Visualizing electron rearrangement in space and time during the transition from a molecule to atoms

Abstract

Imaging and controlling reactions in molecules and materials at the level of electrons is a grand challenge in science, relevant to our understanding of charge transfer processes in chemistry, physics, and biology, as well as material dynamics. Direct access to the dynamic electron density as electrons are shared or transferred between atoms in a chemical bond would greatly improve our understanding of molecular bonding and structure. Using reaction microscope techniques, we show that we can capture how the entire valence shell electron density in a molecule rearranges, from molecular-like to atomic-like, as a bond breaks. An intense ultrashort laser pulse is used to ionize a bromine molecule at different times during dissociation, and we measure the total ionization signal and the angular distribution of the ionization yield. Using this technique, we can observe density changes over a surprisingly long time and distance, allowing us to see that the electrons do not localize onto the individual Br atoms until the fragments are far apart (∼5.5 Å), in a region where the potential energy curves for the dissociation are nearly degenerate. Our observations agree well with calculations of the strong-field ionization rates of the bromine molecule.

Fig. 1.

(Upper) Calculated electron density (molecular orbitals) as a function of Br₂ internuclear separation and time after a 400-nm dissociating pulse causes the bond to rupture on the dissociative C neutral state. Note that contributions from all 10 electrons in the valence shells are included. (Lower) Measured angular dependence of the Br⁺ ion yield from strong-field ionization of the Br₂ molecule, as a function of time after the dissociating pulse (see also Movie S1 for a movie of the data). Both experiment and theory indicate that the electrons localize onto individual atoms on a time scale ≈140 fs after the dissociating pulse.

Fig. 2.

Experimental Br⁺ total ion yield as a function of time delay between the 400-nm dissociating pulse and the 800-nm probe ionizing pulse.

Fig. 3.

Experimental (A) and calculated (B) total KER (i.e., recoil energy) of the Br⁺ as a function of time after the dissociating pulse. At short time delays (below 20 fs), Br⁺ ions are generated predominantly by ionizing the inner valence orbital 1π_u of the molecule. After dissociating for about 20 fs, the atoms have gained enough kinetic energy such that ionization from 2σ_g and opens additional channels for the dissociation of .

Fig. 4.

Angular distribution of Br⁺ at different time delays after the dissociating pulse, for both the slow Br⁺ (A) (kinetic energy release below 0.7 eV) and fast Br⁺ (B) (kinetic energy release above 0.7 eV). (See text for the fitting method for the anisotropy number n.)