Thursday, February 3, 2011
Scientists study processes using high-intensity ultrashort X-ray pulses: The new method was developed at the free-electron laser FLASH for so-called pump-probe processes. As an example: a first ultrashort pump pulse triggers a photochemical reaction. A second X-ray radiation pulse takes a “photograph” of how the reaction proceeds. For the first time, researchers are now able to determine exactly at what time the picture produced by the second pulse is created. For this new method, they make use of a side effect of the X-ray pulse generation. Indeed, the electron bunch accelerated in FLASH emits both an X-ray flash and an intense terahertz flash at the same time. The researchers separate the two flashes using a perforated, gold-coated mirror. As both pulses are created at the same time and from the same electron bunch, the terahertz flash can be used as a temporal “marker” of the X-ray flash. Using this method, the researchers were able to determine the time at which the X-ray pulse arrived at the sample with a precision of seven femtoseconds.