Femtosecond time-resolved X-ray diffraction is used to probe reversible lattice changes of a 150-nm Au(1 11) crystal on mica heated with 100-femtosecond laser pulses with fluence below the damage and melting threshold. The lattice strain generated throughout the crystal is larger than estimated from the inhomogeneous lattice temperature distribution and acoustic wave produced by nonequilibrium heating. The broadening and shift of the rocking curve observed across the depth of the crystal support the presence of an additional 'blast' wave arising from hot electron gas temperature gradient, which is observed experimentally for the first time. (c) 2005 Elsevier B.V. All rights reserved.