Structural Dynamics Principles
In a chemical reaction, typical time scales of atomic or molecular motions start from femtoseconds, meaning the billionth of a billionth of a billionth of a second. Life relevant motions, like moving a pen during writing, can be as slow as seconds or even down to minutes’ or hours’ time scales (depending on the ideas one desires to write down). How are these time scales connected? To what extend structural motifs “freeze in” in time and dynamics information of chemical reactions? Which type of apparatus needs to be built and which kind of methods needs to be developed for investigating the created femtosecond “time stamps” in the structure of complex matter during the time course of a chemical or biochemical reaction?
Some time ago, in a proof of principle experiment, it has been postulated that high flux, pulsed x-rays – as been created with synchrotrons or Free Electron Lasers – can act as the “photons of choice” for collecting of what has been called the “molecular movie” since then [1]. The top figure summarizes its experimental principle: after the initiation of a reaction with an ultrashort optical pulse, the proceeding reaction’s structural changes are imaged by collecting a series of ultrafast snap shots of X-ray images as a function of time. It has been envisioned that utilizing X-ray photons will allow for the development of methods well beyond energy resolution, temporal resolution and spatial resolution of alternative methods developed so far. X-ray sources of the 3rd and 4th generation should make investigations possible, where laboratory sources reach their limit in resolution.
During the time course of FEL methods development we learned that high flux X-ray sources provide much more fascinating possibilities for the characterization of chemical and biochemical reactions as we first naively thought in the begin of 2000th. When X-ray photons are created in an undulator (which is a special arrangement of magnets where the electrons, which emit the X-ray radiation, are guided through), five very typical properties characterize them:
(i) their extremely high degree of lateral and transversal coherence,
(ii) their very high brilliance (very small nanometer focus combined to a high number of generated photons),
(iii) their time resolution down to 10 femtoseconds or even – now – attoseconds,
(iv) their tunability in energy to a very high precision and
(v) their polarization tunability. So, free electron laser sources provide
all the typical characteristics for pulsed lasers, just in the X-ray regime.
