J. S. Robinson

Probing Proton Dynamics in Molecules on an Attosecond Time Scale

We demonstrate a technique that uses high-order harmonic generation in molecules to probe nuclear dynamics and structural rearrangement on a subfemtosecond time scale. The chirped nature of the electron wavepacket produced by laser ionization in a strong field gives rise to a similar chirp in the photons emitted upon electron-ion recombination. Use of this chirp in the emitted light allows information about nuclear dynamics to be gained with 100-attosecond temporal resolution, from excitation by an 8-femtosecond pulse, in a single laser shot. Measurements on molecular hydrogen and deuterium agreed well with calculations of ultrafast nuclear dynamics in the H2+ molecule, confirming the validity of the method. We then measured harmonic spectra from CH4 and CD4 to demonstrate a few-femtosecond time scale for the onset of proton rearrangement in methane upon ionization.

Dynamic imaging of molecules using high order harmonic generation.

We review recent progress towards imaging the electronic wavefunctions and nuclear dynamics of small molecules using the high order harmonics emitted when a molecule experiences an intense laser field. We illustrate that the essence of high harmonic emission is contained in the recombination amplitude between the continuum portion of the electronic wavefunction, that is formed through field ionization and which is accelerated and driven back to recollide in the laser field, and the bound electronic state. We review for the non-specialist some recent experimental and theoretical work dealing with high harmonic generation (HHG) in molecules. Particular attention is paid to two types of experiment recently performed in our group. The first of these types of experiment is the measurement of signatures of molecular electronic structure using HHG from molecules with a fixed orientation in space. The second is the use of HHG to track extremely fast proton rearrangement following ionization in light molecules, using the intrinsic temporal variation of the recolliding electron energy to extract these dynamics from measurements of the high harmonics.

High resolution imaging of colliding blast waves in cluster media

Strong shocks and blast wave collisions are commonly observed features in astrophysical objects such as nebulae and supernova remnants. Numerical simulations often underpin our understanding of these complex systems, however modelling of such extreme phenomena remains challenging, particularly so for the case of radiative or colliding shocks. This highlights the need for well-characterized laboratory experiments both to guide physical insight and to provide robust data for code benchmarking. Creating a sufficiently high-energy-density gas medium for conducting scaled laboratory astrophysics experiments has historically been problematic, but the unique ability of atomic cluster gases to efficiently couple to intense pulses of laser light now enables table top scale (1 J input energy) studies to be conducted at gas densities of >1019 particles cm−3 with an initial energy density >5 × 109 J g−1. By laser heating atomic cluster gas media we can launch strong (up to Mach 55) shocks in a range of geometries, with and without radiative precursors. These systems have been probed with a range of optical and interferometric diagnostics in order to retrieve electron density profiles and blast wave trajectories. Colliding cylindrical shock systems have also been studied, however the strongly asymmetric density profiles and radial and longitudinal mass flow that result demand a more complex diagnostic technique based on tomographic phase reconstruction. We have used the 3D magnetoresistive hydrocode GORGON to model these systems and to highlight interesting features such as the formation of a Mach stem for further study.

Probing fast nuclear wavepackets in light molecules: monitoring structural rearrangement on an attosecond timescale

A new technique for probing the structural rearrangements of molecules following ionization has recently been demonstrated. Whilst straightforward to implement, this technique offers remarkable time resolution, allowing intramolecular nuclear dynamics to be followed with a precision of 100 as. Here we discuss this technique, termed PACER (probing attosecond dynamics by chirp encoded recollision), examine the processes that affect the measurement, and compare the technique to alternative methods for probing vibrational wavepackets in light molecules.

Characterising spatio-temporal coupling of extreme ultraviolet ultrashort pulses from high harmonic generation

This work has demonstrated a simple method to measure spatio-temporal coupling in XUV high harmonic generation via the use of spatial shearing interferometry. The method is based on a well established technique used in the visible domain which is robust to noise and can also be measured in a single shot that such a method can prove valuable in further understanding of propagation and phase-matching effects in high harmonic generation, as well as providing a tool for performing reference based time-resolved spectroscopy.

Probing Orbital Structure of Polyatomic Molecules by High-order Harmonic Generation

Laser driven high-order harmonic generation (HHG) from molecules depends on the particular symmetry of the highest occupied molecular orbital (HOMO) and its orientation with respect to the laser field. High-order harmonic emission is observed in acetylene and allene molecules with 14 fs laser pulses. The molecules are aligned non-adiabatically producing a time dependent modulation of the harmonic signal as the ensemble undergoes the subsequent alignment revivals. At the points of maximum alignment the harmonic signal is measured as a function of the alignment angle showing a behavior that can be related to the structure of the highest occupied molecular orbitals.

Characterizing spatio-temporal coupling of extreme ultraviolet ultrashort pulses from high harmonic generation

We demonstrate a tool for performing measurements of space-time coupling of ultrashort, extreme ultraviolet pulses from high harmonic generation which can be used to study propagation and phasematching effects during the generation process.

Probing proton dynamics in molecules on an attosecond timescale

A technique for probing ultrafast (attosecond) structural rearrangement in molecules following laser ionization is discussed. The temporal window accessible has recently been extended beyond that previously reported by employing a driving field in the mid-IR.

Probing Attosecond Dynamics by Laser Driven Electron Recollisions

We briefly review recent advances in measurement with ∼100 attosecond temporal resolution using the sub‐cycle electron dynamics inherent to high order harmonic generation. In particular a technique for utilising the electron wave packet chirp is illustrated by the measurement of ultrafast proton dynamics in the H2 molecule.