Nonresonant UV Pumping of IR Vibrational Fluorescence from Cyanide Ion in Alkali Halides

IR vibrational fluorescence from CN^ - in alkali halides, induced by UV laser excitation, has been discovered. The UV laser pump (308 nm XeCl and 248 nm KrF excimer lines) is non resonant with any of the degrees of freedom of the CN^- ion as well as the host crystal. It is shown that the CN ^- fluorescence grows from zero, and is correlated with the growth of photo-generated color centers (F centers, created by the pump laser) in the sample. Data is presented for three hosts, KBr, RbI and KI. IR fluorescence spectra were acquired at 7 K, and on a time scale short compared to the time for energy exchange by CN^- pairs through vibration -vibration exchange. The IR spectra show that the excitation of the CN^- ions by the UV pump produces a distribution of CN^- vibrational populations that is nearly thermal, with characteristic temperatures on the order of 2300 K (0.2 eV). In order to investigate the dependance of the UV pumped CN^- fluorescence on the F-CN^- distance, optical aggregation was used to form F:CN^- pairs. No change in the UV pumped CN^- fluorescence was observed as a result of this aggregation. However, the UV pump did act to destroy the F:CN^ - aggregation. The F center has weak optical transitions (L bands) in the region of the pump wavelengths. These transitions correspond to the promotion of the F electron into the host conduction band. The data presented are consistent with the CN^- fluorescence being driven by a three step process: F center creation by the UV laser pump, optical ionization of the F electron by pumping of the L band, followed by excitation of the CN ^- ions by electron-CN^ - scattering events. The conduction band electrons also provide a mechanism for communication among the CN ^- ions, establishing the observed thermal distribution of CN^- vibrational levels.