The Keldysh theory of above-threshold ionization (ATI) is applied to few-cycle laser pulses in order to explore the potential of a recently published new method to measure "carrier-envelope phase difference" phenomena. In this experiment, the carrier-envelope phase difference dependent left-right asymmetry of few-cycle ATI was measured and investigated with a correlation technique. Here, we explore spectral features of the asymmetry, present a theoretical analysis of the experiment, and establish a method to determine the duration of few-cycle pulses whose carrier-envelope phase differences are not controlled.

Laser-assisted electron-ion recombination is investigated with an emphasis on the spectrum of the emitted high-energy photons and its modification due to the recollision of the incident electron and the ion. Numerical results for the soft-x-ray power spectra, added up over all intermediate laser photon channels, are presented as a function of the incident electron energy for different laser field intensities. For strong laser fields, maxima, and additional structures are found in these spectra for incident electron energies of the order of magnitude of the ponderomotive energy. We show that the laser-assisted electron-ion recombination, that includes the rescattering of the electron at the ion before the recombination, is a process complementary to the well-known processes of high-order harmonic generation and high-order above-threshold ionization. All these processes can be explained, using the three-step scenario. A semiclassical analysis is presented which shows that for the laser-assisted electron-ion recombination real solutions of the saddle-point equations exist, contrary to what is found with high-order harmonic generation and high-order above-threshold ionization when only complex solutions are permitted. For low incoming electron energies, the cutoff of the emitted soft-x-ray photon energies, including the process of rescattering, is higher than in the case of the direct recombination process. The height of the rescattering plateau is 6\char21{}7 orders of magnitude lower than that of the direct process. However, for higher incident electron energies we obtain the unexpected result that the difference between the height of the rescattering plateau and the height of the direct plateau can be less than one order of magnitude.

High-order harmonic emission is investigated by numerical solution of the weakly relativistic, two-dimensional Schr?dinger equation for the case of ultra-intense laser-driven tightly bound systems (for example, multiply charged ions such as O7+ exposed to laser fields of the order of 1018?W?cm-2 at 248?nm). In contrast to their usual substantial decrease, the low-order harmonics having an energy less than the ionization potential exhibit a high-efficiency (i.e.?intense) plateau with a well defined cutoff. The shape of this plateau is found to depend on the shape of the binding potential. A classical `surfing' mechanism for the generation of these harmonics is proposed that does not involve tunnelling and that nevertheless explains the observed cutoff. Thus we call them `nontunnelling harmonics'. The significance of relativistic effects for these harmonics is investigated and found to be small, despite the high laser intensity, because of the absence of tunnelling.

A recently proposed quantum theory of relativistic ultrahigh-order harmonic generation [Milosevic, D.B., Hu, S.X., and Becker, W., 2001, Phys. Rev. A, 63, 011403(R)] is analyzed in more detail. Several variants of this theory are presented. Our numerical results show the appearance of a multiplateau structure in the harmonic spectrum in the weakly relativistic regime. For stronger fields, the interference pattern familiar from the nonrelativistic case disappears and the height of the plateau decreases towards high harmonic orders. The cutoff of the harmonic spectrum is slightly lower compared to the nonrelativistic case. These features are confirmed for stronger fields and explained in terms of complex relativistic quantum orbits.

The homogeneity of typical electrolessly plated Pd / Ag membranes for H 2 separation was investigated with XAFS and XPS depth profiling. Next the crucial mixing of the separate Pd andAg layers, obtained after the two sequential plating steps, was followed in situ with XRD.The results show that some low temperature alloying procedures found in the field are insufficient for obtaining a well-mixed Pd / Ag alloy membrane. Finally the Pd / Ag-H XANES was determined for the first time. In contrast to the Pd-H and Pd / Ag systems, shifts in electron density from the Ag or H atoms to the Pd could not satisfactorily explain the observed XANES. Not only is the intensity of the whiteline higher than expected, the shape and the intensity of the anti-bonding Pd 4d H 1s feature have also significantly changed. Local geometry is expected to account for these deviations.

Atoms interacting with intense laser fields can emit electrons and photons of very high energies. An intuitive and quantitative explanation of these highly nonlinear processes can be found in terms of a generalization of classical Newtonian particle trajectories, the so-called quantum orbits. Very few quantum orbits are necessary to reproduce the experimental results. These orbits are clearly identified, thus opening the way for an efficient control as well as previously unknown applications of these processes.

The generation of circularly polarized high-order harmonics is considered by a bichromatic laser field whose two components with frequencies ω and 2ω are circularly polarized in the same plane, but rotate in opposite directions. Both exact and approximate (obtained by using the saddle-point method) results for the harmonic spectra are presented. The effect of the ratio of the intensities of the two components on the harmonic spectrum is investigated. For the case where the high-frequency field has twice the intensity of the low-frequency field, the possibility of generating a train of attosecond pulses is pointed out that consists of circularly polarized harmonics all having the same helicity.

Theoretical results for high-harmonic generation by a two-color elliptically polarized laser field are presented. Special emphasis is placed on coherent radiation near 13 nm in connection with the development of extreme-ultraviolet lithography. Linearly polarized radiation at this wavelength can be obtained with a linearly polarized bichromatic laser field, whereas circularly polarized radiation can be generated with a bichromatic circularly polarized field with counterrotating coplanar components. In both cases the harmonic emission efficiencies at the saturation intensity are more than 1 order of magnitude larger than for harmonic generation with a monochromatic linearly polarized field.

A quasi-classical three-step model for high-harmonic generation is presented. The results for the harmonic emission rate obtained using this model agree very well with the exact results. For an elliptically polarized laser field a staircase of plateaux is observed. Both the heights and the cut-off positions of these plateaux are explained. An analytical formula for the cut-off law is derived and applied to determine the cut-off of the harmonic spectrum for each of the observed plateaux. We also present the electron orbits, obtained as solutions of the classical equation of motion with the complex time.

We analyze the use of strong static electric and magnetic fields for controlling two intense laser-atom processes: laser-assisted x-ray-atom scattering and high-order harmonic generation. We find that x-rays scattered from atoms in the presence of both a strong laser field and a strong static electric field can be boosted in energy many-fold by absorption of energy from the laser field. The spectrum of scattered x-ray intensity vs. the number of laser photons absorbed exhibits a rich plateau structure, whose key features may be understood using a classical analysis. We find also that the intensities of high-order harmonics can be increased by orders of magnitude in the presence of strong static magnetic or parallel electric and magnetic fields and also that the static electric field can introduce additional plateaus and cutoffs. The maximum values of the harmonic intensity correspond to values of magnetic field for which the return time of the ionized electron wave packet to the atomic core under the infl...