Self-Referenced Coherent Diffraction X-Ray Movie of Ångstrom- and Femtosecond-Scale Atomic Motion

Abstract

Time-resolved femtosecond x-ray diffraction patterns from laser-excited molecular iodine are used to create a movie of intramolecular motion with a temporal and spatial resolution of 30 fs and 0.3 A. This high fidelity is due to interference between the non-stationary excitation and the stationary initial charge distribution. The initial state is used as the local oscillator for heterodyne amplification of the excited charge distribution to retrieve real-space movies of atomic motion onÅngstrom and femtosecond scales. This x-ray interference has not been employed to image internal motion in molecules before. Coherent vibrational motion and dispersion, dissociation, and rotational dephasing are all clearly visible in the data, thereby demonstrating the stunning sensitivity of heterodyne methods.High brightness ultrafast hard x-ray free electron lasers (FELs) can perform time-resolved x-ray diffractive imaging. Recent demonstrations of time-resolved crystal diffraction or time-resolved non-periodic imaging illustrate the power of these sources to trackÅngstrom-scale motion [1,2]. These have spurred new insights in broad areas of science, but have not fully realized the potential of x-ray FELs to image molecules with simultaneous sub-Ångstrom and few-femtosecond resolution. Previous x-ray or electron scattering experiments have used correlations between simulations and data to extract femtosecond molecular dynamics information [3][4][5][6][7].Here we propose and demonstrate an imaging method that employs a universal but unappreciated feature of time-resolved hard x-ray scattering that dramatically improves reconstructed images of charge motion, and enables femtosecond and sub-Ångstrom x-ray movies. The method relies on the "pump-probe" protocol, where motion is initiated by a short "start" pulse, and then interrogated at a later time by a "probe" pulse. The pumped fraction is small, and the unexcited fraction is our heterodyne reference [8].

Time-resolved ultrafast x-ray scattering from an incoherent electronic mixture

“…Now let us consider TRXS from a molecule consisting of a heavy element like iodine. This corresponds to the situation of the recent TRXS experiment [12] on the iodine molecule. As there are many electrons in iodine, one gets a large cross section if the electrons do not change their state in the x-ray scattering process.…”

“…Recently, Glownia and co-workers imaged ultrafast vibrational motion in photoexcited molecular iodine at LCLS * gdixit@phy.iitb.ac.in † robin.santra@cfel.de using TRXS with a spatial and temporal resolution of 0.3Å and 30 fs, respectively. Moreover, the idea of holographic (heterodyne) detection, based on the assumption of interference between the ground-state stationary charge distribution and the nonstationary excitation, was used to analyze the data in this experiment [12]. Also, plasma-based ultrashort x-ray sources have been used to image various nonequilibrium phenomena in matter using TRXS [13].…”

“…If the pump pulse fluctuates from shot to shot (for instance, there is typically no carrier envelope phase stabilization), an incoherent electronic superposition is created. An incoherent electronic ensemble in molecular iodine was created in the recent experiment by Glownia and co-workers [12]. There was no coherence between the ground and excited electronic states, and only full or partial vibrational coherence in each electronic state was present.…”

“…There was no coherence between the ground and excited electronic states, and only full or partial vibrational coherence in each electronic state was present. The notion of heterodyne detection used to interpret the TRXS signal [12] was recently challenged by Mukamel and co-workers in a brief comment [14]. Note that the concept of heterodyne detection has been used to interpret the TRXS signal from an electronically excited solid [13,15], but there is no consistent quantum theory of TRXS from a nonstationary solid showing that the heterodyne detection is feasible.…”

Fundamental Limits on Spatial Resolution in Ultrafast X-ray Diffraction

“…The recent development of ultrashort pulsed electron and X-ray beams has now expanded the scope of structure determination to the time domain [4,5] (and references therein). Important applications include the determination of molecular structures in excited states and the observation of structural molecular dynamics, i.e., the time-resolved determination of transient molecular structures during chemical reactions [6][7][8][9][10][11][12][13]. The advent of ultrafast pulsed X-ray Free-Electron Lasers (XFELs) in particular has increased the intensity of X-rays while decreasing pulse durations to below 30 fs [14].…”