What Causes a Stationary State to Decay in Quantum Mechanics?

[MiGUi]

I've been searching the answer for the called spontaneous de-excitation or free decay.

We solve Time Independent Scrödinger's Equation for particles cause we know that stationary states evolves with a well defined frequency determined by de Broglie-Einstein's relations, etc.

And when we are learning this bussiness for atoms, someone stands 'if an electron of an excited level decays to another level with low energy, it emit an energy which is exactly the Bohr frequency' and so and so...

If the atom (or the system) is perturbed with a time-dependent harmonic perturbation, the electron can access levels with an energy equal to $$\hbar \omega$$ but my question is: why a electron decide to decay? Moreover, why a stationary state, which is supposed to be a stationary state decay?

I'm an spanish undergraduate student of 4th year, so don't have fear to use your best QM's weapons

Thanks in advance,
MiGUi

 

[vanesch]

The simple answer is that your stationary state is not really stationary, but is only stationary in an approximation where the interaction with the EM radiation field has not been taken into account.
As such, the small coupling between the EM field (the photons) and the atom will result in the decay of the pseudo-stationary state.

 

[MiGUi]

What EM field? Electrostatic interaction between nucleus and electrons?

MiGUi

 

[vanesch]

What EM field? Electrostatic interaction between nucleus and electrons?

No, that part is taken into account. The radiation field. Photons.