Short, intense laser pulses can be used to access the transition regime between classical and quantum optical responses in dielectrics. In this regime, the relative roles of inter- and intraband light-driven electronic transitions remain uncertain. We applied attosecond transient absorption spectroscopy to investigate the interaction between polycrystalline diamond and a few-femtosecond infrared pulse with intensity below the critical intensity of optical breakdown. Ab initio time-dependent density functional theory calculations, in tandem with a two-band parabolic model, accounted for the experimental results in the framework of the dynamical Franz-Keldysh effect and identified infrared induction of intraband currents as the main physical mechanism responsible for the observations.

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Transition metals with their densely confined and strongly coupled valence electrons are key constituents of many materials with unconventional properties 1 , such as high-Tc superconductors, Mott insulators and transition-metal dichalcogenides 2 . Strong electron interaction offers a fast and efficient lever to manipulate their properties with light, creating promising potential for next-generation electronics 3-6 . However, the underlying dynamics is a fast and intricate interplay of polarization and screening effects, which is poorly understood. It is hidden below the femtosecond timescale of electronic thermalization, which follows the light-induced excitation 7 . 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 effects 8 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 whole 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 prior to electron thermalization, suggesting that the ultimate speed of electronic phase transitions is limited only by the duration of the controlling laser pulse. Furthermore, external control of the local electronic density serves as a fine tool for testing state-of-the art models of electron-electron interactions. We anticipate our study to facilitate further investigations of electronic phase transitions, laser-metal interactions and photo-absorption in correlated electron systems on its natural timescale.A characteristic thermalization time of laser-excited hot electrons in solids is in the femtosecond regime and becomes faster with stronger electron interactions 7 . Therefore, attosecond time resolution is required to resolve coupled electron dynamics during the lasermatter interaction before electron thermalization has occurred. Attosecond transient absorption spectroscopy revealed electric-field-guided electron dynamics in simple dielectrics and semiconductors 9-14 . However, transient absorption studies of localized and strongly interacting electrons, such as on d-and f-orbitals of transition metals, have been limited to the fewfemtosecond regime 15 . Transition metal elements are the key constituents of many materials exhibiting remarkable properties, such as Mott insulators, high-Tc superconductors and transition metal dichalcogenides 2 . Understanding the coupled-electron dynamics in these systems is central for their applications in optoelectronics, energy-efficient electronics, magnetic-memory devices, spintronics and new solar cells 16 . Transition metal elements were studied with attosecond photoemission spectroscopy [17][18][19] . However, state-of-the-art theoretical

What is the spatiotemporal limit of a macroscopic model that describes the optoelectronic interaction at the interface between different media? This fundamental question has become relevant for time-dependent photoemission from solid surfaces using probes that resolve attosecond electron dynamics on an atomic length scale. We address this fundamental question by investigating how ultrafast electron screening affects the infrared field distribution for a noble metal such as Cu (111) at the solid-vacuum interface. Attosecond photoemission delay measurements performed at different angles of incidence of the light allow us to study the detailed spatiotemporal dependence of the electromagnetic field distribution. Surprisingly, comparison with Monte Carlo semiclassical calculations reveals that the macroscopic Fresnel equations still properly describe the observed phase of the IR field on the Cu(111) surface on an atomic length and an attosecond time scale.

We present a combined experimental and computational study of the relaxation dynamics of the ethylene cation. In the experiment, we apply an extreme-ultraviolet-pump/infrared-probe scheme that permits us to resolve time scales on the order of 10 fs. The photoionization of ethylene followed by an infrared (IR) probe pulse leads to a rich structure in the fragment ion yields reflecting the fast response of the molecule and its nuclei. The temporal resolution of our setup enables us to pinpoint an upper bound of the previously defined ethylene-ethylidene isomerization time to 30 ± 3 fs. Time-dependent density functional based trajectory surface hopping simulations show that internal relaxation between the first excited states and the ground state occurs via three different conical intersections. This relaxation unfolds on femtosecond time scales and can be probed by ultrashort IR pulses. Through this probe mechanism, we demonstrate a route to optical control of the important dissociation pathways leading to separation of H or H2.