Benji Wales

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.

N2O ionization and dissociation dynamics in intense femtosecond laser radiation, probed by systematic pulse length variation from 7 to 500 fs

We have made a series of measurements, as a function of pulse duration, of ionization and fragmentation of the asymmetric molecule N2O in intense femtosecond laser radiation. The pulse length was varied from 7 fs to 500 fs with intensity ranging from 4 × 10(15) to 2.5 × 10(14) W∕cm(2). Time and position sensitive detection allows us to observe all fragments in coincidence. By representing the final dissociation geometry with Dalitz plots, we can identify the underlying breakup dynamics. We observe for the first time that there are two stepwise dissociation pathways for N2O(3+): (1) N2O(3+) → N(+) + NO(2+) → N(+) + N(+) + O(+) and (2) N2O(3+) → N2 (2+) + O(+) → N(+) + N(+) + O(+) as well as one for N2O(4+) → N(2+) + NO(2+) → N(2+) + N(+) + O(+). The N2 (2+) stepwise channel is suppressed for longer pulse length, a phenomenon which we attribute to the influence which the structure of the 3+ potential has on the dissociating wave packet propagation. Finally, by observing the total kinetic energy released for each channel as a function of pulse duration, we show the increasing importance of charge resonance enhanced ionization for channels higher than 3+.

Ultrafast molecular dynamics of dissociative ionization in OCS probed by soft x-ray synchrotron radiation

Soft X-rays (90-173 eV) from the 3rd generation Canadian Light Source have been used in conjunction with a multi coincidence time and position sensitive detection apparatus to observe the dissociative ionization of OCS. By varying the X-ray energy we can compare dynamics from direct and Auger ionization processes, and access ionization channels which result in two or three body breakup, from 2+ to 4+ ionization states. We make several new observations for the 3+ state such as kinetic energy release limited by photon energy, and using Dalitz plots we can see evidence of timescale effects between the direct and Auger ionization process for the first time. Finally, using Dalitz plots for OCS

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

^{4+}

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

we observe for the first time that breakup involving an O

Dissociative ionization dynamics of triatomic molecules induced by soft X-rays

^{2+}

Fragmentation processes of OCS in collision with highly charged ions

ion can only proceed from out of equilibrium nuclear arrangement for S(2p) Auger ionization.

Coulomb explosion dynamics of triatomic molecules in laser pulses ranging from 7 to 200fs

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.

Coincidence momentum imaging of the Coulomb explosion of OCS induced by collision of 15keV/q Ar4+ Ar8+ ions

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.

A coincidence detection algorithm for improving detection rates in coulomb explosion imaging

Soft X-rays (50eV to 170eV) have been used in conjunction with a triple coincidence time and position sensitive detection apparatus to observe the Coulomb explosion of OCS and CS2. By varying the X-ray energy we can access ionization channels which result in two or three body break up, for 3+, such as O++C++S+ and for 4+ such as S++C++S2+. We can compare the kinetic energy release to the value calculated for a purely Coulombic break up from near equilibrium. The extent to which channels are concerted or stepwise is also determined.