Subject: What is the reference point when measuring ionization potentials of atoms

Take Hydrogen for example. It has an ionization potential of 13.6ev. If an 
electron or photon with that much kinetic energy strikes the electron, it will 
be knocked out the potential well. But what about an electric field. At what 
point is the 13.6ev referenced to? In terms of volts/meter. Is the 13.6ev 
referenced to the Bohr radius for each orbital? Certainly 13.6ev/meter won't 
pull the electron off. At what distance from the electron you want to remove is 
the voltage measured? Does it take an e-field of 2.72 x 10(11) v/m to ionize 
hydrogen, which is 27.2 ev/Angstrom?

Re: What is the reference point when measuring ionization potentials of atoms