Vincent Wanie

Tabletop imaging of structural evolutions in chemical reactions demonstrated for the acetylene cation

The introduction of femto-chemistry has made it a primary goal to follow the nuclear and electronic evolution of a molecule in time and space as it undergoes a chemical reaction. Using Coulomb Explosion Imaging, we have shot the first high-resolution molecular movie of a to and fro isomerization process in the acetylene cation. So far, this kind of phenomenon could only be observed using vacuum ultraviolet light from a free-electron laser. Here we show that 266 nm ultrashort laser pulses are capable of initiating rich dynamics through multiphoton ionization. With our generally applicable tabletop approach that can be used for other small organic molecules, we have investigated two basic chemical reactions simultaneously: proton migration and C=C bond breaking, triggered by multiphoton ionization. The experimental results are in excellent agreement with the timescales and relaxation pathways predicted by new and quantitative ab initio trajectory simulations.

10 mJ 5-cycle pulses at 1.8 μm through optical parametric amplification

We report the generation of 10 mJ, 5-cycle pulses at 1.8 μm (30 fs) at 100 Hz repetition rate using an optical parametric amplifier pumped by a high energy Titanium-Sapphire laser system (total energy of 23 mJ for Signal and Idler). This is the highest reported peak power (0.33 TW) in the infrared spectral range. This high-energy long wavelength laser source is well suited for driving various nonlinear optical phenomena such as high harmonic generation for high flux ultrafast soft X-ray pulses.

0.42 TW 2-cycle pulses at 1.8 μm via hollow-core fiber compression

By employing pulse compression with a stretched hollow-core fiber, we generated 2-cycle pulses at 1.8 μm (12 fs) carrying 5 mJ of pulse energy at 100 Hz repetition rate. This energy scaling in the mid-infrared spectral range was achieved by lowering the intensity in a loose focusing condition, thus suppressing the ionization induced losses. The correspondingly large focus was coupled into a hollow-core fiber of 1 mm inner diameter, operated with a pressure gradient to further reduce detrimental nonlinear effects. The required amount of self-phase modulation for spectral broadening was obtained over 3 m of propagation distance.

Coherent control of D2/H2 dissociative ionization by a mid-infrared two-color laser field

Steering the electrons during an ultrafast photo-induced process in a molecule influences the chemical behavior of the system, opening the door to the control of photochemical reactions and photobiological processes. Electrons can be efficiently localized using a strong laser field with a well-designed temporal shape of the electric component. Consequently, many experiments have been performed with laser sources in the near-infrared region (800 nm) in the interest of studying and enhancing the electron localization. However, due to its limited accessibility, the mid-infrared (MIR) range has barely been investigated, although it allows to efficiently control small molecules and even more complex systems. To push further the manipulation of basic chemical mechanisms, we used a MIR two-color (1800 and 900 nm) laser field to ionize H2 and D2 molecules and to steer the remaining electron during the photo-induced dissociation. The study of this prototype reaction led to the simultaneous control of four fragmentation channels. The results are well reproduced by a theoretical model solving the time-dependent Schrodinger equation for the molecular ion, identifying the involved dissociation mechanisms. By varying the relative phase between the two colors, asymmetries (i.e., electron localization selectivity) of up to 65% were obtained, corresponding to enhanced or equivalent levels of control compared to previous experiments. Experimentally easier to implement, the use of a two-color laser field leads to a better electron localization than carrier-envelope phase stabilized pulses and applying the technique in the MIR range reveals more dissociation channels than at 800 nm.

Isotope Effect in the three Break-up Channels of the Acetylene Cation

The dynamics and the isotope effect on the symmetric (CH++CH+), the deprotonation (C2H++H+), and the isomerization channel (CH2 ++C+) is studied systematically by pump (four 266 nm photons) probe (800nm) excitation.

Single shot absorption measurements in the water window XUV region via HHG

Using 6-cycle, 7mJ pulses at 1.8µm we exploited limit intensities and drove HHG from 50eV up to the oxygen edge (550eV) with sufficient flux for single shot absorption measurements at the carbon k-edge (280eV).

High peak-power (0.42TW) mid-IR pulses achieved through hollow-core fiber compression

A novel scheme of flexible hollow-core fiber compression is used to achieve 2-cycles pulses at 1.8µm with 5mJ pulse energy. This source is shown to greatly enhance the efficiency of the high harmonic generation process.

Mid-IR 0.4TW pulses achieved through hollow-core fiber compression

By employing hollow-core fiber compression using a stretched flexible fiber, we achieved 2-cycles pulses centered on 1.8μm with more than 5mJ energy per pulse.

Tabletop imaging using 266nm femtosecond laser pulses, for characterization of structural evolution in, single molecule, chemical reactions

We have demonstrated a generally applicable tabletop approach utilizing a 266nm femtosecond laser pulse pump, 800nm pulse probe, coupled with Coulomb explosion imaging (CEI). We have investigated two simple chemical reactions in C2H2+ simultaneously: proton transfer and C=C bond-breaking, triggered by multiphoton ionization to excited states. Too and fro proton migration results are in excellent agreement with new ab initio trajectory simulations which predict isomerization timescales and pathways.