Abstract. The plasmon frequencies of a cube shaped metal cluster are calculated by two different methods. Unlike the spherical cluster, which has a single plasmon frequency, cop/~, the cube has a series of optically active plasmon frequencies. The two dominant peaks appear at frequencies which are considerably lower than that of a spherical cluster.
PACS: 36.40The plasma resonance absorption in metal clusters is usually interpreted in terms of the Mie resonance [1], which is characteristic of a spherical cluster. For a free electron like spherical cluster in air, the resonance occurs at the frequency co = cop/,~. In optical measurements on metal clusters, the spherical shape is indeed prevalent, but other geometrical shapes may occur [2]. We calculate here the optical absorption spectrum of a cube shaped metal cluster, having a frequency dependent dielectric constant of the formwhere cop is the plasma frequency and ? is a damping constant. Two different methods will be employed.
The method of FuchsFuchs [3] discussed the case of ionic crystal cubes, and developed a method for calculating their infi'ared absorption spectrum. However, his approach is quite general, and is applicable to any material described by a frequency dependent dielectric constant e(co). The only assumption is that the cube is much smaller than the wavelength of light, so that retardation can be neglected. Fuchs has shown that the susceptibility of the cube can be written as a sum over normal modes in the formwhere eh is the dielectric constant of the surrounding medium. The depolarization factors nm determine the normal mode resonance frequencies, whereas C(m) gives the strength of the m-th peak. The C(m) satisfy the sum rulem For a sphere only the uniformly polarized mode contributes to the optical absorption, and its depolarization factor is n = 1/3, so that the susceptibility assumes the form 1 1
Z((]J) = 4T(, (,f,/,S h --This susceptibility yields the resonance at the frequency cop/V/3 (assuming that eh = 1). Assuming an external uniform field, the surface polarization at a discrete set of points on the surfaces of the cube was obtained by We have calculated the absorption cross section of a cube of edge a = 3 nm, in air, assuming a dielectric constant of the form (1), with cop = 8.65 x 1015, the sodium value. For the damping constant the value 7 = 0.02c% was used. In