For the first time, scientists have generated x-ray pulses so brief that they capture the quicksilver behavior of electrons inside atoms. Using these pulses like flashbulbs--each one lasts less than a millionth of a billionth of a second--the researchers have traced how electrons in a gas hop between energy levels.
Scientists have known how to capture the breaking of chemical bonds between atoms for decades, using ultraspeedy strobe lights. But bond breaking is languid compared with the lightning-fast activity of electrons inside atoms, which zip around the nucleus and hop between energy shells in less than 200 attoseconds. (An attosecond is 1 billionth of a billionth of a second.)
Now, in the 29 November issue of Nature, researchers from the Vienna University of Technology in Austria, the National Research Council Canada, and Bielefeld University in Germany report that they have produced isolated x-ray pulses 650 attoseconds long. To generate these pulses, the researchers shined a beam of laser light at neon gas to produce x-ray photons of various frequencies. The frequencies generally canceled each other out through destructive interference, but a few added up.
Measuring the duration of this pulse was more challenging. The researchers shot the pulse and a laser beam into a chamber of krypton gas. When the x-ray pulse ionized the krypton, electrons flew out of their parent atoms with a certain kinetic energy. By shooting the x-ray pulse into the gas at different points in the laser wave cycle, the team could delay the x-ray pulse's impact with the krypton atoms, which would affect the electrons' kinetic energy. They observed that the change in kinetic energy fit a wavelike pattern, or modulation. This proved that the time of the krypton's ionization--and hence the duration of the x-ray pulse--fell within a window of time less than half the 1200-attosecond period of the laser cycle. From the modulation, the researchers calculated that the x-ray pulse lasted a fleeting 650 attoseconds and that the krypton atoms released their electrons in less than 150 attoseconds.
"This is an important experiment," says Anne L'Huillier, a physicist at the Lund Institute of Technology in Sweden. "It opens the door to the study of extremely fast electronic processes occurring inside atoms and molecules."