26th Australian Institute of Physics Congress and the Australian and New Zealand Conference on Optics and Photonics

Watching a molecular bond break

Not scheduled

20m

Belinda Hutchinson Building (The University of Sydney )

Contributed Oral AIP | Atomic and Molecular Physics (ATMOP)

Speaker

Dr Nida Haram (Joint Attosecond Science Lab, National Research Council and University of Ottawa, ON K1A 0R6, Canada)

Description

Polarization and correlation effects in molecular photodynamics are intimately tied to the coupled motion of electrons and nuclei. Understanding these interactions on their natural femtosecond timescales requires measurements that resolve both electronic structure and nuclear configuration simultaneously. Here, we present a complete imaging study of the UV-initiated dissociation of $\text{Br}_2$ using a femtosecond pump-probe scheme combined with COLTRIMS-based electron–ion coincidence detection [1]. Using 400 nm pump and 800 nm probe pulses, we track the time-resolved evolution of molecular fragmentation through both electronic and structural observables, providing a direct, molecular-frame view of how electron correlation, orbital coherence, and ionic polarization influence dissociation dynamics at the few-femtosecond scale.The 400 nm excitation populates the dissociative $C$-state of $\text{Br}_2$, launching a neutral dissociation process [2]. A delayed 800 nm probe pulse ionizes the evolving fragments, allowing us to correlate the kinetic energy release with photoelectron momentum distributions. The use of COLTRIMS enables full 3D momentum imaging, revealing ionization dynamics and internuclear distance evolution with sub-cycle temporal resolution [3].Measured photoelectron distributions exhibit striking delay-dependent features—angular asymmetries, ATI ring modulation, and high-momentum enhancements—that encode coherence and polarization effects. These features reflect the progressive change in parent ion polarizability and the transition from multi-center to single-center scattering regimes. Our semiclassical two-step model and TDDFT simulations confirm that tunnel ionization probes a superposition of molecular and atomic polarization channels. We find a $\sim 50$ fs temporal offset between electronic orbital reconfiguration and structural dissociation, indicating that electron dynamics precede and drive nuclear rearrangement. These results emphasize the necessity of molecular-frame coincidence measurements to determine true reaction timescales in correlated systems.References[1] T. Wang et al., in preparation.[2] W. Li et al., PNAS 107, 20219 (2010).[3] J. Ullrich et al., Rep. Prog. Phys. 66, 1463 (2003).