[Media Release] Tunnelling ionization studies by researchers in Japan and Russia show changes in electron distributions between ground- and excited-state in laser tunnelling ionization of molecules
May 09, 2016
The change in electronic distributions in molecules as they are photoexcited can offer useful insights for photochemistry. Studying the position and momentum of molecular fragments after ionization by “tunnelling” in an intense laser field can provide a means of mapping these electron distributions, but excited state molecules can be awkward to probe in this way. Now a collaboration of researchers at Nagoya University, The Open University of Japan, the University of Electro-communications, Tokyo (UEC, Tokyo), and the Moscow Institute of Physics and Technology, has successfully applied the approach to excited states of nitrogen oxide (NO).
The researchers, including Toru Morishita at UEC, Tokyo, observed a change in the peak of the momentum of N+ fragments depending on the initial state: 45° with respect to the polarization of the laser beam when probed in the ground state and 0° for the excited states. The researchers compared the results with weak-field asymptotic theory, which is established by the group of Morishita, and noted “excellent agreement for both the ground and excited states”.
The main difficulty in probing the electron distribution of excited state molecules in this way is the diminished tunnelling potential. As a result multiphoton processes are more likely to contribute to ionization. The researchers probed the mechanism at work by altering the polarization of the laser field. A linearly polarised field sends the electron towards and an electron impact process causes the excited ionized state. For more circularly polarised fields, the electron trajectory shifts away from the nucleus so that fragment yields decrease, as observed.
In their report of the results the researchers conclude, “The present study provides a deeper understanding of laser tunnelling ionization of molecules, a key step of important applications such as high-order harmonics generation and self-electron diffraction, and paves the way for real-time visualization of electron dynamics in chemical reactions.”
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