Sivakumar et al. Nanoscale Research Letters 2011, 6:78 http://www.nanoscalereslett.com/content/6/1/78 Page 3 of 5 Figure 2 TEM-EDX analysis of the nanoparticles in the aggregate structure. showed broadened peaks of Au 4f7/2 and Au 4f5/2 lines. Deconvolution of these lines showed the presence of two peaks in each line. For instance, the peaks of Au 4f7/2 line appear at 85.33 eV, which is a characteristic of metallic gold, and at 83.9 eV, 1.43 eV higher in BE can be ascribed to silicide [21-23]. Moreover, the 83.9 eV peak is due to the interaction of gold with silicon at the interface [24] by laser irradiation, resulting the forma- tion of gold silicide in the nanoparticles aggregate. The presence of elemental gold with laser-irradiated samples is due to untreated areas of the sample around the laser spot above which nanoparticles aggregate was formed. Since the thickness of gold layer deposited on sample surface is about 200 to 400 nm, XPS analysis of untreated sample has not showed the silicon peak. However, investigations on the laser-irradiated sample surface revealed the presence of Si 2p line. The decon- voluted spectrum of the laser-treated sample is shown in Figure 5. The peaks are due to silicon nanoparticles in the aggregate structure. Irradiation of metal films using femtosecond laser radiation results in fast nonequilibrium processes such as laser melting and film disintegration [7,25]. The energy from laser radiation is absorbed by the conduc- tion band electrons and results in a sharp increase in electronic temperature near the irradiated front sur- face. Since the heat capacity of electrons in a metal is much smaller than that of lattice, an ultrashort laser pulse can heat electrons to a very high temperature while leaving the lattice relatively cool. The fast tem- perature-dependent electron heat conduction leads to the redistribution of the deposited energy within the film. This process occurs simultaneously with a more gradual energy transfer from electrons to the lattice vibrations due to electron-phonon coupling. The time duration of energy transfer and the ensuing equili- brium processes depends on electron-phonon coupling in gold. For Au, the coupling is 2.1 × 1016 W/m3 K, and the energy is transferred to the lattice within 15 ps. The equilibrium between hot electrons and Figure 3 X-ray diffractograms of both treated and untreated samples. The peak at 39.7° in the treated sample is attributed to the Au-Si alloy phase. Figure 4 XPS spectra of untreated and laser treated samples for Au 4f system.