Local fields reveal atomic-scale nonadiabatic carrier-phonon dynamics

Understanding nonadiabatic carrier-lattice interactions at the atomic scale remains a fundamental challenge, yet these processes govern energy transfer in materials and ultimately set limits in microelectronics. We combined attosecond core-level transient absorption spectroscopy with many-body theory to uncover how nonadiabatic electron-phonon coupling drives ultrafast relaxations in a titanium-carbide MXene. Phonon-driven changes in carrier localization modulated local field effects (LFEs), yielding carrier-, site-, and orbital-specific absorption signatures. LFEs served as sensitive fingerprints of electron-phonon coupling strength across the phonon spectrum and revealed a breakdown of the Born-Oppenheimer approximation: Electrons lagged lattice oscillations by 32 ± 8 femtoseconds, whereas holes responded almost instantaneously (7 ± 7 femtoseconds). Our results establish a framework for probing and controlling nonadiabatic carrier-phonon interactions with orbital and site specificity.