Influence of Ion Pairing on the UV-Spectral Behavior of KI Dissolved in Supercritical NH3: From Vapor Phase to Condensed Liquid

The UV-spectroscopic behavior of KI dissolved in supercritical ammonia enabled us to identify two species that contribute to the optical absorption depending on the fluid density rho1 and the temperature T. At low rho1 and high T, contact ion pairs (CIPs) prevail, while at high density of ammonia, solvent separated ion pairs (SSIPs) and free iodide ions dominate the optical absorption of the solute. The features of the electron excitation process depend on the state of the K+ I- species present. Starting with isolated KI in the vapor, where the process is an interionic charge transfer, when the CIP becomes solvated the UV absorption shifts strongly to the blue. As rho1 increases, the amounts of SSIP and of free iodide increase progressively and their electronic excited states become those characteristic of the charge-transfer-to-solvent process. This study suggests there is a strong influence of the cation on the electronic transition of dissolved iodide when it is forming CIPs. Moreover, the fact that K+-NH3 interaction is much larger than that of I(-)-NH3 suggests that the electronic photoinduced excited state of CIPs is similar to the ground state observed for alkali metals in NH3 clusters.