We report on the optical absorption properties of as prepared gold
naoparticles of different shapes and sizes measured by photoacoustic (PA) method.
The gold nanoparticles of two different shapes (dots, rods) have been prepared
using the seed mediated growth method. The shape and the size of these different
nanoparticles were determined by STM measurements. PA spectra show the
splitting of the surface plasmon resonance (SPR) into two modes (transverse and
longitudinal) in case of gold nanorods. The increase in the aspect ratio of the
nanorods leads to clear redshifts of the longitudinal SPR. These shifts were used
to determine the dielectric constant of the surrounding medium and its variation
with the aspect ratios.