Photoelectron elliptical dichroism spectroscopy of resonance-enhanced multiphoton ionization via the 3s, 3p and 3d Rydberg series in fenchone.

The resonance-enhanced multiphoton ionization of chiral molecules by elliptically polarized laser pulses produces photoelectron angular distributions that are forward/backward asymmetric with respect to the light propagation axis. We investigate this photoelectron elliptical dichroism in the (2 + 1)-photon ionization of fenchone molecules, using wavelength tunable femtosecond UV pulses. We show that the photoelectron elliptical asymmetry is extremely sensitive to the intermediate resonant states involved in the ionization process, and enables electronic couplings to be revealed that do not show up so clearly when using circularly polarized light.

High-power sub-15 fs nonlinear pulse compression at 515 nm of an ultrafast Yb-doped fiber amplifier.

We present an efficient and robust scheme to produce energetic sub-15 fs pulses centered at 515 nm with a peak power exceeding 3 GW. Combining efficient second-harmonic generation of a 135 fs, 50 W Yb-doped fiber amplifier with a low-loss capillary-based visible pulse compression stage, we reach an overall efficiency higher than >20%. The system is also designed to take advantage of the repetition rate flexibility of the fiber amplifier, leading sub-15 fs pulse generation from 166 to 500 kHz with an average power exceeding the 10 watt level. The combined reduction of the laser wavelength and pulse duration is expected to highly improve the yield of high-order harmonic generation to provide high photon flux of ultrashort extreme ultraviolet radiation.

Attosecond impulsive stimulated X-ray Raman scattering in liquid water.

We report the measurement of impulsive stimulated x-ray Raman scattering in neutral liquid water. An attosecond pulse drives the excitations of an electronic wavepacket in water molecules. The process comprises two steps: a transition to core-excited states near the oxygen atoms accompanied by transition to valence-excited states. Thus, the wavepacket is impulsively created at a specific atomic site within a few hundred attoseconds through a nonlinear interaction between the water and the x-ray pulse. We observe this nonlinear signature in an intensity-dependent Stokes Raman sideband at 526 eV. Our measurements are supported by our state-of-the-art calculations based on the polarization response of water dimers in bulk solvation and propagation of attosecond x-ray pulses at liquid density.