Fig. 1. Illustration of phonon-electron energy transfer in laser-excited nickel.

(A) Energy in the laser-excited electronic system (blue line) is transferred to phonon modes at the Brillouin zone (BZ) boundary (purple line) via electron-phonon (e-p) coupling (24). When the electronic energy becomes less than the energy in a particular phonon mode, p-e energy back-transfer can occur. (B) Calculated non-equilibrium energy distribution of BZ-boundary phonons (red: more; blue: less; see Materials and Methods and fig. S13) compared to equilibrium conditions (24). The purple arrow indicates a phonon momentum vector, q_p, in the BZ of nickel. (C) Measured photoemission intensity maps near the Fermi level, E_F, in the BZ of Ni within the L-W-U (green border) and Γ-K planes (blue border), respectively. White indicates higher and brown lower intensity. (D) Illustration of the change in electron occupation of states upon absorption of phonons with momentum, q_p, and scattering of electrons from states with momentum vector, k_e, to unoccupied states at k_e′. The vector diagram illustrates momentum conservation.