Laser-induced optical-emission studies of niobium clusters

The laser-induced optical emission of niobium clusters generated via, the standard laser-vaporization technique is studied in the wavelength range from 350 to 750 nm. Particles with a radius of approximately 4 nm were heated by pulsed Nd:YAG laser operating at the fourth harmonic at 266 nm. The emission spectra are recorded as a function of delay time after the excitation and show a broad structureless continuum similar to blackbody radiation. Using the emissivity of a small spherical particle given by the Mie theory together with Planck's law the experimental spectra can be reproduced quite well by varying the temperature of the particle. Thus temperatures can be deduced from the spectra. Increasing delay time leads to decreasing temperature of the particles which can be attributed to radiation cooling.