Verification of the dominant role of resonant enhancement in short-pulse multiphoton ionization

We report experimental results which imply that observed peaks in the short-pulse photoelectron spectrum of atoms in high fields result only from a resonant enhancement of the multiphonon ionization rate at the peak of the laser pulse, rather than a two-step process of real population transfer and subsequent single-photon ionization. These results consist of the first identification, in a high intensity (>${10}^{13}$ W/${\mathrm{cm}}^2$) short-pulse photoelectron spectrum, of low-lying states of argon with an ac Stark shift significantly different from the ponderomotive energy.