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

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 Å. This high fidelity is due to interference between the nonstationary 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 on ångstrom 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.

Figure 1

Left: Half of the LCLS 2.5 megapixel array detector (CSPAD [23]) showing the fractional deviation from the mean scattering signal recorded in each pixel at a pump-probe delay of 150 fs, integrated over 100 x-ray pulses. Right: The Legendre polynomial fit obtained by applying Eq. (5) to the data at this time delay. The scattering vs time delay appears as a movie in the Supplemental Material [24].

Figure 2

${\beta }_2(Q,t)$ as defined in Eq. (5). This term captures most of the excited state scattering signal. The time-averaged value is subtracted, and the scale is the fraction of modulation due to the holographic interference between the excited state and the reference ground state. The principal features are long-period oscillations in $Q$ that are due to the $B$ state, and much shorter period oscillations in $Q$ caused by dissociation.

Figure 3

Extracted excited-state charge distribution vs time, for $R$ from 2.3 to 7 Å and time delays out to 2 ps. This “movie” was extracted from ${\beta }_2(Q,t)$ (see Fig. 2) using Eqs. (1)–(9). Bound-state wave-packet oscillations, dissociation, and rotational dephasing are clearly visible. Letters refer to features described in the text. The scale is proportional to the excitation in the ${\beta }_2$ channel. Inset: Excitation path and iodine molecular potentials. A video is in the Supplemental Material [24].