Effect of Pressure on the Optical Absorption of the Activator System in KCl: Tl

The optical absorption spectrum of thallium-activated potassium chloride has been measured at 2000 atmospheres. The absorption band corresponding to the ${}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}\to {{}_{}{}^3{}_{}{}^{}P_1^{}{}_{}{}^{}}^0$ transition of ${\mathrm{Tl}}^{+}$ shifts 4 A to longer wavelengths at this pressure. The spectral shift is calculated theoretically from the quantitative configuration coordinate model of KCl: Tl. The occupation probabilities of accessible atomic configurations of the activator system are dependent on pressure, whereas the transition energy for a particular configuration is independent of pressure. The pressure is considered to act hydrostatically on six pistons corresponding to the ${\mathrm{Cl}}^{-}$ nearest neighbors. The theoretical shift in absorption under 2000 atmospheres pressure is 5 A to longer wavelengths.