X-rays put molecules into a spin

The past decade has witnessed the emergence of a new X-ray light source technology that has been under development for some decades: the so-called self-amplified spontaneous emission-based free-electron laser (FEL). Such lasers provide pulses with durations from just below 1 fs (10⁻¹⁵ s) to over 100 fs and with energies from a few microjoules to a few millijoules, permitting the study of, for example, nonlinear optical effects (1), ultrafast processes (2), coherent diffractive imaging of biological systems such as protein crystals (3, 4), and mimivirus (5). After the opening of the first extreme-UV to soft X-ray user facility, namely FLASH in the Deutsches Elektronen-Synchrotron at Hamburg, Germany (6), a number of other machines covering the extreme-UV (FERMI FEL-1, DALIAN), soft X-ray (FERMI FEL-2, FLASH1/FLASH2), and hard X-ray spectral ranges (LCLS, PAL-XFEL, SACLA, and XFEL) have been opened to users from a wide range of scientific and engineering disciplines. Many more are under construction (e.g., SwissFEL and the Shanghai FEL), with an up-to-date list available in ref. 7. In PNAS, Céolin et al. (8) focus on an experimental methodology aimed at the study of ultrafast molecular rotation; the rotation is initiated by X-ray pulses, from a synchrotron, that are photoabsorbed by the molecule and probed by electrons emitted from the molecule. Below, the innovative nature …

↵¹To whom correspondence should be addressed. Email: john.costello{at}dcu.ie.