Ultraviolet–visible absorption spectra of the colloidal metallic elements
The ultraviolet–visible absorption spectra are given for 10 nm diameter colloidal particles of 52 of the metallic elements, calculated from the optical constants of the metals by means of Mie theory. For most of the elements the spectra cover the range 200–900 nm. Well resolved absorption bands are observed for colloidal Sc, Ti, V, Y, Cd, Eu, Yb, Hg and Th as well as for colloids of Cu, Ag, Au and the s-block metals. However, for the majority of the colloidal metallic elements in this size range there is only a continuous absorption in the visible range, rising to broad and poorly resolved absorption bands in the ultraviolet near 200 nm. The difference in the way that the spectra of colloidal particles of different metals change when the particle shape is varied from spherical to spheroidal is investigated systematically in the dipole approximation. This is achieved by means of contour plots of the absorbance cross-section for the particles vs. the real and imaginary parts of the dielectric function for the metals, and this method of investigation is extended also to hollow spherical particles. The results suggest that Ca, Sr, Ba, Eu, Yb, Th and possibly Sc, Ti, V and Y may merit experimental investigation as new metals for exhibiting surface-enhanced Raman scattering.