I have been studying Rydberg-Stark State Atoms and their Stark Manifolds (like the one on Wikipedia: http://en.wikipedia.org/wiki/File:Hfspec1.jpg) and I was wondering, Why does the y-axis (of Energy of Rydberg Atom) only show Negative Energies? I thought it was impossible for something to have a negative energy? - How is this possible?
Thanks in advanced for any help!
BTW, I understand the concept of a Stark Manifold (which shows the Stark Shifting of Ryderg atoms in Different Electric Field Strengths), and the concept of High and Low-Field-Seeking States of these atoms.