the spacing between two adjacent energy levels, we obtain
We see that this spacing is indeed independent of mass (and, in fact, of 
as well).
Back to WKB Approximation