Evidence For Resonant Effects In High-order Ati Spectra

We have observed resonant phenomena in the intense-field ionization of argon that do not easily fit into the standard models of above-threshold ionization. High-resolution angle- resolved measurements of the energy region around 6-10 times the ponderomotive potential UP in the ATI electron spectrum show at least three separate series of peaks, each with a distinct intensity threshold for their onset. These series do not shift in energy and have characteristic values of intensity for their appearance, pointing to an intensity-selective resonant enhancement effect. In order to further investigate this phenomenon, we have observed VUV fluorescence photons from the focal region following ionization. Current theories for the creation of high-energy electrons do not seem to explain these phenomena without significant modifications. We discuss possible physical mechanisms and implications for extending the standard model. Above-threshold ionization (ATI) in noble gases produces highly structured electron spectra, consisting of one or more series of regularly spaced narrow energy peaks. Most low-energy (less than 10 eV) features of these spectra have been successfully explained by considering the dynamics of a single-active-electron (SAE) wavepacket driven by the laser field in the presence of a static ion potential (1-3). When the ionization probability per optical cycle is not too high, the spectrum is dominated by narrow peaks due to excited states which AC Stark shift into multiphoton resonance with the laser. For higher intensities, most electrons are emitted below an energy of twice the ponderomotive energy 2UP , which is the maximum energy a free classical electron can extract from the laser field if it starts from rest (4). UP is the classical wiggle energy for a free electron in an oscillating field ( UPD.e 2 F 2 /=.4me! 2 /), which amounts to about 3.5 eV in our experiment at 6 10 13 Wc m 2 . Recent experiments (5-8) have discovered an additional region of enhanced ionization in the high-energy part of the electron spectrum above 20 eV, separated from the low-order peaks by a gap of several eV. The energies of electrons in the enhanced region significantly exceed 2UP . In this energy region, the electron spectrum often shows a non-monotonic decrease or 'plateau' (3, 5, 7-10). One explanation for this enhancement within the SAE framework involves rescattering of the photoelectrons that are driven back by the laser field to re-encounter their parent ion (11-13). This process can produce electrons with the observed energies (up to 11UP ). In this, and other models that neglect the (Coulomb) electron-ion interaction up to the very moment of collision, virtually all rescattering happens