The lattice expansion and relaxation of noble-metal nanoparticles heated by intense femtosecond laser pulses are measured by pump-probe time-resolved X-ray scattering. Following the laser pulse, shape and angular shift of the (111) Bragg reflection from crystalline silver and gold particles with diameters from 20 to 100 nm are resolved stroboscopically using 100 ps X-ray pulses from a synchrotron. We observe a transient lattice expansion that corresponds to a laser-induced temperature rise of up to 200 K, and a subsequent lattice relaxation. The relaxation occurs within several hundred picoseconds for embedded silver particles, and several nanoseconds for supported free gold particles. The relaxation time shows a strong dependence on particle size. The relaxation rate appears to be limited by the thermal coupling of the particles to the matrix and substrate, respectively, rather than by bulk thermal diffusion. Furthermore, X-ray diffraction can resolve the internal strain state of the nanoparticles to separate non-thermal from thermal motion of the lattice.

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