A new method allowed scientists to observe the recombination of electron shells in molecules in "real time."
The feat is impressive because this process occurs over the course of only a few attoseconds, which are a billionth of a billionth of a second, Moscow Institute of Physics and Technology reported.
To make these incredible findings, a research team used the pump-probe method. In this process a molecule is oriented with a laser pulse, and then a second powerful, low-frequency laser pulse is used to ionize it, generating high harmonic radiation. Looking at this high harmonic spectrum allowed the scientists to observe the restructuring of the molecule's electron shell caused by the ionized pulsing's field.
"With this method, we were able to track structural changes in the electron shells of methyl fluoride (CH3F) and methyl bromide (CH3Br)molecules," said Oleg Tolstikhin, associate professor at MIPT's Theoretical Physics Section. "These processes are even faster than chemical reactions, in which atomic nuclei move. In this experiment, we were able to see the restructuring of the electron shell."
The experiment was conducted using a sapphire laser with a wavelength of 800 nanometers that generated short, intense pulses. The laser hit methyl fluoride and methyl bromide gas molecules in a vacuum chamber. The high harmonics spectrum was analyzed using an X-ray and ultraviolet spectrometers.
"This was the first time ever that evidence of the restructuring of a molecule's electron shell caused by its interaction with the strong field of an ionizing laser pulse was observed in the high harmonic spectrum," Tolstikhin said. "The observed processes lasted a few tens of attoseconds. Identifying the traces of such processes in high harmonic spectra was possible thanks to our asymptotic theory of the tunneling ionization of molecules in the case of degenerate electronic states. Our theoretical model describes the experimental results pretty well."
The findings revealed how the electron cloud "migrated" within the molecule. The team found the migration involves a permanent dipole moment and degenerate states of outer electrons in the molecule. The new observations method will be used in the future to study fine chemical processes.
