Effect of THz-bandwidth incoherent laser radiation on bulk damage in potassium dihydrogen phosphate crystals.

The laser-damage performance characteristics of potassium dihydrogen phosphate (KDP) samples under exposure to a distinctive broadband incoherent laser pulse are investigated. A laser system providing such pulses is intended to explore improved energy-coupling efficiency on the target in direct-drive inertial confinement fusion experiments and provides incoherent bandwidths as large as 10 THz in a nanosecond pulse. A consequence of this bandwidth is very rapid fluctuations in intensity capable of reaching maxima much larger than the average intensity within the pulse. A custom damage-test station has been built to perform measurements with broadband incoherent pulses in order to determine what effect these fast and high-intensity oscillations have on laser damage. A set of experiments under different bandwidth and beam configurations shows the effect to be minimal when probing bulk damage in KDP. Modeling indicates this behavior is supported by long electron-relaxation times compared to the source-field fluctuations, following excitation of individual electrons in the conduction band. The results help better understand the laser-induced-damage mechanisms in KDP, and its ability to operate in broadband temporally incoherent high-energy lasers that may be particularly suitable for future laser-fusion energy systems.

Enhancement of strong-field multiple ionization in the vicinity of the conical intersection in 1,3-cyclohexadiene ring opening.

Nonradiative energy dissipation in electronically excited polyatomic molecules proceeds through conical intersections, loci of degeneracy between electronic states. We observe a marked enhancement of laser-induced double ionization in the vicinity of a conical intersection during a non-radiative transition. We measured double ionization by detecting the kinetic energy of ions released by laser-induced strong-field fragmentation during the ring-opening transition between 1,3-cyclohexadiene and 1,3,5-hexatriene. The enhancement of the double ionization correlates with the conical intersection between the HOMO and LUMO orbitals.

Transient X-ray fragmentation: probing a prototypical photoinduced ring opening.

We report the first study of UV-induced photoisomerization probed via core ionization by an x-ray laser. We investigated x-ray ionization and fragmentation of the cyclohexadiene-hexatriene system at 850 eV during the ring opening. We find that the ion-fragmentation patterns evolve over a picosecond, reflecting a change in the state of excitation and the molecular geometry: the average kinetic energy per ion fragment and H(+)-ion count increase as the ring opens and the molecule elongates. We discuss new opportunities for molecular photophysics created by optical pump x-ray probe experiments.

The laser shock station in the dynamic compression sector. I.

The Laser Shock Station in the Dynamic Compression Sector (DCS) [Advanced Photon Source (APS), Argonne National Laboratory] links a laser-driven shock compression platform with high energy x-ray pulses from the APS to achieve in situ, time-resolved x-ray measurements (diffraction and imaging) in materials subjected to well-characterized, high stress, short duration shock waves. This station and the other DCS experimental stations provide a unique and versatile facility to study condensed state phenomena subjected to shocks with a wide range of amplitudes (to above ∼350 GPa) and time-durations (∼10 ns-1 µs). The Laser Shock Station uses a 100 J, 5-17 ns, 351 nm frequency tripled Nd:glass laser with programmable pulse shaping and focal profile smoothing for maximum precision. The laser can operate once every 30 min. The interaction chamber has multiple diagnostic ports, a sample holder to expose 14 samples without breaking vacuum, can vary the angle between the x-ray and laser beams by 135°, and can translate to select one of the two types of x-ray beams. The x-ray measurement temporal resolution is ∼90 ps. The system is capable of reproducible, well-characterized experiments. In a series of 10 shots, the absolute variation in shock breakout times was less than 500 ps. The variation in peak particle velocity at the sample/window interface was 4.3%. This paper describes the entire DCS Laser Shock Station, including sample fabrication and diagnostics, as well as experimental results from shock compressed tantalum that demonstrate the facility's capability for acquiring high quality x-ray diffraction data.