Recent advances in attosecond science have relied upon the nearly instantaneous response of free electrons to an external field. However, it is still not clear whether bound electrons are able to rearrange on sublaser cycle time scales. Here, we probe the optical Stark shifts induced by a few-cycle near infrared laser field in helium bound states using isolated attosecond pulses in a transient absorption scheme and uncover a subcycle laser-induced energy level shift of the laser-dressed 1s3p state.

Spherical shock-ignition experiments on OMEGA used a novel beam configuration that separates low-intensity compression beams and high-intensity spike beams. Significant improvements in the performance of plastic-shell, D 2 implosions were observed with repointed beams. The analysis of the coupling of the high-intensity spike beam energy into the imploding capsule indicates that absorbed hot-electron energy contributes to the coupling. The backscattering of laser energy was measured to reach up to 36% at single-beam intensities of $8 Â 10 15 W/cm 2. Hard x-ray measurements revealed a relatively low hot-electron temperature of $30 keV independent of intensity and timing. At the highest intensity, stimulated Brillouin scattering occurs near and above the quarter-critical density and the two-plasmon-decay instability is suppressed. V

Spin-orbit coupling of a multiply charged ion is investigated in an intense laser field with a nonnegligible magnetic field component. The Lorentz force induces an enhanced angular motion of the bound electron, especially in the vicinity of the nucleus and, consequently, an orbital angular momentum and spin-orbit coupling significantly larger than without the presence of the intense laser field. This gives rise to clear deviations in the electronic wave packet motion and to a strongly increased splitting of resonant lines in the corresponding radiation spectrum. PACS numbers: 31.30.Jv, 32.80.Rm, 42.50.Hz High power laser systems have become available in recent years in an intensity regime where the interaction with electrons and atoms has entered the relativistic regime [1]. Free electrons in such intense laser fields were predicted long ago to propagate with velocities near the speed of light c in a plane spanned by the laser polarization and propagation directions [2]. Recently, experimentalists were able to observe the transition from Thompson to Compton [3] and nonlinear Thompson [4] scattering of such fast laser accelerated electrons. In extremely powerful laser fields the generation of electro-positron pairs [5] and neutrons [6] was observed and, most recently, laser induced nuclear fusion for [7] up to the forefront of presently achievable intensities [8]. In the high harmonic radiation spectrum, which has meanwhile entered the soft x-ray regime beyond the water window [9], Doppler shifts of the harmonics were predicted due to the relativistic mass shift [10]. Similar effects in the radiation spectrum were shown to arise from the magnetic field component of an intense laser field, in particular if modified by an additional external magnetic field [11]. The magnetic field of the laser field, even if oscillatory, may be larger than any static magnetic field prepared in a laboratory up to date. Its influence on the spin degree of freedom of the laser driven bound electron was investigated numerically via the Pauli [12] and Dirac equation [13] with clear evidence of spin flipping due to the laser field, however no substantial influences on the electron motion and radiation were put forward. Small quantitative deviations due to the spin were predicted in an analytic approximate treatment of the scattering of a laser driven electron at a nucleus [14].In this Letter, we investigate numerically the effect of the spin degree of freedom on bound electron dynamics and radiation in an intense laser field. Under the irradiation of the laser pulse, the bound electron obtains a large velocity in its polarization direction and then due to the magnetic field and the Lorentz push a partially angular motion with considerable orbital angular momentum L with respect to the origin set by the nucleus. We show that the resulting enhanced spin-orbit coupling gives rise to observable effects in the electron dynamics and radiation. In particular, we note a significant splitting of the nonsymmetric bound states due to this addi...