Non-thermal melting in semiconductors measured at femtosecond resolution

Ultrafast time-resolved optical spectroscopy has revealed new classes of ph ysical(1), chemical(2) and biological(3) reactions, in which directed, dete rministic motions of atoms have a key role. This contrasts with the random, diffusive motion of atoms across activation barriers that typically determ ines kinetic rates on slower timescales. An example of these new processes is the ultrafast melting of semiconductors, which is believed to arise from a strong modification of the inter-atomic forces owing to laser-induced pr omotion of a large fraction (10% or more) of the valence electrons to the c onduction band(1,4-12). The atoms immediately begin to move and rapidly gai n sufficient kinetic energy to induce melting-much faster than the several picoseconds required to convert the electronic energy into thermal motions( 13). Here we present measurements of the characteristic melting time of InS b with a recently developed technique of ultrafast time-resolved X-ray diff raction(14-19) that, in contrast to optical spectroscopy, provides a direct probe of the changing atomic structure. The data establish unambiguously a loss of long-range order up to 900 Angstrom inside the crystal, with time constants as short as 350 femtoseconds. This ability to obtain the quantita tive structural characterization of non-thermal processes should rnd widesp read application in the study of ultrafast dynamics in other physical, chem ical and biological systems.