In Brief
Terahertz Flashes for Exact X-ray Measurements
Scientists from the HZB, DESY, the European XFEL GmbH and the Helmholtz Institute Jena have now found a way to measure the arrival time of X-ray pulses with a precision of less than ten femtoseconds. This new method was developed at the free electron laser FLASH for so-called "pump probe" procedures. In these procedures, a first ultrashort pump pulse triggers, for example, a photochemical reaction. A second pulse of X-ray light "takes a photo" of how the reaction develops. Researchers now can determine exactly at which moment in time the picture is made by the second pulse. In their new method, the scientists use a side effect of X-ray pulse generation: In addition to the X-ray flash, the parcel of electrons accelerated in FLASH emits an intensive terahertz flash at the same time. The scientists separated both flashes by using a gold-coated mirror with holes. Since both pulses are created at the same time by the same parcel of electrons, the terahertz flash serves as a temporal "marker" for the X-ray flash and can be used as a time reference. Thus the researchers were able to determine with an accuracy to seven femtoseconds when the X-ray flash reaches the probe.
With only minimal modifications, the new method now can be used in all existing and planned new FEL sources. In combination with corresponding experiments, it opens up the possibility to fully exhaust the potential of these large-scale facilities. For the first time, phenomena can now be investigated with X-ray pulses in the relevant femtosecond time scale. Scientists have long been waiting for this to become reality.
