Scientists used a powerful X-ray laser to heat water from room temperature to 100,000 degrees Celsius in less than a tenth of a picosecond (millionths of a millionth of a second). The experimental setup, which can be considered the world's fastest water heater, created an exotic state of water that researchers hope to learn more about the peculiar properties of Earth's most important fluid. The observations also have practical utility for probing biological and many other samples with X-ray lasers. The team led by Carl Caleman from the Center for Free-Electron Laser Research (CFEL) at DESY and the University of Uppsala (Sweden) reports his findings in the Journal Proceedings of the National Academy of Sciences (19659002)] The researchers used the Linac Coherent Light Source LCLS X-ray free-electron laser at the SLAC National Accelerator Laboratory in the US to record extremely intense and ultra-short X-ray flashes with a stream of water. "It's not the usual way to cook your water," Caleman said. "When you heat water, the molecules are usually shaken stronger and stronger." At the molecular level, heat is movement ̵
"Our heating is fundamentally different," explains Caleman. "The high-energy X-rays emit electrons from the water molecules, destroying the balance of the electrical charges, and suddenly the atoms feel a strong repulsive force and begin to move violently." In less than 75 femtoseconds, which is 75 millionths of a billionth of a second or 0,000,000,000,000 075 seconds, the water undergoes a phase transition from liquid to plasma. A plasma is a matter state in which the electrons are removed from the atoms, resulting in a kind of electrically charged gas.
"As water moves from liquid to plasma, it still remains in the density of liquid water because the atoms have not had time to move significantly," said co-author Olof Jönsson of Uppsala University. This exotic state of matter is not something that could be found naturally on Earth. "It has similar properties to some plasmas in the Sun and the gas giant Jupiter, but it has a lower density and is now hotter than the Earth's core."
Scientists used their measurements to validate simulations of the process. Together, the measurements and simulations allow to study this exotic state of water to learn more about the general properties of the water. "Water is really a strange liquid, and if it did not have its peculiar properties, many things on Earth would not be what they are, especially life," Jönsson said. Water has many anomalies, including its density, heat capacity and thermal conductivity. It is these anomalies that will be investigated in the future DESY Center for Water Research (CWS) and the results obtained are of great importance for the activities there.
In addition to its fundamental importance, the study also has immediate practical significance. X-ray lasers are commonly used to study the atomic structure of minute samples. "It's important for any experiment with liquids in X-ray lasers," said co-author Kenneth Beyerlein of CFEL. "In fact, any sample you place in the X-ray is destroyed in the way we've observed, and if you analyze something that is not a crystal, you have to take that into account."
The measurements show almost no structural changes in the water for up to 25 femtoseconds after the X-ray pulse starts to hit him. But even at 75 femtoseconds changes are visible. "The study gives us a better understanding of what we do with different samples," said co-author Nicusor Timneanu of Uppsala University, one of the key scientists who developed the theoretical model used. "His observations are also important for the development of techniques that can be used to image single molecules or other tiny particles with X-ray lasers."