A one-dimensional Gaussian Wave Packet

[Anden]

Homework Statement

I have been given the function
<br /> \varphi_{G}(z,t) = \frac{2}{\Delta k_0\sqrt{\pi}} \int_{-\infty}^{\infty}dk\ e^{-\frac{4(k-k_0)^2}{\Delta k_0^2}}e^{i(kz-\omega t)}<br />
and been told to do the integration and then to specify the phase and group velocity of the wave package. I also have to decide if there is dispersion or not.

Homework Equations

<br /> \omega = ck <br />
<br /> \int_{-\infty}^{\infty}dx e^{-ax^2} = \sqrt{\frac{\pi}{a}},\quad a &gt; 0<br />

The Attempt at a Solution

Using the method of completing the square I get
<br /> \begin{align*}<br /> \varphi_{G}(z,t) <br /> &amp;= \frac{2}{\Delta k_0\sqrt{\pi}} \int_{-\infty}^{\infty}dk\ \exp{\left(-\frac{4(k-k_{0})^2}{\Delta k_{0}^{2}} + i(kz - kct)\right)} \\<br /> &amp;= \frac{2}{\Delta k_0\sqrt{\pi}} \int_{-\infty}^{\infty}dk\ \exp{\left(-\frac{4}{\Delta k_{0}^{2}}(k^2 - 2k(k_0 + i\frac{\Delta k_{0}^{2}}{8}(z-ct)) + k_{0}^{2})\right)} \\<br /> &amp;= \frac{2}{\Delta k_0\sqrt{\pi}} \int_{-\infty}^{\infty}dk\ \exp{\left(-\frac{4}{\Delta k_0^2}( (k-(k_0 + i\frac{\Delta k_0^2}{8}(z-ct)))^2 - 2k_0 i\frac{\Delta k_0^2}{8}(z-ct) + \frac{\Delta k_0^4}{64}(z-ct)^2)\right)} \\<br /> &amp;= \frac{2}{\Delta k_0\sqrt{\pi}} \exp{\left(ik_0(z-ct) - \frac{\Delta k_0}{16}(z-ct)^2\right)} \int_{-\infty}^{\infty}dk\ \exp{\left(-\frac{4}{\Delta k_0^2}(k-(k_0 + i\frac{\Delta k_0^2}{8}(z-ct)))^2\right)} \\<br /> &amp;= \frac{2}{\Delta k_0\sqrt{\pi}} \exp{\left(ik_0(z-ct) - \frac{\Delta k_0}{16}(z-ct)^2\right)} \frac{\sqrt{\pi}\Delta k_0}{2} \\<br /> &amp;= \exp{\left(-\frac{\Delta k_0^2}{16}(z-ct)^2 + ik_0 (z-ct)\right)}<br /> \end{align*}<br />

Now, I don't really have a lot of experience doing things like this (in fact this is the first time). Is the result I got correct or have I done an error somewhere? Also, is there maybe an easier way to calculate the integral?

 

[member 553083]

Finally, I would be grateful if someone could help me with the other parts of the problem, i.e. determining the phase and group velocity, and deciding if there is dispersion or not.