Optical Studies of Alkalides and Electrides
Abstract
A high vacuum system was built for photoemission measurements. The system allows one to load alkalides and electrides under vacuum and at low temperatures with minimum contact with air or moisture. The vacuum of the measuring system can reach better than 10^ {-8} torr. The temperature of the sample holder can be controlled to +/-2 ^circC and cooled down to -130 ^circC. The 1000-W xenon light source is monochromatized, and light in the range from 200 to 900 nm is used. The response time of the system is ~40 ms, and the gain of the preamplifier is 10^9. A data acquisition system is employed to control the scanning monochromator and the electrometer, and to record data with respect to wavelength or time. The quantum yield spectra of the alkalides, rm K^+(15C5)_2cdot K^-, Rb^+(15C5)_2cdot Rb^-, Na^+C222cdot Na^-, K^+C222 cdot K^-, Cs^+C222cdot Cs ^- and K^+C222cdot Na^ -, show the common features of main emission peak at an energy higher than 4 eV and a low energy tail, and sometimes the high energy peak has a shoulder on the low energy side and peaks at lower energies. The high energy peak is less temperature dependent and is assigned as the emission from the valence electrons. The low energy emission strongly depends on temperature and the experimental conditions, and is presumed to be emission of trapped electrons. Photoemission threshold energies were determined, and depend not only on the alkali metal anion but also on the environment in which the anions reside. Laser pulses had appreciable effects on polycrystalline rm Na^+C222cdot Na^- samples. Most samples of rm Na^+C222 cdot Na^- contain both defect sites and defect electrons. As a result, the photoemission spectrum yields an additional shoulder at 360 nm (3.4 eV) and peaks and shoulders in the range of 500-700 nm, besides the main emission peak of Na^- at 290 nm (~4.3 eV). The 6 ns, high intensity laser pulses result in transient depletion of shallowly trapped electrons and/or occupancy of defect sites, and also lead to apparent modifications of the surface layers. Two kinetic models are developed to explain the mechanisms of the photobleaching effects and population-depopulation of defect electron traps. In order to study the anisotropic properties of rm Na^+C222cdot Na^-, nearly-normal incidence reflectivity on various faces of single-crystal rm Na^+C222 cdot Na^- was measured in the photon energy range 0.5-3.1 eV. The polarization-dependent spectra were determined for the electric field vector parallel and perpendicular to the C axis, based on the orientation -dependent results, known structure and the shapes of the single-crystals. A Kramers-Kronig analysis of the reflectivity yielded the spectral dependence of the real and imaginary parts of the complex dielectric constant and refractive index, as well as the absorption coefficient and the energy loss function. Comparison of the polarization-dependent spectra permitted identification of the orientation-dependent interband transitions. A peak with energy 1.64 eV appears irregularly, which is presumably due to trapped electrons. The polarization dependence of the spectra provide us with further insight into the nature of anisotropic energy states of rm Na^+C222cdot Na^ -..
- Publication:
-
Ph.D. Thesis
- Pub Date:
- 1994
- Bibcode:
- 1994PhDT.......193K
- Keywords:
-
- PHOTOEMISSION;
- Chemistry: Physical; Physics: Condensed Matter