Photolysis of Na+(Cryptand[2.2.2])Na-: Photobleaching of Absorbance and Quenching of Fluorescence

The optical properties of the alkalide Na + (cryptand[2.2.2])Na - are sensitive to the presence of defect electrons and can be dramatically altered by irradiation with light. Fluorescence intensities decrease markedly at excitation power densities above about 1 mW cm -2, even though this power level is some 10 7 times lower than that required to affect the absorbance. Partial recovery occurs in powder samples over a period of several minutes at 30−100 K. The enhanced quenching of fluorescence is attributed to the presence of photoproduced trapped electrons. Pronounced changes occur in the optical absorbance spectra of vapor-deposited thin films of Na + (cryptand[2.2.2])Na - following irradiation by high intensity doubled- and tripled-YAG laser pulses. The effects are attributed to the intermediate formation of a “p-band metal” in which half of the electrons in the ground s 2 state of Na - are promoted to the p-level, resulting in a high concentration of electrons trapped at some distance from the parent anion site.