Imaging Molecular Motion: Femtosecond X-Ray Scattering of an Electrocyclic Chemical Reaction

Structural rearrangements within single molecules occur on ultrafast time scales. Many aspects of molecular dynamics, such as the energy flow through excited states, have been studied using spectroscopic techniques, yet the goal to watch molecules evolve their geometrical structure in real time remains challenging. By mapping nuclear motions using femtosecond x-ray pulses, we have created real-space representations of the evolving dynamics during a well-known chemical reaction and show a series of time-sorted structural snapshots produced by ultrafast time-resolved hard x-ray scattering. A computational analysis optimally matches the series of scattering patterns produced by the x rays to a multitude of potential reaction paths. In so doing, we have made a critical step toward the goal of viewing chemical reactions on femtosecond time scales, opening a new direction in studies of ultrafast chemical reactions in the gas phase.

Figure 1

Experimental setup for ultrafast time resolved x-ray scattering studies. Low pressure 1,3-cyclohexadiene vapor (green) is introduced into the scattering cell via a needle valve from a room temperature sample reservoir. CHD is excited to the photoactive state by the UV optical pump pulse (blue), inducing the chemical reaction that leads to several conformers of 1,3,5-hexatriene. The reacting molecules are probed by diffracting photons from an 8.3 keV x-ray pulse (red) that arrives with variable time delay relative the pump pulse, onto a large area pixel array detector. The time delay between the optical and x-ray pulses are varied to obtain the complete molecular movie.

Figure 2

The time-dependent pump-probe signal, defined as the percent difference. The scattering patterns for positive delay times versus negative delay times [100*(laser on-laser off)/laser off], as a function of pump-probe delay time. The scattering signals change on the order of 1% to 2% as indicated by the color bar.

Figure 3

Time-dependent scattered signal intensities. (a) Line traces of the scattering signals at 2.1–2.5 Å (red squares) and 2.9–3.1 Å (blue circles), and fits (red and blue lines, respectively). (b),(c) The derivatives of the fits show transients of $82\pm 24$ and $75\pm 35\mathrm{fs}$, respectively. Error bars for the fits in (a) and the uncertainties in (b),(c) are reported to $3\sigma$, calculated from the frame-to-frame shot noise from the CSPAD detector.

Figure 4

Experimental (black lines) and computational (thick colored lines) scattering signals for the first 250 fs of the ring-opening reaction of 1,3-cylohexadiene.