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[Phys-l] BuckyBall Resonance Absorption (was Nano-Particle: Color)



Tony wrote:
I was just reading that when the particles of gold are 100nm it appears
yellow. But when the particles are 25nm, the color changes to red. This is
the way medieval artisans made red stained glass. My question is by what
mechanism do these smaller particles produce this color?
-Tony
Following John D's suggestion to Google with "plasmon" for a more pertinent story on the
differential absorption of light by small particles with mobile electrons in glass, I came on this note with amusing news of an effect looking for a story, and a story looking for an effect at Berkeley at this URL:

http://newscenter.lbl.gov/feature-stories/2005/05/13/making-the-buckyballs-ring/

...Ron Phaneuf visited his collaborators in Germany, where Alfred Müller presented the results at a workshop in Berlin. A group of theorists from the Max Planck Institute for Complex Systems in Dresden was in attendance; on the basis of theoretical calculations, Jan-Michael Rost, Himadri Chakraborty, and Mohamed Elamine Madjet had predicted a higher-energy resonance than the known 22 eV giant resonance in C-60, but because of a lack of experimental evidence they had not published their prediction.

“We had an experiment looking for an explanation and they had an explanation looking for an experiment,” says Phaneuf. “It was a scientific marriage made in heaven.”

When stimulated by photons at an energy of about 20 electron volts, a buckyball displays collective electron motion as a surface plasmon. But when stimulated by photons of about 40 eV, the result is a different mode of collective electron motion, a volume plasmon.

The second resonance in C-60, occurring at a photon energy of 38 eV, is called a volume plasmon — not a back-and-forth oscillation of the valence electron cloud but rather an in-and-out contortion, like squeezing a beach ball. Such collective motion would be impossible if C-60 were a solid sphere instead of a hollow charged shell; for this reason it is not been observed in metal clusters like gold nanoparticles, where surface plasmons are common.

When a 22-eV photon smacks into a singly charged buckyball having 239 outer electrons, often the whole spherical cloud surrounding the cage structure oscillates with enough energy to eject another electron: this is photoionization. The same thing happens when a 38-eV photon smacks into a charged buckyball, except that the electron cloud wobbles in and out, penetrating the cage — a phenomenon unique to charged buckminsterfullerenes. Like hitting a big bronze bell with a clapper, it’s a way to make the buckyballs ring.

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