We present a generalization of the S-matrix theory of above-threshold
ionization (ATI), which includes both the Coulomb and rescattering eff
ects. The Coulomb effects are responsible for a considerable increase
of the ionization rates, while the rescattering effects give rise to t
he appearance of the second plateau of the ATI spectrum. Coulomb effec
ts change the behavior of the rates at the first plateau, but they do
not influence the qualitative behavior of the spectrum of the high-ene
rgy electrons in the second plateau. Our results are in excellent agre
ement with recent experiments. We show that the sidelobe positions cor
respond to the last rounded top before the cutoff of the ATI spectrum
for the corresponding angle. Our model is simple, and can be easily ge
neralized to different inert gases. [S1050-2947(98)08310-3].