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arXiv:2512.16132

Resolving electronic evolution during bond dissociation

Tian Wang, Nida Haram, Zack Dube, Kyle A. Hamer, Yonghao Mi, Fatemeh Karimi, Andrei Yu. Naumov, Giulio Vampa, Caterina Vozzi, Xiaojun Liu, Albert Stolow, Michael Schuurman, Nicolas Douguet, David Villeneuve, Paul B. Corkum, Andre Staudte

physics.chem-phphysics.atom-phphysics.opticsquant-phSubmitted Dec 18, 2025• Announced Dec 19, 2025• Revised Sep 15, 2026 (v2)

v1 · Dec 18, 2025 v2 · Sep 15, 2026

Abstract

Coupled electronic and nuclear motions govern chemical reactions, yet resolving how electronic structure evolves during bond dissociation remains a central challenge. Here we investigate the photodissociation of Br2 using correlated photoelectron photoion coincidence measurements. A 400 nm pulse initiates dissociation, while strong field ionization probes the evolving molecular system. Coincident measurement of three dimensional photoion and photoelectron momenta provides simultaneous access to the internuclear separation and the accompanying electronic evolution. We identify multiple distinct stages of electronic evolution during bond dissociation. The reshaping of the ionizing molecular orbital occurs first, followed by redistribution and localization of the electronic charge density, and finally by the gradual decay of residual electronic coherence between the separating atomic centers. Between the molecular and atomic limits, we observe an intermediate bond-breaking state in which localized atomic character coexists with a partially delocalized electronic response. By combining correlated observables with semiclassical modelling, we resolve the temporal ordering of these coupled electronic and nuclear processes and determine their associated dynamical timescales. These results demonstrate how correlated momentum observables can disentangle different aspects of the molecular-to-atomic transition.

Comments: 21 pages, 4 figures

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Abstract Photoionization of a weakly bound van-der-Waals dimer, composed of two atomic speciesAandB, is studied theoretically. AtomAcan be ionized either directly or via an indirect two-center channel, where atomBis first resonantly photoexcited and, afterwards, transfers the excitation energy via long-range electron–electron correlations radiationlessly to atomA, causing its ionization. By taking the vibrational nuclear motion of the dimer during the process into account, we analyze the contributions to the photoionization cross section from bound and dissociative molecular states in the final A+ –Bsystem. For the specific example of a LiHe dimer we show (a) that substantial contributions to both ionization mechanisms result from dissociative Li ++ He states and (b) that the two-center pathway strongly dominates over the direct photoionization channel for photon energies close to one of the molecular LiHe resonances.

In a double-slit experiment with a bipartite system, the visibility of interference fringes depends on the availability of which-way information. Here, we report a Bell-like state of photoelectron and residual ion in the multiphoton dissociative ionization of the D

_2

molecule. Evidence for entanglement is provided by the correlated emission directions of photoelectron and ion, which is observed using a COLTRIMS reaction microscope. In the presence of this correlation, the holographic interference fringes contained in the photoelectron momentum distributions are suppressed, indicating the existence of which-way information. The interference pattern persists when a single ionic state is populated and thus no which-way information is available. The experimental observations and conclusions are supported by the numerical solution of the electronic-nuclear time-dependent Schrödinger equation. Our work demonstrates that coincidence spectroscopy of ions and electrons is a powerful method for studying fundamental concepts of quantum information science within the context of ultrafast light-matter interactions.

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Photoassociation in laser-assisted collisions of ultracold alkali atoms has been used recently to perform high-precision spectroscopy of very-long-range states. Rovibrational spectra close to the Na(3

^{2}

S)+

{\mathrm{Na}}^{\mathrm{*}}

(3

^{2}

{\mathit{P}}_{3/2}

) dissociation limit have been obtained by monitoring an enhanced

{\mathrm{Na}}_{2}^{+}

ion signal. We propose the use of pump-probe techniques to study the dynamics of the ionization process. At very large distances (typically 70 a.u.) two colliding Na atoms absorb a photon and form a coherent vibrational wave packet. We follow the packet in time and probe two possible mechanisms that are proposed to yield molecular ions: photoionization, where the second photon is absorbed at a small internuclear distance R directly ionizing the molecule, and associative ionization with excitation at intermediate R to the nuclear continuum of a doubly excited state followed by a half collision and autoionization at small R. The latter event can also lead to dissociation of the complex. The two mechanisms lead to distinct differences in the ion signals as a function of delay time between the pump and the probe pulses. Estimates of the ion rates to be expected in actual pump-probe experiments are given, which tend to favor the autoionization mechanism as the dominant reaction path.

Using high-order harmonic attosecond pulse trains, we investigate the photoionization dynamics and transient electronic structure of a helium atom in the presence of moderately strong (∼10(12) W cm(-2)) femtosecond laser pulses. We observe quantum interferences between photoexcitation paths from the ground state to different laser-dressed Floquet state components. As the intensity ramps on femtosecond time scales, we observe switching between ionization channels mediated by different atomic resonances. Using precision measurements of ion yields and photoelectron distributions, the quantum phase difference between interfering paths is extracted for each ionization channel and compared with simulations. Our results elucidate photoionization mechanisms in strong fields and open the doors for photoabsorption or photoionization control schemes.

Electronic decay processes provide a fascinating window into correlated electronic rearrangements occurring on ultrafast timescales. Following these dynamics in real time has become increasingly accessible experimentally, but extracting the underlying electronic dynamics from measured spectra requires understanding how nuclear motion shapes the observable signal. Here, we extend an analytical description of time-resolved electronic decay spectra to include dissociative nuclear dynamics and apply it to Interparticle Coulombic Decay (ICD) in the neon dimer. The resulting spectra reproduce the experimentally observed spectral shape and reveal an unexpectedly important role of interference between pathways involving different vibronic resonance states. We further establish a clear connection between the temporal build-up of the spectral structure and the nuclear wavepacket dynamics in the decaying electronic state. Most strikingly, our analytical expressions reveal that the time-dependent integrated ICD signal contains contributions proportional to both

\exp(-t/τ)

and

\exp[-t/(2τ)]

. Nevertheless, a conventional mono-exponential fit can describe the temporal signal remarkably well while yielding a decay lifetime that differs substantially from the underlying value. Applying the theoretically derived fitting model to experimental data for the neon dimer yields an ICD lifetime

τ

of 73 fs, rather than the previously extracted 150(50) fs, placing the experimental value within the range of previous theoretical predictions. Since the underlying temporal structure is common to electronic decay processes, our findings have implications for extracting lifetimes from time-resolved decay spectra well beyond ICD.