One technique for controlling electron motion uses the strong, oscillating electric field of an infrared laser to ionize a cloud of atoms. During each cycle, the field points up, then down, then up again, so it can pull an electron downward and then slam it back up against its atom. The electron scatters off its atom and is then yanked to the right by a steady, sideways electric field, toward a detector......
But this technique lacks precision. ....So the resulting image on the detector is fuzzy and represents electrons in a variety of scattering conditions.
Now a team led by Anne L’Huillier and Johan Mauritsson of Lund University in Sweden has combined this laser technique with precisely timed trains of ultrashort light pulses to cleanly image electron motion. ...
.... The images are the first of their kind that show such controlled electron-atom scattering. The team calls their system a stroboscope, after another device that uses periodic flashes of light to capture a still image of a hummingbird's wings, for example.
Each experiment generated a "bullseye" pattern ....
... If electrons were ionized at a time when the laser field gave them an extra boost upward, the pattern shifted upward; if the ionizing pulse came a half-cycle later, the bullseye shifted downward. This pattern shifting wouldn't have been possible with longer-lasting ionization periods.
Previously it was impossible to photograph electrons because of their extreme speediness, so scientists had to rely on more indirect methods. These methods could only measure the effect of an electron's movement, whereas the new technique can capture the entire event.....
...."It takes about 150 attoseconds for an electron to circle the nucleus of an atom. An attosecond is 10-18 seconds long, or, expressed in another way: an attosecond is related to a second as a second is related to the age of the universe," said Johan Mauritsson of Lund University in Sweden.