# Hydrogen excitation, ionisation and photon absorption

1. ### Raezeman

8
I need help with something that i seemed to remember was fairly easy, but just can't seem to find the simple formula for:

I have an amount of hydrogen atoms at a temperature of 10000 Kelvin. What percentage of them are ionised and what percentage of them are in the n=2 excited state.

Secondly, where could i find the absorption cross section of a hydrogen atom in the n=2 state for the wavelength 656.28 nm? (aka Halpha line)

### Staff: Mentor

That will depend on the density.

See "Boltzmann factor".

The NIST database could have that, not sure.