Resonant above-threshold ionization at quantized laser intensities

We argue that quantum electrodynamics dictates resonance phenomena in multiphoton processes as the laser intensity varies. A perturbation theory is developed in which the coupling between an electron and the second quantized laser mode is treated nonperturbatively. As an example, we predict that the above-threshold ionization rate can exhibit resonance at intensities with integer ponderomotive parameter. Such quantum effects may be exploited to calibrate laser intensities.