Magnetic circular dichroism spectra for colloidal gold nanoparticles in xerogels at 5.5 K

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.