X-ray photoemission studies of the lead halide valence bands

✍ Scribed by M.G. Mason; L.J. Gerenser

• Publisher: Elsevier Science
• Year: 1976
• Tongue: English
• Weight: 432 KB
• Volume: 40
• Category: Article
• ISSN: 0009-2614
• DOI: 10.1016/0009-2614(76)85122-6

✦ Synopsis

X-ray photoemission spectra of rhe lead halide valence bands are reported..Assignments based on B point charge model unambii;uously show the Pb 6s level to occur several volts below the top of the valence band.

Detailed studies of the excitonic transitions near the fundamental gap in the lead halides have indicated that these transitions are localized on the Pb2+ ion [l-12] (6s + 6p cation exciton). In 1he case of Pbl,, which has the hi&est crystal symmetry of the lead halides, tight binding 1131 and empirical pseudopotential 114,151 calculations seem to support this cation exciton model. We report here, however, the results of X-iay photoemission studies (XPS or ESCA) which show that previous assignments were incorrect, and they also illustrate quite clearly rhat the top of the valence band is primarily halogen p like for PbCl2, PbBr, and PbI2 _ Spectra were recorded on a Hewlett-Packard 595OA ESCA spectrometer using monochromatized Al Ka (1486.6 eV) radiation. The lead halides were preparkd as pressed powders or were evaporated in si& in the sample preparation chamber at a base pressure of less than 2 X 10e8 torr. The stainless steel substrates were held at 300 K during the evaporation and cooled to less than 100 K prior to exposure to '. the X-ray source. Apart from slight changes in line widths, no significant differences were observed in the two types of samples. Small amounts of carbon and oxygen could be observed in the core level spectra of the evaporated films after about one hour, but no detectable changes could be seen in the valence region. Sample charging waS minimized by prepzring very thie samples (Cl000 A) and using a low energy electron flood gun for charge neutralization. SampleS were examined by X-ray. diffraction to ensure proper _. -476 :.-I