No abstract

The sodium anion, Na-, both in solution and in crystalline sodides, has a broad intense absorption band at 1.7 - 1.9 eV, which is assigned to the [MATH] optical transition. In single crystals of Na+(cryptand[2.2.2])Na-, a narrow emission band (fwhm = 0.03 eV) occurs from a highly mobile excited state (exciton-polariton). The peak position is time-dependent, shifting from 1.86 eV to 1.835 eV over a 30 ns time span. The emission intensities, peak shapes and time-profiles have now been measured for a number of salts of Na-. The photophysical properties of alkalides are complicated and very dependent on the synthesis method and purity of the compound. Defect electrons appear to be effective fluorescence quenchers and most sodides show much lower emission intensities than the nearly defect-free "parent" alkalide, Na+(C222)Na-. A model for light absorption and emission by crystalline sodides is discussed and related to the experimental results

The optical properties of the alkalide Na + (cryptand[2.2.2])Naare 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 vapordeposited thin films of Na + (cryptand[2.2.2])Nafollowing 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 Naare promoted to the p-level, resulting in a high concentration of electrons trapped at some distance from the parent anion site.