Molecular Dynamics & Imaging

Fig. 1: (Left) A few-femtosecond soft x-ray pulse excites a molecule by ejecting an electron, leaving it in a super-excited state. (Right) A short laser pulse can then probe the pathways through which the super-excited molecule explodes.

Fig. 2. (Left) Time-resolved dynamics in N2O4 probed using high harmonic generation. At the inner turning point of N2O4 vibration, the ionization favors the “dark” state cation (HOMO-1)+ and the harmonic yield is strongly suppressed.  At the outer turning point, the ground state cation(HOMO)+ produces bright harmonics. (Right) Cover of Science, Nov. 21 issue, 2008.

This research thrust has the broad goal of capturing and manipulating coupled electron and nuclear dynamics in molecules by combining ultrafast X-ray and laser pulses with advanced molecular imaging techniques. This work will help address grand challenge science questions such as controlling matter at the level of electrons, understanding emergent properties from complex correlations of atomic and electronic constituents, and controlling matter very far from equilibrium. Example projects include -

Radiation driven femtochemistry: How do triatomic and larger molecules, which have rich potential energy landscapes, breakup after being exposed to ionizing radiation? In the soft X-ray regions of the spectrum, light-matter interaction is dominated by photoionization. The sudden removal of an electron from a triatomic or larger molecule initiates coupled electronic and nuclear dynamics that are very fast because the molecule is left in a super-excited state. Combining ultrafast X-ray pulses with advanced coincident molecular imaging techniques (COLTRIMS) now makes it possible to probe ionization-driven dynamics in highly excited molecules, where electron and nuclear dynamics occur on comparable time scales, and non-Born-Oppenheimer dynamics are the norm.

Advanced molecular imaging: Can strong field ionization and high harmonic generation from molecules be developed as a broadly-used tool to capture electronic and structural dynamics in molecules? Atoms and molecules exposed to strong laser fields exhibit a universal response where part of the electron wave function undergoes tunnel ionization, and driven by the laser field, can later recombine with the parent ion. Thus, monitoring either the resultant electron angular distribution, or the high harmonics emitted by the recombining electron, may yield potential in-situ probes of the fastest electronic orbital and nuclear structural dynamics.

Capturing coupled electronic and structural dynamics in molecules: By using mid-infrared lasers to drive the high harmonic generation process, bright, keV-bandwidth, coherent soft x-rays, which are perfectly synchronized to the driving laser, can be produced. The HHG spectrum emerges as an x ray super continuum, spanning photon energies from the EUV into the keV region of the spectrum and covering many elemental absorption edges simultaneously. This source is ideal for a variety of very powerful soft x-ray spectroscopies that can probe multiple site-specific dynamic charge density changes simultaneously (e.g. core level chemical spectroscopies, photoelectron diffraction, or extended x-ray absorption fine structure (EXAFS)). Dynamics in excited electronic states, bond breaking, and charge transfer in the gas, liquid and condensed phases are being explored.

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