Radiative cooling of free metal clusters

The laser induced optical emission of Tungsten, Niobium, and Hafnium clusters generated via standard laser vaporization technique is studied in the wavelength range from 440 up to 830 nm for different size distributions. The emission shows a broad structureless continuum similar to a black-body radiator. Using the emissivity of a small spherical particle given by the Mie theory together with the Rayleigh approximation and Planck's law, the experimental spectra can be reproduced and thus the temperature of the particle can be deduced. Varying the delay between excitation and light detection, the time evolution of the cluster temperature can be obtained in detail. The cooling from initially close to 4000 K down to about 2900 K can be detected. As expected, the cooling behavior shows a dramatic clusters size dependence and will be analyzed for times up to 20 mu s after the excitation.