Energy Conservation Law in Strong-Field Photoionization by Circularly Polarized Light

We establish an energy conservation law for tunneling electrons driven by a circularly polarized light in a rotationally invariant potential. This conservation law emerges from the adiabaticity of the ionization process when described in the frame rotating with the laser field. It offers a clear picture of nonlinear phenomena observed in strong fields for initial states carrying a magnetic quantum number. It provides unambiguous sets of trajectories to model and interpret cutting-edge experiments using semiclassical approaches.