Multiphoton ionization in circularly polarized standing waves

Strong-field Kapitza-Dirac scattering of neutral atoms

Laser induced strong-field phenomena in atoms and molecules on the femtosecond (fs) time scale have been almost exclusively investigated with traveling wave fields. In almost all cases, approximation of the strong electromagnetic field by an electric field purely oscillating in time suffices to describe experimental observations. Spatially dependent electromagnetic fields, as they occur in a standing light wave, allow for strong energy and momentum transfer and are expected to extend strong-field dynamics profoundly. Here we report a strong-field version of the Kapitza-Dirac effect for neutral atoms where we scatter neutral He atoms in an intense short pulse standing light wave with fs duration and intensities well in the strong-field tunneling regime. We observe substantial longitudinal momentum transfer concomitant with an unprecedented atomic photon scattering rate greater than 10(16)s(-1).

Spectral minimum and giant enhancement in photoelectron spectra from xenon atoms driven by intense midinfrared laser fields

Our theoretical study shows that the spectral minimum and the giant enhancement structures observed in the high harmonic spectra also exist in the photoelectron spectra from driven Xe atoms. They are attributed to the inherent property of the radial part of the wave function of the Xe 5p subshell in momentum space. The spectral minimum is caused by the nodal point in the modulus of the radial wave function in momentum space, and the giant enhancement reflects the increase in magnitude of the modulus of the wave function. To observe these structures, midinfrared lasers of about 0.2 PW/cm(2) intensity are preferred. Employing circularly polarized laser light is suggested for exhibiting these structures in photoelectron spectra.

Half Kapitza-Dirac effect of H2+ molecule in intense standing-wave laser fields

The half Kapitza-Dirac effect of H2+ molecule in an intense standing-wave laser field is studied with a focus on the influence of the molecular orbital symmetry and the molecular alignment on the photo-electron angular distributions (PADs). In standing-wave laser fields, the PADs split along the scattering angle due to the momentum change of electron with photons when it escapes from the laser fields. The structures and the symmetry of PADs are severely affected by the molecular orbital symmetry and the molecular alignment. For H2+ molecule in ground state (σg), the PADs are severely changed by the molecular alignment only when the photoelectron kinetic energy is sufficiently high. For H2+ molecule in the first excited state (σμ), the molecular alignment distinctively changes the PADs, irrelevant to the kinetic energy of photoelectrons. When the molecules are aligned either parallel with or perpendicular to the laser polarization, the PADs are symmetric about an axis. In other cases, the PADs do not show any symmetry. These results indicate that the molecular alignment can be used to control the splitting in the half Kapitza-Dirac effect.

Photoelectron angular distributions from above threshold ionization of hydrogen atoms in strong laser fields

We apply a scattering theory of nonperturbative quantum electrodynamics to study the photoelectron angular distributions (PADs) of a hydrogen atom irradiated by linearly polarized laser light. The calculated PADs show main lobes and jetlike structure. Previous experimental studies reveal that in a set of above-threshold-ionization peaks when the absorbed-photon number increases by one, the jet number also increases by one. Our study confirms this experimental observation. Our calculations further predict that in some cases three more jets may appear with just one-more-photon absorption. With consideration of laser-frequency change, one less jet may also appear with one-more-photon absorption. The jetlike structure of PADs is due to the maxima of generalized phased Bessel functions, not an indication of the quantum number of photoelectron angular momentum states.

Theory of the Kapitza-Dirac diffraction effect

We treat the Kapitza-Dirac diffraction effect observed recently by Batelaan et al. using a newly developed nonperturbative quantum-field scattering theory. Our theory shows that an electron beam passing perpendicularly through a focused standing light wave can produce diffraction patterns. Our theory predicts (1) the minimum value of the ponderomotive energy is (Planck's over 2 pi omega)(2)/m(e)c(2), (2) the critical laser intensity above which the first pair of electron diffraction peaks will occur, and (3) the existence of sidebands in the electron spectra separated far from the central band by a momentum of several hundred photons. Our theory provides a unified explanation of the experimental results of Bucksbaum et al. and Batelaan et al.