Attosecond XUV probing of vibronic quantum superpositions in Br$_2^+$

TL;DR

This study uses attosecond transient absorption spectroscopy to observe and analyze the coupled electronic and vibrational quantum dynamics in Br$_2^+$, revealing how nuclear motions influence electronic coherences on ultrafast timescales.

Contribution

It provides the first detailed experimental and theoretical investigation of vibronic quantum superpositions in Br$_2^+$, highlighting nuclear effects on electronic coherence in molecules.

Findings

Electronic coherences decay, revive, and shift frequency due to nuclear motions.

Quantum simulations link decoherence to phase broadening of nuclear wave packets.

Vibronic structure manifests as discrete frequency progressions in electronic beat signals.

Abstract

Ultrafast laser excitation can create coherent superpositions of electronic states in molecules and trigger ultrafast flow of electron density on few- to sub-femtosecond time scales. While recent attosecond experiments have addressed real-time observation of these primary photochemical processes, the underlying roles of simultaneous nuclear motions and how they modify and disturb the valence electronic motion remain uncertain. Here, we investigate coherent electronic-vibrational dynamics induced among multiple vibronic levels of ionic bromine (Br$_2^+$), including both spin-orbit and valence electronic superpositions, using attosecond transient absorption spectroscopy. Decay, revival, and apparent frequency shifts of electronic coherences are measured via characteristic quantum beats on the Br-$3d$ core-level absorption signals. Quantum-mechanical simulations attribute the observed electronic decoherence to broadened phase distributions of nuclear wave packets on anharmonic potentials. Molecular vibronic structure is further revealed to be imprinted as discrete progressions in electronic beat frequencies. These results provide a future basis to interpret complex charge-migration dynamics in polyatomic systems.