Photostimulated Electron Emission Spectra of Vacuum‐Ultraviolet‐Irradiated Ionic Crystals

Abstract

By the investigation of the spectral characteristics of photostimulated electron emission it has been shown that F‐centres are created in NaCl, KCl, KBr, and KI single crystals by ultraviolet irradiation in the 5 to 11 eV region. The F‐centres are formed as a result of the photoproduction of excitons as well as electron–hole pairs, the exciton mechanism being especially effective. Under stimulation with visible light, electron emission appears from UV‐created F‐centres due to a two‐step photothermal process in crystals with sufficiently low electron affinity χ and by the usual photoemission mechanism in crystals with relatively high χ.

Creation spectra of colour centres in KCl and NaCl developed via electron emission

“…Curve 2 resembles closely the creation spectrum obtained for KC1 in our previous work [7] with the aid of a Geiger-Muller counter in methane atmosphere.…”

“…The experimental set-up was principally the same as in [ 7 ] . However, our previous gas-filled measuring chamber with a Geiger-Muller electron counter was replaced by a special vacuum cryostat and the emitted electrons were counted with a multiplier tube (for details see [ll]).…”

Method for the analysis of nonselective spectra of optically stimulated electron emission from irradiated dielectrics

“…As a final result, we obtain an emission analog of Smakula's formula for the concentration of emissionactive defects: 16 1 OSEE…”

“…However, some methodological difficulties limit the development and wider use of OSEE in studies of point defects on the surface of dielectric materials. These difficulties include the nonselectivity of OSEE spectra caused by different mechanisms of emission and a strong effect of the surface potential barrier on the integral intensity of the electron current [16]. This problem considerably complicates the separation and quantitative analysis of the contribution to OSEE from active centers belonging to defects of a particular type.…”

Exciton‐Induced Colour Centre Growth in KBr and KI Crystals at 5 °K

“…It is well known that excitons created in alkali halides by ultraviolet light of suitable wavelengths give rise to F-centres (see, e.g., [l] to [5]). Beside different interactions with defects such as vacancies and clusters of vacancies being initially in crystals there are some possibilities for destroying the exciton in a regular crystal site accompanied by the formation of a new defect or colour centre.…”