# Time scale of photon absorption in electron

1. Jun 7, 2008

### I_wonder

Does anyone know, when an electron in an atom is exited by absorbing a photon, how long does it take for the electron to absorb the entire energy of the photon? Does the time scale depend on the energy level?

Thanks!

2. Jun 7, 2008

### f95toli

It is not that simple, mainly because the only thing we can calculate is the probability to find the electron in a given state. If you drive a two level system (e.g. an atom where the energy of the photons are chosen so that only one transition is active) using e.g. a laser what you find is that this probability changes a rate proportional to the amplitude of the drive field and the dipole moment of the transition(meaning yes, it does depend on the levels); this rate is just the inverse of the Rabi frequency.

see e.g.
http://physics.schooltool.nl/quantumoptics/rabi_jaynesmodel.php [Broken]

Note that the main difference between the "semi-classical" Rabi model and the QED version (using a J-C Hamiltonian) is just the presence of the vacuum Rabi oscillations in the latter; i.e. the semi-classical model with a classical drive field is actually quite good.

I know that this is not what you asked for, but the point is that Rabi rate is the only time-scale we can actually talk about or indeed observe in experiments.

Last edited by a moderator: May 3, 2017