Gigavolt bound-free transitions driven by extreme light

The photoelectron spectrum in the ultra-relativistic limit of tunneling ionization is strongly affected by wave-particle resonance and finite spot-size effects, in contradistinction with the usual assumptions of strong field physics. Near term laser facilities will access a regime where ionized electrons are abruptly accelerated in the laser propagation direction, such that they stay in phase with the laser fields through a substantial portion of the confocal region. The final momentum of the electron depends significantly on where in the confocal region it originated. By manipulating the target and collection geometry, it is possible to obtain low emittance, low energy spread, gigavolt photoelectrons. Radiation reaction effects play a negligible role in near term scenarios, but become interesting in the multi-exawatt regime. A significant advance in numerical particle tracking is introduced.