Nonlinear four-wave mixing processes are a powerful technique to unravel ultrafast dynamics in solid-state systems. Here, we employ attosecond four-wave mixing spectroscopy with one extreme ultraviolet (XUV) pump and two independently delayed, noncollinear near-infrared (NIR) probes to resolve the ultrafast decoherence of both dipole-allowed and dipole-forbidden core excitons at the ${\mathrm{Na}}^{+}{\mathrm{L}}_{2,3}$ edge in sodium fluoride (NaF). The decoherence times of the core excitons are observed to be much faster than the 8 fs limit of the instrument response time, which is attributed to strong exciton–phonon coupling. Furthermore, polarization control of the NIR probes (perpendicular and parallel polarizations) reveals that the bright core excitons exhibit $s$-like orbital angular momentum, while dark core excitons, reached by two-photon excitation, exhibit $p$-like orbital angular momentum.

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• Excitons
• Insulators
• Attosecond laser spectroscopy
• Nonlinear optical techniques