Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Atomic dipole moment for a superposition state

  1. Aug 11, 2015 #1
    Hi all,

    I'm trying to understand how to calculate the time dependent expectation value of the atomic dipole moment for a superposition state, and I have a good guess to check with you. Say we have
    [tex]\psi = \frac{1}{\sqrt{2}} \left[ \psi _{100} + \psi _{310} \right][/tex]
    at t = 0. Then, for t > 0:
    [tex]\Psi (t) = \frac{1}{\sqrt{2}} \left[ \psi _{100} e^{-iE_1 t/\hbar} + \psi _{310} e^{-iE_3 t/\hbar} \right] = \frac{1}{\sqrt{2}} \left[ \psi _{100} e^{-i\omega_1 t} + \psi _{310} e^{-i\omega_3 t} \right] [/tex]
    Given that the time dependent expectation value of the atomic dipole moment is defined as
    [tex]\langle \vec{d}(t) \rangle = -e \langle \vec{r} (t) \rangle[/tex]
    I proceed as follows:
    [tex]\langle \vec{d}(t) \rangle = -e \langle \Psi (t) | \mbox{ } \vec{r} \mbox{ } | \Psi (t) \rangle[/tex]
    [tex]\langle \vec{d}(t) \rangle = -e \langle \left( \frac{1}{\sqrt{2}} \psi _{100} e^{-i\omega_1 t} + \frac{1}{\sqrt{2}} \psi _{310} e^{-i\omega_3 t} \right) | \mbox{ } \vec{r} \mbox{ } | \left( \frac{1}{\sqrt{2}} \psi _{100} e^{-i\omega_1 t} + \frac{1}{\sqrt{2}} \psi _{310} e^{-i\omega_3 t} \right) \rangle[/tex]
    [tex]\langle \vec{d}(t) \rangle = -e \langle \left( \frac{1}{\sqrt{2}} \psi _{100} e^{-i\omega_1 t} \right) | \mbox{ } \vec{r} \mbox{ } | \left( \frac{1}{\sqrt{2}} \psi _{310} e^{-i\omega_3 t} \right) \rangle -e \langle \left( \frac{1}{\sqrt{2}} \psi _{310} e^{-i\omega_3 t} \right) | \mbox{ } \vec{r} \mbox{ } | \left( \frac{1}{\sqrt{2}} \psi _{100} e^{-i\omega_1 t} \right) \rangle[/tex]
    [tex]\langle \vec{d}(t) \rangle = -e \left( \frac{1}{\sqrt{2}} e^{+i\omega_1 t} \right) \langle \psi _{100} | \mbox{ } \vec{r} \mbox{ } | \psi _{310} \rangle \left( \frac{1}{\sqrt{2}} e^{-i\omega_3 t} \right) -e \left( \frac{1}{\sqrt{2}} e^{+i\omega_3 t} \right) \langle \psi _{310} | \mbox{ } \vec{r} \mbox{ } | \psi _{100} \rangle \left( \frac{1}{\sqrt{2}} e^{-i\omega_1 t} \right)[/tex]
    [tex]\langle \vec{d}(t) \rangle = -e \left( \frac{1}{\sqrt{2}} e^{+i\omega_1 t} \right) \langle 100 | \mbox{ } \vec{r} \mbox{ } | 310 \rangle \left( \frac{1}{\sqrt{2}} e^{-i\omega_3 t} \right) -e \left( \frac{1}{\sqrt{2}} e^{+i\omega_3 t} \right) \langle 310 | \mbox{ } \vec{r} \mbox{ } | 100 \rangle \left( \frac{1}{\sqrt{2}} e^{-i\omega_1 t} \right)[/tex]
    [tex]\langle \vec{d}(t) \rangle = -e \left( \frac{1}{\sqrt{2}} e^{+i\omega_1 t} \right) \langle 10 | \mbox{ } r \mbox{ } | 31 \rangle \langle 00 | \mbox{ } \hat{r} \mbox{ } | 10 \rangle \left( \frac{1}{\sqrt{2}} e^{-i\omega_3 t} \right) -e \left( \frac{1}{\sqrt{2}} e^{+i\omega_3 t} \right) \langle 31 | \mbox{ } r \mbox{ } | 10 \rangle \langle 10 | \mbox{ } \vec{r} \mbox{ } | 00 \rangle \left( \frac{1}{\sqrt{2}} e^{-i\omega_1 t} \right)[/tex]
    [tex]\langle \vec{d}(t) \rangle = -e \left( \frac{1}{\sqrt{2}} e^{+i\omega_1 t} \right) (0.517 a_0) \left( 0, 0, \frac{1}{\sqrt{3}}\right) \left( \frac{1}{\sqrt{2}} e^{-i\omega_3 t} \right) -e \left( \frac{1}{\sqrt{2}} e^{+i\omega_3 t} \right) (0.517 a_0) \left( 0, 0, \frac{1}{\sqrt{3}}\right) \left( \frac{1}{\sqrt{2}} e^{-i\omega_1 t} \right)[/tex]
    [tex]\langle \vec{d}(t) \rangle = -e \frac{1}{2} e^{+i\omega_1 t} e^{-i\omega_3 t} (0.517 a_0) \left( 0, 0, \frac{1}{\sqrt{3}}\right) -e \frac{1}{2} e^{+i\omega_3 t} e^{-i\omega_1 t} (0.517 a_0) \left( 0, 0, \frac{1}{\sqrt{3}}\right)[/tex]
    [tex]\langle \vec{d}(t) \rangle = \left[ -e \frac{1}{2} (0.517 a_0) \left( 0, 0, \frac{1}{\sqrt{3}}\right) \right] \left( e^{+i\omega_1 t} e^{-i\omega_3 t} + e^{+i\omega_3 t} e^{-i\omega_1 t}\right)[/tex]
    [tex]\langle \vec{d}(t) \rangle = \left[ -e \frac{1}{2} (0.517 a_0) \left( 0, 0, \frac{1}{\sqrt{3}}\right) \right] \left( e^{+i(\omega_1 - \omega_3) t} + e^{-i(\omega_1 - \omega_3) t} \right)[/tex]
    [tex]\langle \vec{d}(t) \rangle = \left[ -e \frac{1}{2} (0.517 a_0) \left( 0, 0, \frac{1}{\sqrt{3}}\right) \right] 2\cos \left[(\omega_1 - \omega_3) t \right][/tex]
    [tex]\fbox{$\displaystyle\langle \vec{d}(t) \rangle = -e (0.517 a_0) \left( 0, 0, \frac{1}{\sqrt{3}}\right) \cos \left[(\omega_1 - \omega_3) t \right]$}[/tex]

    Is this correct? :smile: Thanks!
     
  2. jcsd
  3. Aug 11, 2015 #2

    Orodruin

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    I did not check your math but the approach is reasonable.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook