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Abstract

Transition metals, with their densely confined and strongly coupled valence electrons, are key constituents of many materials with unconventional properties¹, such as high-temperature superconductors, Mott insulators and transition metal dichalcogenides². Strong interaction offers a fast and efficient lever to manipulate electron properties with light, creating promising potential for next-generation electronics^3,4,5,6. However, the underlying dynamics is a hard-to-understand, fast and intricate interplay of polarization and screening effects, which are hidden below the femtosecond timescale of electronic thermalization that follows photoexcitation⁷. Here, we investigate the many-body electron dynamics in transition metals before thermalization sets in. We combine the sensitivity of intra-shell transitions to screening effects⁸ with attosecond time resolution to uncover the interplay of photo-absorption and screening. First-principles time-dependent calculations allow us to assign our experimental observations to ultrafast electronic localization on d orbitals. The latter modifies the electronic structure as well as the collective dynamic response of the system on a timescale much faster than the light-field cycle. Our results demonstrate a possibility for steering the electronic properties of solids before electron thermalization. We anticipate that our study may facilitate further investigations of electronic phase transitions, laser–metal interactions and photo-absorption in correlated-electron systems on their natural timescales.