So I've been studying forbidden transitions and metastable states in atoms, etc., and I was just wondering if there is such a thing as forbidden bound-free transitions? I've only encountered bound-bound. For example, if you have a neutral Helium atom with two electrons, I understand how forbidden transitions arise within that ion. But are there certain energy levels for which the transition to the ionised state is forbidden? Thanks for any information! I tried googling but wasn't able to figure it out.