Speaker
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 Br₂ using a femtosecond pump–probe scheme combined with COLTRIMS-based electron–ion coincidence detection [1]. Using 400 nm pump and 800 nm probe femtosecond pulses, we track the time-resolved evolution of molecular fragmentation through both electronic and structural observables. Our experiment provides a direct, molecular-frame view of how electron correlation, orbital coherence, and ionic polarization influence dissociation dynamics on the few-femtosecond scale.
The initial 400 nm excitation populates the dissociative C-state of Br₂, launching a neutral dissociation process [2]. A delayed 800 nm pulse ionizes the evolving fragments at controlled delay times, allowing us to correlate the kinetic energy release (KER) with photoelectron momentum distributions. The use of COLTRIMS enables full three-dimensional momentum imaging in coincidence, revealing both ionization dynamics and internuclear distance evolution with sub-cycle temporal resolution [3].
The measured photoelectron distributions show striking delay-dependent features, including angular asymmetries, ATI ring modulation, and high-momentum enhancements that encode coherence and polarization effects tied to the evolving charge environment. These features reflect the progressive change in parent-ion polarizability and the transition from multi-center (molecular) to single-center (atomic) scattering regimes. Semiclassical two-step (SCTS) modeling and time-dependent density functional theory (TDDFT) simulations confirm that tunnel ionization probes a superposition of molecular and atomic polarization channels whose relative contributions evolve during bond breaking.
We find an approximately 50 fs temporal offset between the completion of electronic orbital reconfiguration and structural dissociation, indicating that electron dynamics precede and drive nuclear rearrangement. This decoupling of electronic and nuclear observables is a hallmark of correlated dynamics and emphasizes the need for complete measurements, specifically ion–electron coincidence in the molecular frame, to determine true reaction timescales.
Our results demonstrate how ultrafast correlation, ionic polarization, and multielectron coherence shape molecular dissociation and strong-field ionization pathways. This study exemplifies the power of multi-observable approaches for uncovering coherence and correlation in the interaction of intense laser fields with complex molecular systems.
References
[1] T. Wang et al., manuscript submitted (2026)
[2] W. Li et al., 2010, PNAS 107, 20219
[3] J. Ullrich et al., 2003, Rep. Prog. Phys. 66, 1463
| Keyword-1 | Ultrafast molecular dynamics |
|---|---|
| Keyword-2 | Electron–nuclear coupling |
| Keyword-3 | Photodissociation |