Photophysics and spectroscopy of metal particles

Abstract

This paper describes our recent work using ultrafast laser spectroscopy to examine the fundamental properties of metal particles. Two studies are presented. First, the characteristic time scale for electron-phonon coupling in Au particles with sizes between 2 and 120 nm has been examined by bleach recovery measurements. These experiments show that the coupling between the electrons and phonons is independent of particle size, to within the signal-tonoise of our experiments. We have also used transient absorption spectroscopy to examine the low-frequency "breathing" modes of the Au particles. These modes are impulsively excited by the rapid lattice heating that accompanies electron-phonon coupling. The breathing motion contributes to the transient absorption signal because the position of the plasmon band depends on the free electron density and, thus, the particle volume. The measured frequencies are inversely proportional to the radius, and almost exactly match the predictions of classical mechanics calculations for Au.

Photochemistry and Electrochemistry of Nanoassemblies

“…The recovery of the plasmon band on the picosecond timescale arises from the electron-phonon and phonon-phonon relaxation. Similar transient bleaching has also been observed for silver nanoparticles [66,73] and bimetallic particles [74][75][76]. The bleaching recovery of the pristine Au colloids consists of a fast (t ¼ 2:5 ps) and a slow process (t > 50 ps) [71].…”

Plasmonic coupling in noble metal nanostructures

“…Ultrafast electron dynamics measurements using pump-probe techniques have shown that excited electrons in metal nanoparticles thermalize very fast (within 500 fs) to a hot Fermi distribution followed by electron cooling via electron-phonon collisions ($1 ps) and slower cooling of the nanoparticle lattice via phonon-phonon coupling ($100 ps) [2,41,[78][79][80][81]. The hot-electron cooling rate in gold nanoparticles has been found to be $1 ps and is unaffected by changes in the size or shape of the nanoparticles [41].…”

Photophysics of Nanometer Sized Metal Particles: Electron−Phonon Coupling and Coherent Excitation of Breathing Vibrational Modes

“…In Situ Measurements of the Particle Size Distribution. The successful use of time-resolved spectroscopy to analyze thin films suggests that our experiments might be useful for measuring particle size distributions . In fact, there are several advantages of these experiments compared to TEM or STM measurements.…”