Herein we report Raman signals, enhanced greater than 10 5 -fold, from citrate ion adsorbed on colloidal silver. Al though this confirms our recently formulated theoretical model 1,2 in that SERS comparable to that observed on roughened macroscopic surfaces 3 also occurs in colloidal dispersions, 4 the measured wavelength dependence of Raman intensity differs from that predicted by the theoretical model.
In that model, molecules are stimulated by a primary field at the exciting frequency, comprised of the incident and near scattered fields. Emission consists of dipole and scattered fields, each at the shifted frequency. The solution contains complete information about the amplitude, phase, and po larization of the Raman fields at any observer location for molecules at any positions outside of or on the surface of a sphere of any size or optical constants. Results may be ob tained for either coherent or incoherent emission.
A dense red silver hydrosol was prepared by Carey Lea's method. 5 The average particle radius was estimated by comparison of the measured absorbance, using a Cary 14 spectrophotometer, with values calculated from Lorenz-Mie theory 6 for recent values of the optical constants of silver. 7 The best fit shown in Fig. 1 was obtained for a 21-nm particle radius, giving a surface area of 13.6 m 2 /g compared with 70 m 2 /g reported in an earlier study. 8 Broadening of the ex perimental absorption curve is due to some polydispersity that would decrease the calculated surface area only slightly. The particles are covered by a monolayer of citrate, each ion oc cupying an area 8 of about 38 Γ
2 .
The concentration of adsorbed citrate was estimated by assuming that all silver introduced as AgNO 3 (0-5 g per 5.0 ml) is retained throughout the preparative procedure and is converted to colloidal silver (0.32 g per 6.0 ml). Thus the adsorbed citrate concentration in the final hydrosol is 3.2 Γ 10 -3 M. This sample was diluted 500 to 1 prior to each ab sorption and Raman measurement to give 6.4 Γ 10 -6 M ad sorbed citrate.
Raman spectra of freshly diluted sols were measured at five wavelengths. Measurements were also made under identical conditions for a 1.37 M sodium citrate solution. Spectra with the 457.9, 514.5 Ar + and 647.1-nm Kr + lines of Spectra-Physics model 164 lasers were measured with a Jarrell-Ash 25-100 double monochromator employing an RCA C31034 phototube for detection and either a stationary rectangular MENT FROM THE OSA FELLOW (who in effect has served as the referee and whose sponsorship will be indicated in the published Letter), A COMMITMENT FROM THE AUTHOR'S INSTITUTION TO PAY THE PUBLICATION CHARGES, and the signed COPYRIGHT TRANSFER ' AGREEMENT. The Letter will be published without further refereeing. The latest Directory of OSA Members, including Fellows, was published in the Spring 78 issue of Optics News.