Their research allows double ionization events to be observed at the time scale of attoseconds, which are one-billionth of a billionth of a second. The physicists have also shown that these ionization events occur earlier than thought -- a key factor to improving knowledge of correlated electron dynamics, which involve two electrons and their interactions with each other. The work appears in a recent issue of Nature Communications.
"The research involves studying if these correlated electrons, ejected from an atom or a molecule, are traveling in the same or opposite directions," said Nora Johnson, a doctoral student in physics from Dell Rapids, S.D. "We can also determine if one electron has all the energy or if they share energy equally."
Other university researchers involved include Itzik Ben-Itzhak, university distinguished professor of physics, and Matthias Kling, assistant professor of physics. Kling is the principal investigator for the project and is on research leave at the Max Planck Institute of Quantum Optics in Garching, Germany, where he is performing related research. All of the researchers are involved with the university's James R. Macdonald Laboratory.
Double ionization occurs when two electrons are removed from an atom -- a process that can be caused by an intense laser pulse. When double ionization occurs in the laser field it can take the form of a sequential process, in which the laser removes one electron and then removes the other electron. This project focuses on another mechanism -- the nonsequential process for ionization -- in which the laser removes one electron, which is accelerated and hits a second electron to excite it. The laser then knocks out the second electron from the atom.
The researchers sent a four femtosecond-long laser pulse onto argon atoms. A femtosecond is a millionth of a billionth of a second. While most of the argon atoms were singly ionized, approximately every thousandth atom underwent nonsequential double ionization.
"The surprising result is that everybody expected that the second electron becomes excited and then, when the laser field is the strongest, this electron is removed," said Ben-Itzhak, director of the Macdonald laboratory. "But it actually happens earlier."
The researchers discovered that the time between the recollision and the second ionization is about 400 attoseconds. This is about 200 attoseconds earlier than the peak of the field, which is when physicists expected the second ionization to occur.
Johnson conducted her early experiments at the Macdonald Laboratory. She performed more extensive experiments during a 2009 Fulbright Fellowship at the Max Planck Institute of Quantum Optics. The two organizations have an ongoing collaboration and the Kansas State University team is directly funded by a $400,000 National Science Foundation grant.
"The key is that Nora has brought knowledge from Germany about short pulses and we can now continue these experiments in Kansas," Ben-Itzhak said. "We have an ongoing collaboration with them that goes both ways."
Now that the researchers have made an important discovery with atoms, Johnson is performing a similar experiment with molecules. She is performing experiments at the Macdonald Laboratory and will use the laboratory's expertise in imaging molecules.
"A molecule is more complex than an atom, which typically means its reaction dynamics are richer," Johnson said. "We are excited to pursue correlated electron dynamics at the next level of complexity to further understand them."
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