Ma. Zaitoun et al., Magnetic circular dichroism spectra for colloidal gold nanoparticles in xerogels at 5.5 K, J PHYS CH B, 105(29), 2001, pp. 6780-6784
Colloidal gold nanoparticles (similar to 25 nm) and Au-9(PPh3)(8)(3+) compl
ex ions (<2 nm) are separately encapsulated in optically transparent xeroge
ls. The high quality xerogel is found to be isotropic with no material stra
in or birefringence that would interfere with the magnetic circular dichroi
sm (MCD) measurements. The MCD and absorption spectra for the surface plasm
on band at 523 nm for similar to 25 nm gold nanoparticles and for the Au-9(
PPh3)(8)(3+) cluster complex ion in the spectral region of 600-400 nm are e
xamined over the temperature range from 5.5 to 295 K. The MCD spectra for t
he Au-9(PPh3)(8)(3+) ion (D-2h skeletal geometry) exhibit only B terms and
follow the expected temperature dependent patterns for allowed electronic t
ransitions and become better resolved at 5.5 K as compared to those at 295
K. The spectral features at 529, 448, and 402 nm have been assigned previou
sly with the aid of a simple MO scheme to transitions to spin-orbit states
of predominately triplet parentage. For colloidal gold, the observed MCD si
gnal is large, suggesting that both the magnetic and electric in moments of
the surface plasmon resonance are allowed. The MCD spectra for the surface
plasmon transition at 523 nm display an A-term-like feature. To within exp
erimental error, the MCD dispersion in the xerogel is temperature independe
nt between 5.5 and 295 K and, thus, represents the signature of an excited-
state phenomenon. It is further shown that the A-term-like dispersion of th
e MCD spectral feature does not originate from a single purely degenerate e
xcited state but rather from a strong intermixing of the excited spin-orbit
states.