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...

Over the past thirty years, extensive studies of strong-field photoionization of atoms have revealed both quantum and classical aspects including above-threshold ionization 1 , electron wave-packet drift, quiver and rescattering motions. Increasingly sophisticated spectroscopic techniques 2 and sculpted laser pulses 3 coupled with theoretical advances have led to a seemingly complete picture of this fundamental laser-atom interaction. Here, we describe an effect that seems to have escaped observation: the photoelectron energy distribution manifests an unexpected characteristic spike-like structure at low energy, which becomes prominent using mid-infrared laser wavelengths (λ > 1.0 µm). The low-energy structure is observed in all atoms and molecules investigated and thus seems to be universal. The structure is qualitatively reproduced by numerical solutions of the time-dependent Schrödinger equation but its physical origin is not yet identified.Atomic photoionization under intense laser irradiation is considered a well-understood process. In the low-intensity/shortwavelength limit, it is described quantum mechanically as multiphoton absorption: above-threshold ionization (ATI) is the absorption of photons beyond the minimum required for ionization. In the high-intensity/long-wavelength limit, the photoelectron energy distribution can be understood classically according to the Simpleman theory 4 , as the drift kinetic energy of an electron as a function of the phase at which it was released in the laser cycle. Inclusion of the d.c.-tunnelling rate to describe the ionization probability completes a semi-classical theory. A corresponding quantum approach is provided by the Keldysh-Faisal-Reiss [5][6][7] (KFR) strong-field approximation. The KFR theory incorporates the effect of the external field on the continuum state but neglects the influence of the core potential and ignores the atom's excited states. Keldysh linked these two limits in terms of a single dimensionless parameter γ = √ (IP/2U p ), where IP is the ionization potential and U p is the cycle-averaged kinetic energy of an electron quivering in the field. In the limit defined by γ < 1, the electric field of the wave can be considered quasi-static and the total ionization rate approximated by d.c.-tunnelling in accordance with the semi-classical picture. The photoelectron distribution in this case has a classical cutoff energy at 2U p . A more elusive feature, discovered in the mid-nineties 8,9 , is the plateau in the photoelectron distribution extending to 10U p . Understanding the origin of this plateau requires a straightforward extension of the Simpleman theory that enables a returning electron to elastically scatter off the core. As illustrated in the inset of Fig. 1, the photoelectron spectrum exhibits both of these features, 'direct' and 'rescattered'. The amplitude of the plateau can be described by the scattering cross-section and the spread of the electron wave packet 10 . More detailed features associated with rescattering, such as the plateau'...

A number of unexpected features of small molecules subjected to intense laser fields, with wavelengths ranging from infrared to ultraviolet, have been observed or predicted in the past few years: above-threshold dissociation, molecular bond softening, vibrational population trapping. We review these processes for the case of the molecular ion H2+ and discuss the experimental and theoretical tools that are used to study this system. Both electron and proton energy distributions are used to interpret the experimental results. Theoretically, the fragmentation dynamics can be described equivalently as a laser-assisted half-collision process, using solutions of the time-independent Floquet theory, or as the evolution of a wavepacket subjected to a classical radiation held with a given pulse shape, using solutions of the time-dependent Schrodinger equation. A broad range of laser intensity and pulsewidth has been explored, with the short-pulse results (analysed in terms of 'dressed' potential curves) offering the best interface between theory and experiment. We finally report on a promising new avenue for coherent control of fragmentation dynamics, through the use of two-colour phase-locked radiation.

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