We calculate the emission times of the radiation in high-order harmonic generation using the Gabor transform of numerical data obtained from solving the time-dependent Schrödinger equation in one, two, and three dimensions. Both atomic and molecular systems, including nuclear motion, are investigated. Lewenstein model calculations are used to gauge the performance of the Gabor method. The resulting emission times are compared against the classical simple man's model as well as against the more accurate quantum orbit model based on complex trajectories. The influence of the range of the binding potential (long or short) on the level of agreement is assessed. Our analysis reveals that the short-trajectory harmonics are emitted slightly earlier than predicted by the quantum orbit model. This partially explains recent experimental observations for atoms and molecules. Furthermore, we observe a distinct signature of two-center interference in the emission times for H 2 and D 2 .

(2013)], we investigate the origin of this effect theoretically. We show that it is not possible to describe photoelectron circular dichroism with the commonly used strong-field approximation due to its plane-wave nature. We therefore apply the Born approximation to the scattering state and use this as a continuum-state correction in the strong-field approximation. We obtain electron distributions for the molecules camphor and fenchone. In order to gain physical insight into the process, we study the contributions of individual molecular orientations.

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By solving the time-dependent Schrödinger equation for atoms in short laser pulses of different polarizations, it is shown that in strong-field ionization without rescattering, the lateral width of the electron-momentum distribution corresponds adiabatically to the instantaneous laser field on a sub-laser-cycle time scale, as expected in pure tunneling ionization. In contrast to the distributions along the polarization direction, the width is affected little by depletion or Coulomb effects. Over the past three decades, rapid progress has been made in the development of lasers capable of producing strong pulses in the femtosecond regime. This had led to progress in many areas ranging from above-threshold ionization (ATI) [1] over highorder harmonic generation [2] and attosecond pulse generation [3] to laser-induced fragmentation of molecules [4,5], to name just a few. Many of the phenomena induced by strong fields have been explained on the basis of tunneling ionization. Tunneling is a purely quantum mechanical process, which is considered as one of the main differences from classical mechanics. Frequently, a semiclassical two-step model is invoked, which assumes that ionization leads to the birth of an unbound electron at the outer exit of the laser-induced tunneling barrier. After appearing in the classically allowed region, the electron motion is modeled by a Newtonian trajectory [6]. This has been used for many purposes, e.g., to derive cutoff laws in ATI [7,8], to describe Coulomb focusing [9], to correct the strong-field approximation for elliptically polarized fields [10], and to model the momentum distribution in angular streaking [11]. The semiclassical model is also the basis for the three-step model of high-order harmonic generation [12].For an understanding of strong-field processes it is essential to verify the validity of the tunneling and semiclassical pictures. Along these lines, attosecond angular streaking [13][14][15], originally proposed to measure the carrier-envelope phase of few-cycle pulses [16], has been utilized to put a small upper limit on the tunneling delay time between the maximum of the electric field and the appearance of the escaping electron [17], thus giving insight into strong-field ionization on extremely short time scales. The interpretation of the momentum spectra in angular streaking, however, is complicated by Coulomb effects on the electron trajectories after tunneling and furthermore it is restricted to the intensity range below saturation [11]. It is desirable to overcome these limitations of angular streaking.Recently, the importance of the lateral momentum distribution, i.e., the distribution in the direction perpendicular to the laser field, has been recognized [18][19][20]. If not modified by electron recollision, the lateral distribution carries direct information about the ionization step since there is no laserinduced force in this direction. The tunneling ionization rate decreases with increasing lateral momentum approximately as a Gaussian. The width of the Ga...

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