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This report describes the first demonstration of thermal diffusivity measurements using picosecond transient thermoreflectance (TTR). Although previously reported methods of measuring thermal transport properties of thin films require precise knowledge of the thermal properties of the substrate, this technique allows measurements on films as thin as 100 nm without any evidence of substrate interaction. The TTR measurement is modeled with a one-dimensional heat flow equation using a two-parameter fitting routine to determine the thermal diffusivity. The validity of our approach is confirmed by the TTR measured thermal diffusivity of single crystal nickel. We have also measured the thermal diffusivity of sputtered and evaporated single element metal films. Preliminary results from TTR measurements on compositionally modulated structures are also presented.

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Picosecond laser pulses are used to sequentially heat and probe the thermally induced reflectivity transients in copper, and the nonequilibrium heating of electrons to a temperature above that of the lattice is observed. The ability to temporally resolve electronic and lattice heating contributions is of considerable significance to thermomodulation spectroscopy of solids. PACS numbers: 72.15.Eb, 65.90.+ i, 78.20.Nv Thermomodulation spectroscopy is an important method for studying the critical points associated with the interband optical absorption in solids/ The early work of Scouler^ demonstrated the significance of this technique to bandstructure studies in metals, and the use of this technique continues through the present day.^The traditional approach to a thermomodulation measurement is to pass a low-frequency (^50 Hz) alternating current through an optically thin (-40 nm) sample film, and synchronously detect the corresponding heating-induced reflectivity change as a function of optical wavelength. Temperature changes of -10 K result in reflectivity changes on the order of AJR -lO""*. A variety of effects contribute to the reflectivity change, including thermal expansion, electron-photon interactions, and shifting of the Fermi level.^ Scouler^ and Rosei and Lynch'* were the first to observe a sign reversal of A/? when the photon energy was tuned through the

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We report the first time-resolved studies of image-potential states on Ag(lOO). Femtosecond ultraviolet-pump and visible-probe techniques are combined with two-photon photoemission spectroscopy to measure transient photoemission spectra with femtosecond resolution. The lifetime of the n = 1 image-potential state on Ag(100) is observed to be several tens of femtoseconds.

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