Observation of strong optical absorption at the surface of small particles

Abstract

Experiments are presented in which strong optical absorption at the surface of small metal particles is been observed. This was accomplished by the following method. Na clusters with rough surfaces were prepared on a transparent substrate. They provide the possibility of localized electronic excitations in the surface region in addition to the well-known plasmon-polaritons in the bulk. The particles were irradiated with pulsed laser light and their temperature rise was derived from measurements of the kinetic energy distribution of thermally desorbed Na dimers. From the temperature increase, the heat generated in the particles during decay of both surface and bulk excitations was calculated and taken as a measure of the total absorption cross-section. Separation of the bulk and surface contributions was achieved by repeating the experiments after removal of the surface roughness by annealing. We find that the surface contributes as much as 32% to the total optical absorption at 2 = 532 nm for Na clusters of a mean radius R of 21 mm.

Local Field Effects on Laser-Induced Heating of Metal Nanoparticles

“…More accurate experiments, separing these two aspects, should be performed in order to estabilizh which of the two effects (surface or collective effects) plays the leading role. The hypothesis that a detailed description of the surface is necessary to understand the photothermal properties of nanoparticles is supported by previous experimental results, even for slightly different systems . However, our calculations suffer from several drawbacks: as a first point, the mathematical description of heat flow diffusion eq is valid only if the mean free path of the heat carrier is smaller than the characteristic particle size.…”

“…The hypothesis that a detailed description of the surface is necessary to understand the photothermal properties of nanoparticles is supported by previous experimental results, even for slightly different systems. 31 However, our calculations suffer from several drawbacks: as a first point, the mathematical description of heat flow diffusion eq 2 is valid only if the mean free path of the heat carrier is smaller than the characteristic particle size. For gold metal the electron mean free path l is about 20 nm 32 and in a metal nanoparticle it is widely accepted that the effective electron mean free path l eff is given by (1)/(l eff ) ) (1)/(R) + (1)/(l).…”

Photochemistry on Metal Nanoparticles

“…Träger and co-workers reported time-of-flight distributions of Na dimers desorbed by laser (532 nm) heating of Na NPs on a quartz substrate. , They estimated the temperature increase from the peak times in the distributions. By plotting the temperature increase as a function of laser fluence, they obtained a slope of 49 K cm -2 mJ -1 for Na NPs as prepared and of 27 K cm -2 mJ -1 for annealed NPs . The difference was attributed to the contribution from the absorption at the surface.…”

“…By plotting the temperature increase as a function of laser fluence, they obtained a slope of 49 K cm -2 mJ -1 for Na NPs as prepared and of 27 K cm -2 mJ -1 for annealed NPs. 165 The difference was attributed to the contribution from the absorption at the surface. Using these data it is possible to extrapolate the temperature rise at small laser fluence.…”

Quantum-size colloid metal systems

“…Absorption on the surface of small particles has been observed experimentally; 220 for this purpose, a special method for the separation of surface and bulk absorption was used. Metal particles shown 221 to have a smooth surface and surface electronic states were applied on a transparent substrate from the vapour phase at 100 K. The particles were exposed to pulse laser radiation, which excited the electrons on the surface and in the bulk.…”