Direct Observation of a Roaming Intermediate and Its Dynamics.

Chemical reactions are often characterized by their transition state, which defines the critical geometry the molecule must pass through to move from reactants to products. Roaming provides an alternative picture, where in a dissociation reaction, the bond breaking is frustrated and a loosely bound intermediate is formed. Following bond breaking, the two partners are seen to roam around each other at distances of several Ångstroms, forming a loosely bound, and structurally ill-defined, intermediate that can subsequently lead to reactive or unreactive collisions. Here, we present a direct and time-resolved experimental measurement of roaming. By measuring the photoelectron spectrum of UV-excited acetaldehyde with a femtosecond extreme ultraviolet pulse, we captured spectral signatures of all of the key reactive structures, including that of the roaming intermediate. This provided a direct experimental measurement of the roaming process and allowed us to identify the time scales by which the roaming intermediate is formed and removed and the electronic potential surfaces upon which roaming proceeds.

A versatile high-average-power ultrafast infrared driver tailored for high-harmonic generation and vibrational spectroscopy.

We report on an ultrafast infrared optical parametric chirped-pulse amplifier (OPCPA), pumped by a 200-W thin-disk Yb-based regenerative amplifier at a repetition rate of 100 kHz. The OPCPA is tunable in the spectral range 1.4-3.9 [Formula: see text]m, generating up to 23 W of < 100-fs signal and 13 W of < 200-fs idler pulses for infrared spectroscopy, with additional spectral filtering capabilities for Raman spectroscopy. The OPCPA can also yield 19 W of 49-fs 1.75-[Formula: see text]m signal or 5 W of 62-fs 2.8-[Formula: see text]m idler pulses with active carrier-to-envelope-phase (CEP) stabilisation for high-harmonic generation (HHG). We illustrate the versatility of the laser design, catering to various experimental requirements for probing ultrafast science.

Ultrafast Triggering of Insulator-Metal Transition in Two-Dimensional VSe2.

The transition-metal dichalcogenide VSe2 exhibits an increased charge density wave transition temperature and an emerging insulating phase when thinned to a single layer. Here, we investigate the interplay of electronic and lattice degrees of freedom that underpin these phases in single-layer VSe2 using ultrafast pump-probe photoemission spectroscopy. In the insulating state, we observe a light-induced closure of the energy gap, which we disentangle from the ensuing hot carrier dynamics by fitting a model spectral function to the time-dependent photoemission intensity. This procedure leads to an estimated time scale of 480 fs for the closure of the gap, which suggests that the phase transition in single-layer VSe2 is driven by electron-lattice interactions rather than by Mott-like electronic effects. The ultrafast optical switching of these interactions in SL VSe2 demonstrates the potential for controlling phase transitions in 2D materials with light.

Photodissociation dynamics of methyl iodide probed using femtosecond extreme ultraviolet photoelectron spectroscopy.

Femtosecond pump-probe photoelectron spectroscopy measurements using an extreme ultraviolet probe have been made on the photodissociation dynamics of UV (269 nm) excited CH3I. The UV excitation leads to population of the 3Q0 state which rapidly dissociates. The dissociation is manifested as shifts in the measured photoelectron kinetic energy that map the extending C-I bond. The increased energy available in the XUV probe relative to a UV probe means the dynamics are followed over the chemically important region as far as C-I bond lengths of approximately 4 Å.

Quantitative and correlative extreme ultraviolet coherent imaging of mouse hippocampal neurons at high resolution.

Microscopy with extreme ultraviolet (EUV) light can provide many advantages over optical, hard x-ray or electron-based techniques. However, traditional EUV sources and optics have large disadvantages of scale and cost. Here, we demonstrate the use of a laboratory-scale, coherent EUV source to image biological samples-mouse hippocampal neurons-providing quantitative phase and amplitude transmission information with a lateral resolution of 80 nm and an axial sensitivity of ~1 nm. A comparison with fluorescence imaging of the same samples demonstrated EUV imaging was able to identify, without the need for staining or superresolution techniques, <100-nm-wide and <10-nm-thick structures not observable from the fluorescence images. Unlike hard x-ray microscopy, no damage is observed of the delicate neuron structure. The combination of previously demonstrated tomographic imaging techniques with the latest advances in laser technologies and coherent EUV sources has the potential for high-resolution element-specific imaging within biological structures in 3D.

