Mapping molecular motions leading to charge delocalization with ultrabright electrons

Abstract

Ultrafast processes can now be studied with the combined atomic spatial resolution of diffraction methods and the temporal resolution of femtosecond optical spectroscopy by using femtosecond pulses of electrons or hard X-rays as structural probes. However, it is challenging to apply these methods to organic materials, which have weak scattering centres, thermal lability, and poor heat conduction. These characteristics mean that the source needs to be extremely bright to enable us to obtain high-quality diffraction data before cumulative heating effects from the laser excitation either degrade the sample or mask the structural dynamics. Here we show that a recently developed, ultrabright femtosecond electron source makes it possible to monitor the molecular motions in the organic salt (EDO-TTF)2PF6 as it undergoes its photo-induced insulator-to-metal phase transition. After the ultrafast laser excitation, we record time-delayed diffraction patterns that allow us to identify hundreds of Bragg reflections with which to map the structural evolution of the system. The data and supporting model calculations indicate the formation of a transient intermediate structure in the early stage of charge delocalization (less than five picoseconds), and reveal that the molecular motions driving its formation are distinct from those that, assisted by thermal relaxation, convert the system into a metallic state on the hundred-picosecond timescale. These findings establish the potential of ultrabright femtosecond electron sources for probing the primary processes governing structural dynamics with atomic resolution in labile systems relevant to chemistry and biology.

Time-dependent resonant scattering: An analytical approach

“…[20][21][22] At the moment, these sources are primarily intended for the use in electron diffraction experiments on molecules. 37 As the electron energies are progressively lowered, effects of resonant scattering such as discussed here should become observable. Finally, we are in the process of adapting the formalism presented in this paper to problems associated with impulsive ionization by optical pulses such as time resolved Fano resonances, 15,16 electron correlation times, and photo-emission time.…”

Implementation and modeling of a femtosecond laser-activated streak camera

“…[15][16][17] These electron pulses have been used in tabletop setups to capture ultrafast induced dynamics in condensed matter. 18,19 In all cases, it is essential to accurately measure the duration of the electron pulses on target, as well as the arrival time of the electron pulses relative to the laser pulses. Different methods have been proposed and developed to measure the duration of electron pulses using streaking fields.…”

“…Measuring the duration of femtosecond pulses requires a very fast changing electric field to provide sufficient resolution. These time-varying streaking fields have been generated using a microwave cavity, 19 a laser standing wave, 21-23 a discharging capacitor, 4,24 a split ring resonator, 25 and a terahertz resonator. 27 A laser-activated streak camera is susceptible to time of arrival jitter between the laser and electron pulses.…”

Diffraction using laser-driven broadband electron wave packets

“…The recent development of femtosecond sources 9,10 of X-rays, for example, free-electron lasers, and electron beams has enhanced the temporal resolving power of these standard probes for ultrafast molecular dynamics [11][12][13] . Femtosecond electron and X-ray diffraction have been successfully demonstrated recently [14][15][16][17] . These schemes offer the potential for performing ultrafast dynamic imaging in condensed-phase systems, but their current fluxes are insufficient to achieve good enough signal-to-noise ratios for more rarified systems, for example gas-phase.…”