Joseph Sanderson

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.

Ultrafast imaging of multielectronic dissociative ionization of CO2 in an intense laser field

Momentum vectors of fragment ions produced by the Coulomb explosion of CO2z+ (z = 3–6) in an intense laser field (~50 fs, 1 × 1015 W cm−2) are determined by the triple coincidence imaging technique. The molecular structure from symmetric and asymmetric explosion channels is reconstructed from the measured momentum vectors. The channel-dependent mean C–O bond length of r = 2.14–3.89 A and the O–C–O bend angle of θ = 170–175° confirm substantial deformation of the molecular skeleton during the laser-molecule interaction. The channel-dependent bond length data are qualitatively described using a classical enhanced ionization model. The kinetic energy spectra reveal interesting structure which is attributed to the existence of excited states in the fragment ions.

Experimental observation of Raman chirped adiabatic rapid passage

In this letter, we present the first experimental results of climbing a vibrational ladder by Raman chirped adiabatic rapid passage. We obtain the bandwidth necessary for this experiment by utilizing pulses from a unique dual-wavelength high-intensity laser. The vibrationally excited molecules are field ionized by an intense 180 fs laser pulse, enabling us to probe the extent of the vibrational excitation of the symmetric stretch based on an enhanced ionization calculation.

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 imaging of polyatomic molecules with simplex algorithm

Abstract A simplex algorithm used to solve molecular structure from asymptotic momentum values is reported. The geometry of CO 2 3 + molecular ions in linear and bent configurations is reconstructed. A polyatomic molecular ion with trigonal planar symmetry is used to demonstrate the scalability of the algorithm.

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

Comparison of ADK ionization rates as a diagnostic for selective vibrational level population measurement

^{4+}

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

we observe for the first time that breakup involving an O

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

^{2+}

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

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