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Homework Help: Selection rules electrostatic field

  1. Mar 8, 2012 #1
    Hi all , I need some help with this problem,
    1. The problem statement, all variables and given/known data

    A hydrogen atom, which is in its ground state |1 0 0 > , is put into a weak time-dependent external electric field, which points into the z direction:
    [tex]\boldsymbol{E}(t,\boldsymbol{r}) = \frac{C\hat{\text{e}}_{z}}{t^{2}+\tau ^{2}}[/tex], where C and [tex]\tau > 0[/tex] and e are constants. This gives rise to a perturbation potential [tex]V(t) = C\frac{e\hat{z}}{t^{2}+\tau^{2}}[/tex].
    Using lowest-order time-dependent perturbation theory, find the selection rules for transitions from the ground state, i.e. find out which final state values for the quantum numbers n, l and m are possible in transitions from the ground state.

    2. Relevant equations

    [tex]P_{fi}(t,t_{0})\equiv |\langle\phi_{f}|\psi(t)\rangle|^{2}\approx \frac{1}{\hbar^{2}}\left|\int_{t_{0}}^{t}\text{d}t _{1}\langle\phi_{f}|V_{S}(t_{1})|\phi_{i}\rangle \text{e}^{\text{i}(E_{f}-E_{i})t_{1}/\hbar}\right|^{2}.[/tex]

    3. The attempt at a solution

    First, I have to see the values of m for which the sandwich vanish i.e. [tex]< n l m| \hat{z} | 1 0 0 > =0 [/tex]

    [tex]< n l m| \hat{z} | 1 0 0 > = I (radial ) χ \int Y^*_{l,m} Cos [\theta]d \Omega Y_{0,0} [/tex]

    The radial part is always non zero,

    Therefore , i have to compute
    [tex]\int d \Omega Y^*_{l,m} \cos (\theta) Y_{0,0} = [/tex] But I don't know what I can use, to compute the terms \cos (\theta) X spherical harmonics
    thanks for your help!
  2. jcsd
  3. Mar 9, 2012 #2


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    Staff Emeritus
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    Hint: Look up Y1,0.
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