Need a formula for Ionization HELP!!

[heliumhigh]

What is the greatest wavelength of radiation that will ionize an unexcited Hydrogen atom?

If someone could give me a fomula to work with I'd be EVER so greatful! I've been working on this for HOURS.

[Tide]

The longest wavelength corresponds to the lowest photon energy which would have to be at least equal to the ground state energy of a hydrogen atom. It's possible (I haven't checked) that longer wavelengths could accomplish this as a multiphoton process using intermediate excited states but those states usually have VERY short lifetimes.