Developing a time-dependent, element-specific understanding of electronic (de)localization behavior, coherent wavepacket evolution, and coherence dephasing processes is an important pre-requisite for tailoring energy flow at the atomic level – on the natural length and time scales of electronic motion. Nonlinear wave-mixing spectroscopies that use ultrafast extreme ultraviolet (XUV) and X-ray pulses are promising approaches to directly measuring complex electronic dynamics in chemical systems occurring on few-femtosecond timescales. A relatively new technique, attosecond FWM spectroscopy uses one XUV pulse obtained by high harmonic generation and two separately timed few-cycle near-infrared (NIR) pulses arranged non-collinearly to generate a spatially distinct background-free emission while maintaining attosecond temporal precision. This study provides the first evidence that tabletop-based non-collinear attosecond four-wave mixing (FWM) spectroscopy can measure electronic coherence dephasing timescales in solid-state materials. The dynamics of the Na+ L2,3 edge core-excitons in NaCl (around 33.5 eV) are characterized. A “core”-exciton is formed by the excitation of a Na+ 2p electron predominantly to a Na+ 3s orbital, leaving a Coulombically bound hole in the Na+ core that traps the electron. This Na+ excitation is highly-localized and nearly Rydberg-atom-like, while embedded in the NaCl crystal lattice. In the time-resolved FWM experiments on NaCl, a greater multiplicity of states is observed to underlie the basic spin-orbit split characteristic doublet of the Na L2,3 edge core-exciton states. An inhomogeneous distribution of core-excitons that underlie this well-known doublet absorption is deconvoluted by exploiting the resonance-enhanced, time-dependent and phase-matched nonlinear FWM signals. Collectively, at least five distinct core-excitonic states are characterized spectrally and temporally by probing the time-dependence of both XUV-bright core-excitonic states and XUV-dark core-excitonic states. In particular, the unique ability of attosecond FWM spectroscopy to separately probe the effects of XUV-dark states enables the inhomogeneous distribution of core-excitonic states to be measured. Approximate sub-10 femtosecond coherence lifetimes of the core-exciton states are extracted. The observed dynamics are discussed in the context of electron-hole exchange interactions, electron-electron correlation, and electron-phonon broadening in this solid.

Localized core-exciton dynamics in NaCl measured by tabletop attosecond four-wave mixing spectroscopy

ACS Fall 2021

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