- #1
Cogswell
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Homework Statement
The problem is given in the attached image. I'm currently trying to work out question one.
Homework Equations
[tex]\phi (k) = \dfrac{1}{2 \pi} \int_{- \infty}^{ \infty} \Psi (x,0) e^{-ikx} dx[/tex]
The Attempt at a Solution
Okay, so the first thing I did was to normalise it, but then I realized it was already normalised, so there's no need to do so there.
Using the above relevant equation:
[tex]\phi (k) = \dfrac{1}{2 \pi} \left( \dfrac{2a}{\pi} \right)^{\frac{1}{4}} \int_{- \infty}^{ \infty} e^{-ax^2} e^{ik'x} e^{-ikx} dx[/tex]
[tex]\phi (k) = \dfrac{1}{2 \pi} \left( \dfrac{2a}{\pi} \right)^{\frac{1}{4}} \int_{- \infty}^{ \infty} e^{-ax^2 + ik'x -ikx} dx[/tex]
[tex]\phi (k) = \dfrac{1}{2 \pi} \left( \dfrac{2a}{\pi} \right)^{\frac{1}{4}} \int_{- \infty}^{ \infty} e^{-[ax^2 + (ik - ik')x]} dx[/tex]
Using the hint, ## ax^2 + (ik - ik')x = u^2 - \dfrac{(ik - ik')^2}{4a} ## where ## u = \sqrt{a} \left( x+\dfrac{(ik-ik')}{2a} \right)##
And therefore ## du = \sqrt{a} dx ##So then the equation becomes:
[tex]\phi (k) = \dfrac{1}{2 \pi} \left( \dfrac{2a}{\pi} \right)^{\frac{1}{4}} \int_{x = - \infty}^{ x = \infty} exp - \left( u^2 - \dfrac{(ik - ik')^2}{4a} \right) dx[/tex]
[tex]\phi (k) = \dfrac{1}{2 \pi} \left( \dfrac{2a}{\pi} \right)^{\frac{1}{4}} \int_{x = - \infty}^{ x = \infty} e^{-u^2} e^{\dfrac{(ik - ik')^2}{4a}} \dfrac{du}{\sqrt{a}}[/tex][tex]\phi (k) = \dfrac{1}{2 \pi} \left( \dfrac{2a}{\pi} \right)^{\frac{1}{4}} \dfrac{1}{\sqrt{a}} e^{\dfrac{(ik - ik')^2}{4a}} \int_{x = - \infty}^{ x = \infty} e^{-u^2} du [/tex]
Now how do I change the bounds of integration? If there were still from negative infinity to positive infinity, I'd get ## \sqrt{ \pi} ## for the integral part, so is that right?
I feel like it's not from negative infinity to infinity though.