Single-Photon-Single-Electron Transition for Interpretation of Optical Spectra of Nonspherical Metal Nanoparticles in Aqueous Colloidal Solutions

Noble metal nanoparticles—especially shape anisotropic particles—have pronounced resonances in the optical spectrum. These sensitive absorption modes attract great interest in various fields of application. For nonspherical particles, no analytic description of the absorption spectra according to the commonly used Mie theory is possible. In this work, we present a semi-empirical approach for the explanation of the optical spectra of shape anisotropic particles such as silver nanoprisms and gold nanorods. We found an interpretation of the optical absorption spectra which is based on a single-photon-single-electron transition. This model is in a better agreement with the basic assumptions of quantum mechanics than the electrodynamic model of a localized surface plasmon excitation. Based on microfluidically obtained Ag nanoprisms and Au nanorods with very high ensemble homogeneities, dependencies between the geometrical properties of the shape anisotropic noble metal nanoparticles and the spectral position of the longitudinal absorption mode could be derived, which show that the assumption of a composed relative permittivity and the inclusion of the Rydberg constant is sufficient to describe the optical properties of the shape anisotropic particles. Within the scope of the measuring accuracy, the calculations furthermore lead to the value of the refractive index of the particle-surrounding medium.