High-order above-threshold ionization in a laser field: Influence of the ionization potential on the high-energy cutoff

High-energy above-threshold ionization by a strong linearly polarized laser field is analyzed using the strong-field approximation and the quantum-orbit formalism. An analytical formula for the cutoff is derived. The maximal electron kinetic energy is 10.007U(P)+0.538I(P). The first term with the ponderomotive energy U-P is equivalent to the known classical cutoff law, while the second term is a new result and depends on the ionization potential I-P. The validity of this formula is confirmed by comparison with the numerical results for the high-energy electron spectra.