It is established by the authors, that in KCl(Ca) crystals grown OH-free and X-irradiated in the temperature interval between -80°C and room temperature the less stable VKM band is observable beside the stable V2M band in the first so-called fast stage of coloration. (The superscript M referring to the divalent metal impurity serves to distinguish the bands from the corresponding V2 and VK bands of pure crystals.) The positions of the band-maxima, measured at -180 °C are 222 and 323 nm, respectively. The VKM centres may be transformed spontaneously into V2M centres, so they can be considered to be "pre-centres" of V2M centres. The opposite process can also be brought about by illumination with white light. If the crystal, X-irradiated at room temperature (RT) is illuminated at -180 °C with white light, the transformation of V2M centres into intermediate centres, denoted by V2, KM, becomes observable. Their absorption band lies at 335 nm. With an increase of temperature the V2, KM centres are transformed into VKM centres. Based on the works of Hayes and Nichols and that of Crawford and Nelson, the authors consider the VKM centre to be a Cl2- molecular ion situated in a vacancy-pair next to a Ca++ ion. Starting from this, and on the basis of experimental results, models are proposed for the structure of V2M and V2, KM centres and for the coloration mechanism at higher temperatures. The oscillator strengths of the VM1 and V2M bands are determined with the aid of the models.
ASJC Scopus subject areas
- Nuclear and High Energy Physics