Photodissociation dynamics of CH3I probed via multiphoton ionisation photoelectron spectroscopy.

The dissociation dynamics of CH3I is investigated on the red (269 nm) and blue (255 nm) side of the absorption maximum of the A-band. Using a multiphoton ionisation probe in a time-resolved photoelectron imaging experiment we observe very different dynamics at the two wavelengths, with significant differences in the measured lifetime and dynamic structure. The differences are explained in terms of changes in excitation cross-sections of the accessible 3Q0 and 1Q1 states and the subsequent dynamics upon each of them. The measurements support the existing literature on the rapid dissociation dynamics on the red side of the absorption maximum at 269 nm which is dominated by the dynamics along the 3Q0 state. At 255 nm we observe similar dynamics along the 3Q0 state but also a significant contribution from the 1Q1 state. The dynamics along the 1Q1 potential show a more complex structure in the photoelectron spectrum and a significantly increased lifetime, indicative of a more complex reaction pathway.

Multi-color carrier-envelope-phase stabilization for high-repetition-rate multi-pulse coherent synthesis.

Using a zero-offset carrier-envelope locking technique, we have synthesized an octave-spanning composite frequency comb exhibiting 132-attosecond timing jitter between the constituent pulses over a one-second observation window. In the frequency domain, this composite comb has a modal structure and coherence which are indistinguishable from those of a comb that might be produced by a hypothetical single mode locked oscillator of equivalent bandwidth. The associated phase stability enables the participating multi-color pulse sequences to be coherently combined, representing an example of multi-pulse synthesis using a femtosecond oscillator.

Lateral shearing interferometry of high-harmonic wavefronts.

We present a technique for frequency-resolved wavefront characterization of high harmonics based on lateral shearing interferometry. Tilted replicas of the driving laser pulse are produced by a Mach-Zehnder interferometer, producing separate focii in the target. The interference of the resulting harmonics on a flat-field extreme ultraviolet spectrometer yields the spatial phase derivative. A comprehensive set of spatial profiles, resolved by harmonic order, validate the technique and reveal the interplay of single-atom and macroscopic effects.

Accuracy measurements and improvement for complete characterization of optical pulses from nonlinear processes via multiple spectral-shearing interferometry.

We demonstrate that multiple spectral-shearing interferometry increases the precision and accuracy of measurements of the spectral phase of a complex pulse (time-bandwidth product = 125) arising from self-phase modulation in a gas filled capillary. We verify that the measured interferometric phase is accurate to 0.1 rad across the full bandwidth by checking the consistency between the spectral phases of each individual shear measurement. The accuracy of extracting pulse parameters (group delay dispersion, pulse duration and peak intensity) for single shear measurements were verified to better than 10% by comparison with the multishear reconstruction. High order space-time coupling is quantified across a single transverse dimension, verifying the suitability of such pulses for use in strong field experiments.

Sub-10 fs pulse characterization using spatially encoded arrangement for spectral phase interferometry for direct electric field reconstruction.

We demonstrate an extremely accurate method for measuring ultrabroadband, sub-10 fs pulses even if they exhibit a highly modulated spectrum, space-time coupling, or both. The method uses a spatially encoded arrangement for spectral phase interferometry for direct electric field reconstruction, which allows a zero additional phase measurement to be performed with a relatively low signal-to-noise ratio in real time and single shot.

Self-referencing, spectrally, or spatially encoded spectral interferometry for the complete characterization of attosecond electromagnetic pulses.

We propose a method for the complete characterization of attosecond duration electromagnetic pulses produced by high harmonic generation in an atomic gas. Our method is based on self-referencing spectral interferometry of two spectrally sheared extreme ultraviolet pulses, which is achieved by pumping the harmonic source with two sheared optical driving pulses. The resulting interferogram contains sufficient information to completely reconstruct the temporal behavior of the electric field. We demonstrate that such a method is feasible, and outline two possible experimental configurations.