**5. Measuring time-resolved dynamics using pump-probe techniques with FELs**

Pump-probe spectroscopy techniques, championed by tabletop lasers [36], allow the measurement of dynamics for any system**—**from atoms to fullerenes and from solids to biological specimens. They are being used extensively, and the ultimate goal is to determine the motions and locations of nuclei and electrons and to determine the energy flow and charge transfer in systems. Recording these motions and making "molecular movies" using pump-probe spectroscopy seem reachable in the near future due to the new technologies.

Pump-probe techniques with FELs offer similar opportunities to measure physical and chemical changes in molecules at an atomic spatial resolution (on the time scale of atomic motion). With attosecond, the goal is to also measure the electronic motion. These techniques can be used for the study of fullerenes, in order to tackle fundamental questions such as, *how do the atoms in the fullerenes move after the photon energy is deposited in the fullerenes?* How do the bonds between the atoms that make the fullerenes break, or what are the pathways for the induced atomic motion in fullerenes? These questions can be asked and answered with FELs using pump-probe techniques. As in standard pump-probe work, to capture the dynamics, the pump pulse initiates the motions, and a probe pulse detects the changes using as many time delays as needed between the pump and probe pulses, ideally in a wide time scale. In order to achieve this goal, one needs two pulses, which can be generated in a few ways: (1) the accelerator scientists have developed several methods, but the most recent one cuts fresh slices from the electron bunch. They manipulate the electron bunch before it enters a split undulator, so that only its tail is lasing, in order to produce the first X-ray pulse. Next, they delay the electron bunch in order to acquire a time delay and spoil the electron bunch orbit further, by having the head of the bunch lasing and thus using a fresh slice from the electron bunch to produce the second photon pulse [52]. This scheme is also capable of providing two X-ray colors [52]; (2) the FEL pulse can be split into two X-ray pulses with an X-ray split and delay tool [53]. These two schemes were used successfully in experiments in the mode of X-ray pump-X-ray probe; and (3) a third scheme utilizes an X-ray pulse from the FEL as either the pump or the probe, and it is paired with a short tabletop pulse laser [54] (IR or UV). All of these schemes have been used in various FELs.

The FEL-based experiments, paired most of the time with pump-probe techniques, are carried out by using various types of spectrometers or imaging detectors for absorption experiments or for diffractive scattering experiments, respectively. The latter holds the promise to achieve imaging of molecules in the gas phase. Double VMI spectrometers paired with X-ray/IR, for a pump-probe scheme, are also used to measure the electrons—as well as the ions using the ionion coincidence techniques. These differential techniques examine both electronic and nuclear dynamics following the interaction of fullerenes with FEL X-ray pulses and also resolve the transient electronic rearrangement that accompanies photoionization [55]. The current repetition (rep) rate for the operational FEL facilities is at most 120 Hz (lower than the current tabletop repetition rate of 1–100 KHz), which does not allow easy FEL-based electron-ion-ion coincidence studies—although some work has been done in this highly differential mode [56]. However, the new XFEL in Hamburg, Germany, which will be completed by the end of this year, will offer ~1 MHz rep rate—thus allowing the measurement of e-ion-ion coincidences to track and precisely determine the decay channels and time it takes for the ultrafast nuclear dynamics to occur. It seems that the dream of making molecular movies is reachable in the next few years.
