Evolution of atomic ionization into the strong-field limit offers the opportunity to study the fundamentals of atom-laser interaction. In this study, we report on high precision measurements of the ion and electron distributions from laser-excited helium and neon atoms which reflect the changing continuum dynamics as the ionization process evolves into the pure tunneling regime. The experiments present evidence of both singleand two-electron ionization. These data provide a direct quantitative test of various theories of strong-field ionization. We show that a relatively simple semiclassical model which includes a description of a field-driven electron elastically rescattering from an accurate ion core potential reproduces the measured electron distributions for both atoms. However, using this model to calculate e-2e inelastic rescattering yields cross sections which are incompatible with the measured two-electron ionization. ͓S1050-2947͑98͒06111-3͔

We describe calculations that predict and analyze distinctive features in the velocity distribution in 1D laser cooling for light shift potential well depths only a few times the recoil energy. These features can be interpreted in terms of populations of energy bands or even Bloch states in the periodic potential. They occur with a cr+ standing wave, with and without a small B field, and for lin 3 lin laser cooling. %'e have observed these features experimentally in a beam of metastable helium atoms cooled on the 2 8& 4-+ 2 Pp transition, with velocity resolution 0.3 recoil.PACS numbers: 32.80.Pj, 42.5G.VkRecent progress in laser cooling has produced atoms with mechanical energies less than the light shifts induced by the laser light [1,2]. This means atoms may be confined in the wells of the periodic light shift potential. At such low energies, the atomic de Broglie wavelength is comparable to the optical wavelength. Motion in these wells is thus quantized: The eigenstates of atomic motion show a band structure analogous to that of electrons moving in the periodic potential of a crystal. Since a theory of this band structure in laser cooling was developed [3], there has been a keen interest in studying its effects experimentally.Recently, the frequency intervals between energy bands have been investigated with a probe laser using four-wave mixing [4], through sidebands in the fiuorescence [5], and through rf spectroscopy [6]. Up to now, however, there have been no observations of quantum effects in the velocity distribution, which is arguably closer

The behavior of atoms in strong fields has been the subject of many investigations, both experimental and theoretical. Previously, the majority of experimental studies have been confined to the multi-photon ionization or mixed regimes while few experiments have been done in the tunneling regime[1,2]. The data we will present is the first to examine the photoionization process over the intensity range from multiphoton ionization to well within the tunneling regime. We have taken photoelectron energy spectra, ion yield curves, and angular distributions of helium and neon over a wide range of intensities using a titanium sapphire laser operating at 1 kHz, 780 nm, and a pulse width of ~120 fs, focused into an UHV chamber with f/4 optics, allowing maximum intensities of 20 PW/cm2. The large dynamic range allowed by the kilohertz repetition rate makes it possible for a quantitative comparison between experiment and theory. Helium and neon were chosen for the study since previous investigations have established that ionization occurs in the tunneling regime [3]. Our investigations show that while the rescattering model [4,5] describes the one electron dynamics quantitatively, the extension of the rescattering model to inelastic rescattering as a mechanism for creating higher charge states fails to provide even a qualitative description.

No abstract