Speaker
Description
The coupled motion of electrons and nuclei is central to understanding fundamental processes in molecules. Here, we investigate the ultrafast photodissociation of bromine (Br$_2$) using a femtosecond pump-probe scheme [1]. In our experiment a weak 400 nm pulse initiates dissociation along the neutral C-state, followed by an 800 nm probe pulse that ionizes the evolving fragments at variable delay. We detect the three-dimensional momenta of both photoions and photoelectrons in coincidence using COld Target Recoil Ion Momentum Spectroscopy (COLTRIMS) [2], in order to correlate the internuclear distance with the transition from a molecule to two separate atoms.
Positive delays mean that the intense, 800 nm probe pulse arrives after the pump pulse. Coulomb explosion is used to infer the internuclear distance in the single ionization channel which is correlated to the photoelectron momentum distributions.
Our measurements reveal a clear transition from molecular-like ionization channels at short pump-probe delays to atom-like channels at longer delays. The extracted photoelectron distributions show the evolution of valence orbitals from Br$_2$-centered wavefunctions to Br atomic orbitals as the internuclear distance increases. Numerical simulations using semiclassical two-step (SCTS) modeling and time-dependent density functional theory corroborate that a redistribution of ion core charge drives these characteristic momentum features [3].
By comparing ion kinetic energy release with the molecular-frame photoelectron momentum distributions, we find that the electronic evolution precedes the full separation by about 50 fs, emphasizing the significance of electron-nuclear coupling [1]. These findings highlight the need for multi-observable approaches to disentangle parallel electronic and structural rearrangements, thereby clarifying benchmark timescales for chemical bond cleavage.
References
[1] W. Li et al. 2010 PNAS 107 20219
[2] J. Ullrich et al. 2003 Rep. Prog. Phys. 66 1463
[3] T. Wang et al. 2025 in preparation
