Multi-messenger tracking of coherence loss during bond breaking

Abstract

Coupled electronic and nuclear motions govern chemical reactions, yet disentangling their interplay during bond rupture remains challenging. Here we follow the light-induced fragmentation of Br$_2$ using a coincidence-based multi-messenger approach. A UV pulse prepares the dissociative state, and strong-field ionization probes the evolving system. Coincident measurement of three-dimensional photoion and photoelectron momenta provides real-time access to both the instantaneous internuclear separation and the accompanying reorganization of the electronic structure, allowing us to determine the timescale of bond breaking. We find that electronic rearrangement concludes well before the nuclei reach the bond-breaking distance, revealing a hierarchy imposed by electron-nuclear coupling. Supported by semiclassical modelling, the results show that the stretched Br$_2$ molecule behaves as a two-centre interferometer in which the loss of coherence between atomic centres encodes the coupled evolution of electrons and nuclei. Our work establishes a general framework for imaging ultrafast electron-nuclear dynamics in molecules.

Figures (4)

• Figure 1: a, Potential energy surfaces of Br$_2$ and its ionic states. The internuclear-distance-dependent bound electron density of the dissociating Br$_2^*$ is shown along the dissociative surface. b, KER of Br$^+$ as a function of pump-probe delay, tracing the neutral dissociation over a wide delay range. c, Photoelectron momentum distributions from the distinct five KER-delay zones (1-5). The middle upper inset shows the time-dependent internuclear-distance R, which is calculated by fitting the Coulomb explosion structure C with a classical nuclear wave packet simulation supp. The lower inset illustrates the configuration of the laser polarization and the molecular alignment, as well as the tunnel electron trajectories undergoing recollision.
• Figure 2: Evolution of photoelectron momentum distributions revealed by ND maps for representative KER-delay zones. a, Zone 2 and b, Zone 4. The black circle marks the blue inner-ring structure in the low-momentum region, while the dashed ellipse and fan highlight the high-momentum enhancement features. c,d, Corresponding raw radial momentum distributions of the ND maps along the transverse (30$^{\circ}$$<$$\left|\alpha\right|$$<$ 150$^{\circ}$) and parallel ($\left|\alpha\right|$$<$ 30$^{\circ}$) directions. In the inset of panel d, the shaded error bars highlight the divergence between zone 4 (blue) and 5 (grey) in the parallel direction.
• Figure 3: Simulated ND plots for the bond-breaking state (BBS): a multicycle and b single-cycle SCTS simulations. Normalized radial momentum distributions from the single-cycle simulation are shown in c the transverse and d the parallel directions. Experimental observations from Zones 4 and 5, along with the simulated distribution for intact Br ionization, are included for comparison.
• Figure 4: State evolution of Br$_2$ breaking bond and its influence on the radial momentum distributions of the ND maps: a, Schematic representation of the sequential states of Br$_2$ dissociation. The red shaded cloud artistically depicts the diffusing electron density in the bond-breaking state, and the corresponding polarizability configurations during the ionization dynamics (recollision, curved arrows) are shown for each state. b, Dependence of the valley depth on the ionization potential contribution of Br$_2$$\pi_u$ orbital. c,d, Evolution of the high-momentum enhancement features as a function of the effective ion charge of the bond-breaking system, shown for the c, transverse and d, parallel directions.