Dynamical Study of Femtosecond-Laser-Ablated Liquid-Aluminum Nanoparticles Using Spatiotemporally Resolved X-Ray-Absorption Fine-Structure Spectroscopy

We study the temperature evolution of aluminum nanoparticles generated by femtosecond laser ablation with spatiotemporally resolved x-ray-absorption fine-structure spectroscopy. We successfully identify the nanoparticles based on the $L$-edge absorption fine structure of the ablation plume in combination with the dependence of the edge structure on the irradiation intensity and the expansion velocity of the plume. In particular, we show that the lattice temperature of the nanoparticles is estimated from the $L$-edge slope, and that its spatial dependence reflects the cooling of the nanoparticles during plume expansion. The results reveal that the emitted nanoparticles travel in a vacuum as a condensed liquid phase with a lattice temperature of about 2500 to 4200 K in the early stage of plume expansion.

Figure 1

Examples of spatially resolved soft-x-ray absorbance images of an aluminum plume. An absorbance image, ${\mu }_d$, is defined as ${\mu }_d=\ln (I_0/I)$, where $I_0$ and $I$ are the reference image without the plume absorption and the transmission spectrum with the plume absorption, respectively. (a) A temporal sequence of absorbance images at a laser irradiation intensity of $1.8\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$. (b) Laser intensity dependence of the absorbance image of the aluminum plume at a delay time of 30 ns. The unit of intensity in the figure is ${10}^{14}\mathrm{W}/{\mathrm{cm}}^2$.

Figure 2

Typical profiles of the near-edge absorbance spectrum of the aluminum nanoparticles at various distances from the surface and of solid aluminum with a thickness of 100 nm. Each profile is obtained by averaging the absorbance image in the $11\mu \mathrm{m}$ range at various distances. The laser irradiation intensity and the time delay are $1.8\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$ and 30 ns, respectively. The blue dotted line shows the result of fitting to the edge slope with an FD distribution function. The nanoparticle lattice temperature was estimated by deconvoluting the broadening of the spectral resolution approximated by a Gaussian function from the fitting result.

Figure 3

Lattice temperature of the nanoparticles estimated from an analysis of the absorption edge slope. The solid curves are an eye guide. (a) The spatial distribution of the nanoparticle temperature in the plume at a time delay of 30 ns for an irradiation intensity of $1.8\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$. (b) The temporal sequence of the nanoparticle temperature near the surface region from 0 to $11\mu \mathrm{m}$ for the same irradiation intensity.

Figure 4

The edge energy of the nanoparticles as a function of the distance from the surface. The laser irradiation intensity and the time delay are $1.8\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$ and 30 ns, respectively.