Since a large range of particle sizes were analyzed, multipole terms
have also been taken into account throughout.
For the numerical calculation of the extinction efficiency Qext
(Qext ) óext/ðR2) the original FORTRAN code of Bohren and
Huffman was used, which can be found in the appendix of ref 10,
together with computational details. The complex refractive index
n(ì) for bulk gold was taken from the experimental work of
Johnson and Christy,16 where the original data were fitted with a
spline-fit to enable the calculation of Qext over a continuous range
of ì. All calculations were performed with water at 20 °C as the
surrounding medium where a wavelength-independent refractive
index was assumed (nmed ) 1.333). The calculated value for the
extinction efficiency can be related to the experimentally observed
absorption (A) by eq 4 via the number density of particles per
unit volume N and the path length of the spectrometer (d0), which
is usually 1 cm.