Electron thermalization and relaxation in laser-heated nickel by few-femtosecond core-level transient absorption spectroscopy

Abstract

Direct measurements of photoexcited carrier dynamics in nickel are made using few-femtosecond extreme ultraviolet (XUV) transient absorption spectroscopy at the nickel M 2,3 edge. It is observed that the core-level absorption line shape of photoexcited nickel can be described by a Gaussian broadening (σ ) and a red shift (ω s ) of the ground-state absorption spectrum. Theory predicts and the experimental results verify that after initial rapid carrier thermalization, the electron temperature increase ( T ) is linearly proportional to the Gaussian broadening factor σ , providing quantitative real-time tracking of the relaxation of the electron temperature. Measurements reveal an electron cooling time for 50 nm thick polycrystalline nickel films of 640 ± 80 fs. With hot thermalized carriers, the spectral red shift exhibits a power-law relationship with the change in electron temperature of ω s ∝ T 1.5 . Rapid electron thermalization via carrier-carrier scattering accompanies and follows the nominal 4-fs photoexcitation pulse until the carriers reach a quasithermal equilibrium. Entwined with a <6 fs instrument response function, carrier thermalization times ranging from 34 fs to 13 fs are estimated from experimental data acquired at different pump fluences and it is observed that the electron thermalization time decreases with increasing pump fluence. The study provides an initial example of measuring electron temperature and thermalization in metals in real time with XUV light, and it lays a foundation for further investigation of photoinduced phase transitions and carrier transport in metals with core-level absorption spectroscopy.

Femtosecond Resolution of the Nonballistic Electron Energy Transport in Warm Dense Copper

“…This hypothesis might be highly questionable with laser fluence at the mJ=cm 2 level [45]. Nevertheless thermalization times shorter than 10 fs are expected in the high flux range investigated here (Q abs ¼ 0.75 AE 0.25 J=cm 2 ) [46,47] and support such assumptions. Other physical coefficients are directly involved in the diffusive electron transport equation C e ∂T e =∂t ¼ ⃗ ∇ðK e ⃗ ∇T e Þ.…”

Using the photoinduced L3 resonance shift in Fe and Ni as time reference for ultrafast experiments at low flux soft x-ray sources

“…Resonant X-ray absorption in 3d-transition metals can change immediately upon photo-excitation by an ultrashort laser pulse -even on a sub-fs scale -as recently shown in Ti [34] and Ni [35] in the EUV spectral range. In the soft X-ray spectral range, transient changes especially of the L 3 absorption resonance have been observed and studied [32,[36][37][38].…”

Drude-Lorentz Model for Optical Properties of Photoexcited Transition Metals under Electron-Phonon Nonequilibrium

“…The metallic elements used in this study are representative to capture the electronic and optical characteristics for a broad variety of transition metals. During the laser excitation, the electrons thermalize and redistribute almost instantly (on the femtosecond time scale) within the DOS [4,31]. The d-block is affected by laser-induced Fermi smearing of electrons extending the energy range available for optical transitions and thus strongly impacting the permittivity.…”