Talk:Phy5646

Question: A monoatomic atom undergo spontaneous emission. It changes from an excited state $|e\rangle$ with $l=0$ to an intermediate state $|i\rangle$ with $l=1$ emitting a photon with wave vector $\mathbf {k}$, and then to the ground state $|g\rangle$ with $l=0$ emitting a photon with wave vector $\mathbf {k'}$. Find the probability that the angle between the two wavevectors to be $\theta$.

Ans:

${\mathcal {H}}={\frac {1}{2m}}\left(p-{\frac {e}{c}}A(r)\right)^{2}+V(r)+\sum _{k,{\hat {\lambda _{k}}}}\hbar \omega _{k}\left({\hat {a}}_{k{\hat {\lambda _{k}}}}^{\dagger }{\hat {a}}_{k{\hat {\lambda _{k}}}}+{\frac {1}{2}}\right)$

where $\mathbf {{\hat {A}}(r)} ={\frac {1}{\sqrt {V}}}\sum _{k,\lambda _{k}}\left[{\sqrt {\frac {2\pi \hbar }{\omega _{k}}}}c\;\left({\hat {a}}_{k,{\hat {\lambda _{k}}}}{\hat {\lambda _{k}}}e^{ik\cdot r}+{\hat {a}}_{k,{\hat {\lambda _{k}}}}^{\dagger }{\hat {\lambda _{k}^{*}}}e^{-ik\cdot r}\right)\right]$

Using second order time dependent perturbation theory , we can write the wavefunction in Dirac picture as,

$|\chi (t)\rangle \approx |\chi _{0}\rangle +{\frac {1}{i\hbar }}\int _{-\infty }^{t}dt'{\mathcal {H}}'_{I}(t')|\chi _{0}\rangle +{\frac {1}{(i\hbar )^{2}}}\int _{-\infty }^{t}dt'\int _{-\infty }^{t'}dt''{\mathcal {H}}'_{I}(t'){\mathcal {H}}'_{I}(t'')|\chi _{0}\rangle$

where

${\mathcal {H}}'_{I}(t)=e^{{\frac {i}{\hbar }}{\mathcal {H}}_{0}^{(at)}t}\left(-{\frac {e}{mc}}A(r,t)\cdot p+{\frac {e^{2}}{2mc^{2}}}A(r,t)\cdot A(r,t)\right)e^{-{\frac {i}{\hbar }}{\mathcal {H}}_{0}^{(at)}t}$

with

$\mathbf {A(r,t)} ={\frac {1}{\sqrt {V}}}\sum _{k,\lambda _{k}}\left[{\sqrt {\frac {2\pi \hbar }{\omega _{k}}}}c\;\left({\hat {a}}_{k,{\hat {\lambda _{k}}}}{\hat {\lambda _{k}}}e^{ik\cdot r-i\omega _{k}t}+{\hat {a}}_{k,{\hat {\lambda _{k}}}}^{\dagger }{\hat {\lambda _{k}^{*}}}e^{-ik\cdot r+i\omega _{k}t}\right)\right]$

Since the system has rotational symmetry, so the internal eigenstate is $|n,l,m\rangle$, where $|n\rangle =|e\rangle ,|i\rangle ,|g\rangle$. The initial photon field is null $|\phi \rangle$. Initially $l=0$, so $m=0$, so intial state is $|\chi _{0}\rangle =|e,0,0;\phi \rangle$ and final state is $|g,0,0;1_{\lambda _{k},k},1_{\lambda '_{k'},k'}\rangle$. Therefore

$\langle g,0,0;1_{\lambda _{k},k},1_{\lambda '_{k'},k'}|\chi (t)\rangle \approx {\frac {1}{i\hbar }}\int _{-\infty }^{t}dt'\langle g,0,0;1_{\lambda _{k},k},1_{\lambda '_{k'},k'}|{\mathcal {H}}'_{I}(t')|e,0,0;\phi \rangle +{\frac {1}{(i\hbar )^{2}}}\int _{-\infty }^{t}dt'\int _{-\infty }^{t'}dt''\langle g,0,0;1_{\lambda _{k},k},1_{\lambda '_{k'},k'}|{\mathcal {H}}'_{I}(t'){\mathcal {H}}'_{I}(t'')|e,0,0;\phi \rangle$

where to change from state $|e\rangle$ to $|i\rangle$ to $|g\rangle$, we need two momentum operator, so the first term must be zero. And so

${\begin{aligned}&\langle g,0,0;1_{\lambda _{k},k},1_{\lambda '_{k'}}|\chi (t)\rangle \\&\approx {\frac {1}{(i\hbar )^{2}}}\int _{-\infty }^{t}dt'\int _{-\infty }^{t'}dt''\langle g,0,0;1_{\lambda _{k},k},1_{\lambda '_{k'},k'}|{\mathcal {H}}'_{I}(t'){\mathcal {H}}'_{I}(t'')|e,0,0;\phi \rangle \\&={\frac {1}{(i\hbar )^{2}}}\int _{-\infty }^{t}dt'\int _{-\infty }^{t'}dt''\sum _{m=-1,0,1}\langle g,0,0;1_{\lambda _{k},k},1_{\lambda '_{k'},k'}|{\mathcal {H}}'_{I}(t')|i,1,m;1_{\lambda ,k}\rangle \langle i,1,m;1_{\lambda ,k}|{\mathcal {H}}'_{I}(t'')|e,0,0;\phi \rangle \\&=\sum _{m=-1,0,1}f(t,w_{k},w_{k}')\langle g,0,0|p|i,1,m\rangle \cdot {\hat {\lambda _{k'}^{'*}}}\langle i,1,m|p|e,0,0\rangle \cdot {\hat {\lambda _{k}^{*}}}\end{aligned}}$