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quasar987

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## Homework Statement

We consider an hydrogen atom in its fundamental state. At t=0, we apply an electric field in the z direction,

[tex]\mathcal{E}(t)=\mathcal{E}_0e^{-t/\tau}[/tex]

What is the probability that the atom be in the state 2p at [tex]t>>\tau[/tex]?

## The Attempt at a Solution

I thought time dependant perturbation theory failed for large time.... So what is going on here?