“In past molecular movies, we have been able to see how atomic nuclei move during a chemical reaction,” said Peter Weber, a chemistry professor at Brown and senior author of the report. “But the chemical bonding itself, which is a result of the redistribution of electrons, was invisible. Now the door is open to watching the chemical bonds change during reactions.”
This spectacular article has been published in Nature Communications recently. The electrons were excited with light adjusted by a laser. They stayed in the excited state about 200 femtoseconds and that was enough to capture the redistribution of electrons by researchers.
The more detailed explanations can be found here: First direct look at how light excites electrons to kick off a chemical reaction