Photo-induced strengthening of weak bonding in noble gas dimers

We demonstrate through extensive first-principles time-dependent density functional calculations that attractive van der Waals interaction between closed-shell atoms can be enhanced by light with constant spatial intensity. We illustrate this general phenomenon for a He dimer as a prototypical case of complex van der Waals interactions and show that when excited by light with a frequency close to the 1s → 2p He-atomic transition, an attractive force larger than 7 pN is produced. This force gain is manifested as a larger acceleration of He-He contraction under an optical field. The concerted dynamical motions of the He atoms together with polarity switching of the charge-induced dipole cause the contraction of the dimer. These findings are relevant for the photo-induced control of weakly bonded molecular species, either in gas phase or in solution.