Charge‐Transfer Spectra of Transition Metal Ions in Silver Halides

Abstract

On AgBr and AgCl crystals containing substitutional Ti3+, V3+, or Cr3+ ions absorption bands are measured in the near UV having oscillator strengths of to lo-'. In a n MO energy level scheme of the MeX:-cluster these absorption bands are interpreted as due t o ligand-metal charge transfer transitions. The electron is transferred from the t 2 u(x) level to the tzg(x*) level (Me = Ti, V) and to the eg(cr*) level (Me = Cr), respectively.An AgBr und AgCl Kristallen, dotiert mit Ti3+, V3+ oder Cr3+, werden im nahen UV intensive Absorptionsbanden mit einer Oszillatorenstlrke von bis 10-1 gemessen. Diese Banden werden in einem MO-Energieniveauschema des MeX:--Komplexes als Ligand-Metall charge transfer ubergange gedeutet. Die Anregung des Elektrons findet statt vom t z u ( x ) Niveau zum t g g ( x * ) Niveau fur Me = Ti, V bzw. zum eg(a*) Niveau fur Me = Cr. IntroductionThe optical spectra of the first row transition metal ions incorporated as dilute impurities in crystals have been investigated in detail. The interest concentrated nearly exclusively on transitions between the crystal field modified d-levels of the metal ions. However, there are only few papers concerning absorptions due to transitions of electrons from the adjacent anions of the host lattice to the metal ion or vice versa. Such transitions are called charge-transfer(ct) transition. Hitherto ct spectra of V2+, Mn2+, and Ni2+ in alkali halides [l, 21 and recently of A1,0, containing trivalent transition metal ions (Tippins [3]) were reported.The investigation of transition metal ions in AgCl and AgBr has shown that Ti3+, Vs+, and Cr3+ incorporate substitutionally on cation sites [4, 51. The optical spectra of these crystals show very intensive absorptions (oscillator strength f x to 10-l) near the absorption edge ofthe host lattice, besides the crystal field transitions (f x 10-6 to 10-4). This paper reports these absorptions and their interpretation as due to ct transitions. The experimental details and the preparation of silver halide crystals containing trivalent transition metal ions are described elsewhere [4]. Fig. 1 shows the absorption spectra of AgBr doped with small amounts of Ti3+, Vs+, and Crs+. I n the Ti3+ and V3+ curves there are resolved peaks a t low temperatures whereas for Cr3+ only the low energy tail of the band is detectable. Results