Ultrafast charge transfer of a valence double hole in glycine driven exclusively by nuclear motion

Atomic motion can drive charge transfer on unexpectedly short time scales. When the electron cloud of a molecule is hit by an ultrashort burst of light, complex processes ensue. One or more of the very light and fast electrons can suddenly be kicked out, forcing the other electrons to quickly relocate on time scales as short as just a few femtoseconds (1 fs = 0.000000000000001 s). Only at much longer times do the heavier atomic nuclei respond and play a role in the redistribution of electrical charge.

In this paper, we challenge this conventional view. We demonstrate through quantum-mechanical calculations on a prototypical amino acid, that small displacements of the molecular skeleton can dictate the fate of the electronic cloud and be responsible for charge transfer across several atoms on time scales of 3 to 4 fs, just as quickly as the purely electronic response. The breakdown of the Born-Oppenheimer approximation, stating that the evolution of electrons and nuclei can be separated owing to their very different masses, lies at the heart of these ultrafast phenomena. As a consequence of this work, common assumptions in x-ray and attosecond science may have to be revised.

Phys. Rev. Lett. 115, 143002 (2015)