# Recent Advances in Ultrafast Structural Techniques
## 1 Introduction  
The present review article summarizes some of the key advances in the development of ultrafast structural techniques over the last decade. Pioneering experiments including a historical compilation can be found elsewhere [1; 2; 3; 4; 5; 6; 7]. After more than 30 years of continuous improvements in femtosecond (fs, 1 fs = 10\({}^{-15}\) s) hard X-ray and fs-electron sources, ultrafast diffractometers confer the prerequisite spatial and temporal resolutions to capture atoms in motion. The characteristic length-scale that such ultrafast cameras had to resolve is evident--this is the size of an atom or a bond length (\(d_{c}\))-1 A = 10\({}^{-10}\) m = 0.1 nm. In order to determine their necessary shutter speed or temporal resolution, however, one must assume a characteristic speed (\(v_{c}\)) for atomic motion. There are faster and slower processes in nature but as a general rule of thumb, \(v_{c}\)-10\({}^{3}\) m/s\({}^{-1}\) = 1 nm ps\({}^{-1}\) (1 ps = 10\({}^{-12}\) s), which is roughly the speed of sound in most solid-state organic materials and liquids results a good estimate [8]. Note that structural chemists, biologists, and condensed matter physicists are usually interested in rather low-frequency, high-amplitude atomic displacements. Soft vibrational modes are those that govern chemical reactivity through the anharmonicities encountered along the potential energy landscape as we approach the bond dissociation threshold or the transition state [9]. Protein function, on the other hand, seems to rely on collective low-frequency vibrational modes that couple to large amplitude protein motions for which thermal bath fluctuations and entropic factors play key roles (vide infra examples in Sections 2.4.2 and 2.4.3). In addition, very interesting condensed matter physics arises from low-frequency collective excitations (vide infra examples in Sections 2.1, 3.5 and 3.6). Two examples are the famous amplitude phonon mode (known as amplitudon) that modulates the charge density wave (CWD) gap in Peierls distorted systems, [10] and the latticevibrations responsible for the formation of Cooper pairs that lead to the foundation of the BCS theory for superconductivity [11]. Thus, a characteristic time (\(\Delta t_{c}\)) or temporal resolution of about \(\Delta t_{c}=d_{c}\ v_{c}{}^{-1}\) = 0.1 nm-(1 nm ps\({}^{-1}\))\({}^{-1}\) = 0.1 ps = 100 fs suffices to monitor a vast variety of structurally relevant phenomena. Ultrafast diffraction belongs to a family of techniques known as pump-probe in which two well-synchronized pulses, a pump and a probe, are implemented. The pump is a fs-optical pulse coming from a master fs-laser and is responsible for initiating structural changes by bringing the system out of equilibrium. The probe is an ultrashort X-ray or electron pulse, which scatters off the irradiated sample volume to generate a diffraction pattern downstream. A series of diffraction patterns obtained for different time delays (\(t\)) between these two pulses is then recorded in a stroboscopic fashion. Structural refinement finally brings time-dependent changes in diffracted intensities to real-space information to render a \"molecular movie\" that unveils details about electron density and nuclear reorganization. The success of ultrafast structural dynamics, therefore, relies on the ability of producing ultrashort hard X-ray pulses or electron bursts as well as maintaining an outstanding synchronization between probe and pump pulses. Recall these are repetitive stroboscopic experiments so both the durations of the pump and probe pulses and their relative timing are vital factors in addition to source brightness. The synchronization noise is referred to as timing jitter and has been one of the key issues when radio frequency (RF) fields are utilized for accelerating or compressing electron bunches.