We have developed a synchrotron-based, time-resolved x-ray microprobe to investigate optical strong-field processes at intermediate intensities (10(14) - 10(15) W/cm2). This quantum-state specific probe has enabled the direct observation of orbital alignment in the residual ion produced by strong-field ionization of krypton atoms via resonant, polarized x-ray absorption. We found strong alignment to persist for a period long compared to the spin-orbit coupling time scale (6.2 fs). The observed degree of alignment can be explained by models that incorporate spin-orbit coupling. The methodology is applicable to a wide range of problems.

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We review the status of and comment on current developments in the field of two-photon decay in atomic physics research. Recent work has focused on twophoton decays in highly-charged ions and two-photon decay of inner-shell vacancies in heavy neutral atoms. We emphasize the importance of measuring the shape of the continuum emission in two-photon decay as a probe of relativistic effects in the strong central fields found in heavy atomic systems. New experimental approaches and their consequences will be discussed.

Detection of parity-violating asymmetries in 1s 1/2 2p 1/2 3 P 0 Ϫ1s 2 1 S 0 two-photon transitions in high-Z Helike ions is discussed. The circular polarization asymmetry of one of the two photons emitted in the two-photon decay of the 2 3 P 0 level in He-like uranium is of order 3ϫ10 Ϫ4 . Parity-violating asymmetries in the laserdriven processes of singly stimulated two-photon emission and anti-Stokes Raman scattering are also discussed. Further experimental and theoretical research is needed to determine if parity experiments based on these processes are feasible. ͓S1050-2947͑96͒07111-9͔

The Lyman- alpha transitions of hydrogenlike uranium associated with electron capture were measured in collisions of stored bare U (92+) ions with gaseous targets at the storage ring ESR. By applying the deceleration technique, the experiment was performed at slow collision energies in order to reduce the uncertainties associated with Doppler corrections. From the measured centroid energies, a ground state Lamb shift of 468 eV+/-13 eV is deduced which gives the most precise test of quantum electrodynamics for a single electron system in the strong field regime. In particular, the technique applied paves the way towards the 1 eV precision regime.