Strong-field control of H3 + production from methanol dications: Selecting between local and extended formation mechanisms.

Using the CD3OH isotopologue of methanol, the ratio of D2H+ to D3 + formation is manipulated by changing the characteristics of the intense femtosecond laser pulse. Detection of D2H+ indicates a formation process involving two hydrogen atoms from the methyl side of the molecule and a proton from the hydroxyl side, while detection of D3 + indicates local formation involving only the methyl group. Both mechanisms are thought to involve a neutral D2 moiety. An adaptive control strategy that employs image-based feedback to guide the learning algorithm results in an enhancement of the D2H+/D3 + ratio by a factor of approximately two. The optimized pulses have secondary structures 110-210 fs after the main pulse and result in photofragments that have different kinetic energy release distributions than those produced from near transform limited pulses. Systematic changes to the linear chirp and higher order dispersion terms of the laser pulse are compared to the results obtained with the optimized pulse shapes.

A combined experimental and theoretical study on realizing and using laser controlled torsion of molecules.

It is demonstrated that strong laser pulses can introduce torsional motion in the axially chiral molecule 3,5-difluoro-3('),5(')-dibromobiphenyl. A nanosecond laser pulse spatially aligns the stereogenic carbon-carbon (C-C) bond axis allowing a perpendicularly polarized, intense femtosecond pulse to initiate torsional motion accompanied by a rotation about the fixed axis. We monitor the induced motion by femtosecond time-resolved Coulomb explosion imaging. Our theoretical analysis corroborates the experimental findings and on the basis of these results we discuss future applications of laser-induced torsion, viz., time-resolved studies of deracemization and laser controlled molecular junctions based on molecules with torsion.

Manipulating the torsion of molecules by strong laser pulses.

We demonstrate that strong laser pulses can induce torsional motion in a molecule consisting of a pair of phenyl rings. A nanosecond laser pulse spatially aligns the carbon-carbon bond axis, connecting the two phenyl rings, allowing a perpendicularly polarized, intense femtosecond pulse to initiate torsional motion accompanied by an overall rotation about the fixed axis. We monitor the induced motion by femtosecond time-resolved Coulomb explosion imaging. Our theoretical analysis accounts for and generalizes the experimental findings.

Clustered gases as a medium for efficient plasma waveguide generation.

Clustered gas jets are shown to be an efficient means for plasma waveguide generation, for both femtosecond and picosecond generation pulses. These waveguides enable significantly lower on-axis plasma density (less than 10(18) cm(-3)) than in conventional hydrodynamic plasma waveguides generated in unclustered gases. Using femtosecond pump pulses, self-guided propagation and strong absorption (more than 70%) are used to produce long centimetre scale channels in an argon cluster jet, and a subsequent intense pulse is coupled into the guide with 50% efficiency and guided at above 10(17)W cm(-2) intensity over 40 Rayleigh lengths. We also demonstrate efficient generation of waveguides using 100 ps axicon-generated Bessel-beam pump pulses. Despite the expected sub-picosecond cluster disassembly time, we observe long pulse absorption efficiencies up to a maximum of 35%. Simulations show that in the far leading edge of the long laser pulse, the volume of heated clusters evolves to a locally uniform and cool plasma already near ionization saturation, which is then efficiently heated by the remainder of the pulse.

Plasma waveguides efficiently generated by Bessel beams in elongated cluster gas jets.

We demonstrate the efficient generation of plasma waveguides in elongated cluster gas jets using 100 ps axicon-generated Bessel beam pump pulses. The plasma waveguide space and time evolution is measured using picosecond interferometry. Small radius waveguides with central densities as low as approximately 10(18) cm(-3) can be generated with this technique. Despite the expected subpicosecond cluster disassembly time, we observe long pulse absorption efficiencies that can be more than a factor of 10 greater than in unclustered gas targets of the same volume average atomic density. The maximum long pulse absorption observed in cluster jets under our range of conditions was 35%. The explanation for the enhanced absorption is that in the far-leading edge of the laser pulse, the volume of heated clusters evolves to a locally uniform and cool plasma already near ionization saturation, which is then heated by the remainder of the pulse. From this perspective, the use of clustered gases is equivalent to a supercharged preionization scheme for long duration laser pulses.

Guiding of intense laser pulses in plasma waveguides produced from efficient, femtosecond end-pumped heating of clustered gases.

We demonstrate intense pulse guiding in efficient femtosecond end-pumped waveguides generated in clustered gases. This novel scheme provides a route to significantly lower on-axis plasma density (< 10(18) cm(-3)) more than is feasible in conventional hydrodynamic plasma waveguides. Self-focused propagation and strong absorption of intense femtosecond laser pulses are used to produce long centimeter scale channels in an argon cluster jet, and a subsequent pulse is guided with 3 x 10(17) W cm(-2) intensity and approximately 50% coupling efficiency. Preliminary results with hydrogen clusters also show guiding.

Asymmetric high-energy ion emission from argon clusters in intense laser fields.

Clusters of 2x10(3) to 4x10(4) Ar atoms are Coulomb exploded in intense (up to 8x10(15) W cm(-2)) laser fields. The dependence of multiply charged argon ion energies on the polarization state of light is probed. A directional asymmetry in the ion-explosion energies is observed for the highest charge states. The ion-energy distribution consists of a low-energy isotropic component, and a high-energy anisotropic one. The results are discussed in terms of an asymmetric Coulomb-explosion scenario.