We present high sensitivity electron energy spectra for xenon in a strong 50 ps, 1.053 pm laser field. The above threshold ionization distribution is smoothly decreasing over the entire kinetic energy range (0 -30 eV), with no abrupt changes in the slope. This is in direct contrast to the sharp cutofI observed in xenon optical harmonic generation spectra. Calculations using the single active electron approximation show excellent agreement with the observed electron distributions.These results directly address the unresolved relationship between the electron and photon emission from an atom in an intense field.PACS numbers: 31.90.+s, 32.80.Fb, 32.80.Rm Both high-order optical harmonic generation (OHG) and above threshold ionization (ATI) occur when a bound electron absorbs many more photons from a strong laser field than the minimum number necessary for weakfield ionization.An electron that has absorbed many photons, and is possibly in the continuum, can emit one shorter wavelength photon and make an optical transition back to a lower (usually the ground) bound state (OHG), or it may ionize and emerge from the laser focus with some excess kinetic energy (ATI). Theoretical rnodels [1, 2] have emphasized that both ATI and OHG are essentially single-atom phenomena which have their origin in the response of a single, strongly driven electron to an oscillating electric field. Therefore, one might reasonably predict that electron and photon spectra will have many similar features [2 -4]. Although this issue is fundamental to the understanding of strong-field laser-atom interactions, the exact relationship between ATI and OHG has remained largely an unanswered question. This is in part due to the absence of any experiments on ATI distributions over a large dynamic range which correlate with OHG experiments. The purpose of this Letter is to address this relationship using new experimental evidence from high sensitivity electron energy measurements.OHG spectra have a distinctive shape: a rapid decrease for the low-order harmonics consistent with perturbation theory, followed by a "plateau" region where the harmonic intensity drops more slowly, and then an abrupt cutoff, beyond which no harmonics are observed [5]. Because of the inversion symmetry of an atom in a linearly polarized field, only odd harmonics are produced. A simple formula predicting the harmonic cutoff was recently proposed [6] and has been verified experimentally [7]. Similarly, ATI electron spectra [8,9] show a series of peaks separated by one photon energy. In this Letter we report ATI spectra for xenon atoms using a 50 ps, 1.053 pm laser at intensities of a few times 10rs W/cm2, for which comparable OHG data [10] exist. We also present electron energy distributions from time-dependent calculations for these conditions. These same calculations were earlier found to give excellent agreement with OHG experiments [10] in xenon for the same intensity range considered here. The laser used operates at a kilohertz repetition rate, making it possible to measur...

We present calculated optical harmonic spectra for atoms and ions in the high intensity regime relevant to current short-pulse experiments. We find that ions can produce harmonics comparable in strength to those obtained from neutrals, and that the emission extends to much higher order. Simple scaling laws for the strength of the harmonic emission and the maximum observable harmonic are suggested. These results imply that the photoemission observed in recent experiments in helium and neon contains contributions from ions as well as neutrals.

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