Excited States of Neutral Silver Center in Alkali Halides Embedded in Continuum

Abstract

Optical absorption bands associated with the Ag⁰ center in NaCl, KCl and KBr are studied. In the wavelength region between two intense bands observed in the visible and ultraviolet region, four weak bands, of which oscillator strengths are less than that of visible band by two order of magnitude, are observed. These four weak bands are ascribed to the transitions from the ²S ground state to the excited states ²S, ²D_3⁄2 and ²D_5⁄2, the last being split into Γ₇ and Γ₈ due to the cubic crystal field. Photoconductivity measurements are also made in KBr and it is shown that the bottom of the conduction band is separated from the 5s level of the neutral silver by 3.50 eV and situated above the 5p level. It is concluded that the excited states for four weak bands are embedded in the continuum state consisting of core electrons (4d¹⁰) of Ag⁰ and an electron in the conduction band and the asymmetrical shape of these bands is discussed in terms of a resonance effect between the localized states of the Ag⁰ and the conduction band.