Showtime for Molecular Movies

T he scientist Ahmed Zewail (1946–2016) was awarded the Nobel Prize in Chemistry in 1999 for his contributions to femtochemistry, the field that studies chemical changes on the time scales on which atoms move—femtoseconds (fs). Femtochemistry experiments follow the electronic or structural dynamics of a molecule by first exciting it with a femtosecond “pump” laser pulse, and then observing it using a delayed “probe” pulse [1]. Most femtochemistry experiments, however, probe molecules through indirect observables: They rely on the fact that the chemical dynamics are accompanied by changes in the molecule’s absorption spectrum. Two independent groups now use more direct approaches to the visualization of atomic motions, recording molecular “movies” by time-resolved x-ray diffraction [2] and time-resolved electron diffraction [3], respectively. Both experiments image, in real time, the changes of the distance separating the two atoms in a vibrating iodine (I2) molecule. The results represent important progress in our ability to record ultrafast molecular movies with increasing spatial and temporal resolution without relying on complex models.