Cavity ring-down spectrometer for measuring the optical response of supported size-selected clusters and surface defects in ultrahigh vacuum

A cavity ring-down spectrometer designed to investigate optical properties of size-selected clusters on surfaces under ultrahigh vacuum (UHV) conditions is presented. Clusters are produced using a laser vaporization cluster source with typical size-selected cluster currents of about 100 pA. The size of the deposition area can be controlled by means of a focusing octopole. Using the UHV compatible mirror exchanger, it is possible to have up to ten ring-down cavities and to adjust them while in vacuum. With ten cavities it is possible to cover a continuous spectral range as broad as 600 nm. The sensitivity of the method is shown to be about 5 ppm, which is two orders of magnitude better than commercial techniques. The optical spectra of small Ni-N clusters (N=7, 10, and 20) supported at amorphous SiO2 in the range between 417 and 669 nm are presented. Simple Mie theory calculations and oscillator-strength sum rule are in good agreement with the experimental data. The method proved to be sensitive enough for detection of defect sites and therefore makes it possible to study the interaction between the clusters and different surface defects. Furthermore the cryogenic sample holder can be used to perform cavity ring-down spectroscopy at low temperatures.