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'...

In 1964 Keldysh1 helped lay the foundations of strong-field physics by introducing a theoretical framework that characterized atomic ionization as a process that evolves with the intensity and wavelength of the fundamental field. Within this context, experiments 2 have examined the intensity-dependent ionization but, except for a few cases, technological limitations have confined the majority to wavelengths below 1 µm. The development of intense, ultrafast laser sources in the midinfrared (1 µm < l < 5 µm) region enables exploration of the wavelength scaling of the Keldysh picture while enabling new opportunities in strong-field physics, control of electronic motion and attosecond science. Here we report a systematic experimental investigation of the wavelength scaling in this region by concurrently analysing the production of energetic electrons and photons emitted by argon atoms interacting with few-cycle, mid-infrared fields. The results support the implicit predictions contained in Keldysh's work, and pave the way to the realization of brighter and shorter attosecond pulsed light sources using longer-wavelength driving fields. Keldysh 1 described the two main effects an intense lowfrequency laser field has on an atom as (1) a bending of the Coulomb potential by the field, forming a sufficiently narrow barrier for the electron to tunnel into the continuum, and (2) an oscillating motion of the free electron induced by the field of strength E and frequency ω. The cycle-averaged kinetic energy of the oscillating electron (ponderomotive energy) is given in atomic units as U p = E 2 /4ω 2 . The limit of validity of the Keldysh approach is defined by the condition that the adiabaticity parameter γ = √

We present a theoretical study of H þ 2 ionization under strong IR femtosecond pulses by using a method designed to extract correlated (2D) photoelectron and proton kinetic energy spectra. The results show two distinct ionization mechanisms-tunnel and multiphoton ionization-in which electrons and nuclei do not share the energy from the field in the same way. Electrons produced in multiphoton ionization share part of their energy with the nuclei, an effect that shows up in the 2D spectra in the form of energy-conservation fringes similar to those observed in weak-field ionization of diatomic molecules. In contrast, tunneling electrons lead to fringes whose position does not depend on the proton kinetic energy. At high intensity, the two processes coexist and the 2D plots show a very rich behavior, suggesting that the correlation between electron and nuclear dynamics in strong field ionization is more complex than one would have anticipated. DOI: 10.1103/PhysRevLett.110.113001 PACS numbers: 33.20.Xx, 33.60.+q, 33.80.Rv The interaction of atoms and molecules with intense infrared laser pulses has been the object of continuous research for more than two decades [1][2][3][4][5][6][7][8][9]. Since the potential induced by such lasers on the electrons is comparable to or even stronger than that generated by the nuclei, the resulting electron dynamics is significantly different from that of the isolated system, which makes these lasers ideal tools to achieve electronic control [10][11][12][13]. Strong fields can efficiently excite and ionize atoms and molecules. The electrons, which can be ejected following either multiphoton absorption or tunneling, can either directly reach the detector after having been repeatedly accelerated and decelerated by the field [direct electrons (DE)] or recollide with the ionic core within an optical cycle [rescattered electrons (RE)] [14,15]. Only a small fraction of the ejected electrons rescatter, but this fraction is responsible for important nonlinear phenomena such as high-harmonic generation (HHG). In this process, high-energy photons are emitted as a result of electron recombination with the ionic core. HHG is currently used to produce ultrashort extreme ultraviolet laser pulses and trains of these pulses [16][17][18][19], and also to uncover multielectron dynamics in atoms and molecules [13,20] or the structure of atomic and molecular orbitals in the so-called orbital tomography [10,21,22].Rescattered electrons that do not recombine with the ion also leave their signature in the photoelectron spectra at relatively high energies, typically between 2U p and 10U p [23,24], where U p ¼ I=4! 2 is the electron ponderomotive energy (in a.u.), I is the laser intensity, and ! its frequency. Because of their high energy, in contrast with that of direct electrons which is 2U p , RE can be used as signal and DE as reference to image atomic and molecular structure by photoelectron holography [20,25].Compared to atoms, the study of strong-field electron dynamics in molecules, in particular ...