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The recent advances concerning high-order harmonic generation processes in strong laser fields are discussed. We show how the width of the plateau inherent to the production of high harmonics varies with the pulse width, the laser wavelength and the atomic medium. Phase matching effects are discussed in terms of coherence length owing to the laser focusing. Finally, we show that the idea of genera
We apply the random-phase approximation to two-photon ionization. We develop a formalism which, by means of screened electron-photon interactions, includes the nonlinear response and, in particular, double-excitation processes. The theory is illustrated by numerical calculations of the two-photon ionization cross section of the external (5p and 5s) shell of xenon, over a large energy range: from t
We present experimental results of harmonic generation at 1064 nm using a 36-ps neodymium-doped yttrium aluminum garnet laser tightly focused in a gas jet of xenon. The harmonic yields are studied as a function of the position of the focus in the jet at several laser intensities and as a function of intensity at several focus positions. These results bring information about how the high harmonics
We review the main experimental results obtained in the multielectron multiphoton ionization of rare gases. Multiple ionization is interpretated as a sequential process. We study the influence of many‐electron effects in multiphoton ionization within the framework of the random phase approximation. This is applied to a calculation of the two‐photon ionization cross section of xenon. Finally, the r
Multielectron ionization of Xe is described in terms of multistep processes, driven by a laser field which is screened by the motion of the outer 5p shell. In stepwise multiple ionization of the 5p shell, screening is successively reduced. The effective, local intensity will therefore increase during the stripping of the outer shell. In the 4d inner-shell region the effective intensity is very low
We study the influence of many-electron effects in multiphoton ionization within the framework of diagrammatic many-body perturbation theory. We renormalize the electron-dipole coupling by summing to infinite order both many-electron interactions using the random-phase approximation and higher-order intensity terms. We introduce an effective intensity, which takes into account the screening of the
We apply multichannel-quantum-defect theory (MQDT) to multiphoton ionization of rare gases in lowest-order perturbation theory. We determine MQDT parameters of J=1,3 odd-parity states, J=0,2 even-parity states in xenon and krypton by fitting experimental energy levels and constructing Lu-Fano plots. We present calculations of two- and three-photon ionization of xenon and krypton in the autoionizat
The production of molecular and atomic ions has been measured for CO, N2 and O2 with 1064 and 532 nm 40 ps pulses in the 1012-1014 W cm-2 intensity range. A simultaneous ionization-dissociation process occurs at lower intensities, while a sequential process appears in oxygen at higher intensities.
We present experimental measurements of harmonic generation in xenon using a 1064-nm 40-psec Nd-doped yttrium aluminum garnet laser. The harmonic yield is studied as a function of the position of the laser focus in the atomic beam. It shows regular oscillations whose period decreases with increasing harmonic order and which can be interpreted as phase-matching effects due to the tight focusing geo
We present a completely ab initio calculation of harmonic generation in xenon exposed to a strong laser field. The time-dependent Schrödinger equation for the atomic response and the propagation equation are numerically integrated yielding excellent agreement with experiment. The weaker variation with pump intensity of the induced dipole in the high-field regime leads to an enormous enhancement in
We present an approach to describe the phase matching of high harmonics emitted by laser driven atoms in a nonperturbative regime, for which the atomic response displays an intrinsic intensity-dependent phase. We show that the traditional phase-matching conditions involving conservation of wave vectors should be modified by taking into account the gradient of this atomic phase. We investigate vari
Singly, doubly, triply, and quadruply charged krypton ions are formed by multiphoton absorption processes in krypton atoms. They are induced by a 50-psec laser pulse at 1.064 m in the 1013-1014-W-cm-2 intensity range. The percentage ratio between the numbers of doubly and singly ionized Kr atoms is 10% at 8×1013 W cm-2. It is shown that Kr2+ ions result from a direct 33-photon absorption process.
Multiply charged ions are easily formed in rare gases by multiphoton absorption processes. For Kr and Xe up to quadruply charged ions are formed. They are induced by a bandwidth-limited 50 ps laser pulse at 1.064 W cm -2 intensity range. Doubly charged ions are formed through the absorption of a very large number of photons (29 for Xe) in a direct transition from the ground state of the atom to th
We present a theoretical interpretation of the experimental results on three-photon-resonant four-photon ionization of Kr reported by Landen, Perry, and Campbell [Phys. Rev. Lett. 59, 2558 (1987)] and Perry and Landen [Phys. Rev. A 38, 2815 (1988)]. Our calculations are based on multichannel quantum-defect theory combined with a density-matrix formalism describing the spatiotemporal development of
Xe**2** plus ions can easily be formed by multiphoton absorption in Xe atoms at 0. 53 mu m. A kinetic model is developed which is based on the rate equations describing the evolution of populations of Xe atoms, Xe** plus and Xe**2** plus ions. The result is that the ratio of the number of Xe**2** plus to Xe** plus ions varies with the pulse duration tau as tau ** minus **3**/**2.
Multiphoton ionisation of He at 532 nm is investigated at about 1013 W cm-2 by measuring both electron energy distribution, and the laser intensity dependence of the production of He+ and He2+ ions. He2+ ions solely come from multiphoton ionisation of He+ ions.
A retarding-potential method has been used to analyse the energy of electrons produced in the process of generating singly and doubly charged ions in Xe and Ne at 532 and 1064 nm. Energetic electrons observed in Xe at 1064 nm imply that up to 30 photons have been absorbed in the ionisation process, while only 11 photons are necessary to release one electron. The laser intensity dependence of energ
We present results of experiments testing the influence of elliptical polarization on the production of high-order harmonics. Experiments were conducted both with a 600-nm, 1-psec dye laser and with an 825-nm, 140-fsec Cr:LiSAF6 (Cr:LiSAF) laser system, over a wide range of intensities and target gases (xenon, argon, and neon), using a detection system with a dynamical range of more than three ord
