Optical and Raman investigation of NH4Cl:Cu2+

Abstract
The authors have explored the centre II formed in NH4Cl:Cu2+ crystals by means of optical absorption and Raman spectroscopy. The crystal-field spectrum of centre II is shown to be composed of three bands, consistent with its D4h symmetry. An analysis of peak positions suggests NH3-Cu2+ and Cl--Cu2+ distances close to 1.9 and 2.4AA respectively. At 14K the crystal-field bands assigned as b2g(xy) to a1g and eg(xz;yz) to a1g show nice vibronic progressions corresponding to Omega =400+or-15 cm-1. Raman spectroscopy reveals the presence of a sharp peak corresponding to Omega O=445cm-1. Both Omega and Omega O frequencies are associated with the NH3-Cu2+ symmetric stretching of the (CuCl4(NH3)2)2- unit. This difference reflects higher NH3-Cu2+ distances for the excited states due to the promotion of an additional electron to the strongly anti-bonding a1g level.