Configuration Coordinate Model for KCl: Tl Including Spin-Orbit Interaction

The effect of the crystalline field on the spin-orbit interaction of the ${}_{}{}^3{}_{}{}^{}P_{}^{\circ }{}_{}{}^{}$ and ${}_{}{}^1{}_{}{}^{}P_{}^{\circ }{}_{}{}^{}$ states of ${\mathrm{Tl}}^{+}$ in KCl has now been included in the configuration coordinate model. The potential energy curves for the pure spin states are required to satisfy the condition that after the application of spin-orbit interaction the resulting crystal states account for the absorption and emission spectra, explained by the earlier model, and for the oscillator strengths of the absorption bands. In accordance with the adiabatic approximation, the potential energy curves for the states involved in absorption and emission do not cross and the electronic matrix elements for radiative transitions depend on the configuration coordinate. The curve for the lower excited state has two minima which account for the 3050 A and the 4750 A emission bands. From the single minimum of the upper excited state an additional emission band near the 2470 A absorption band is predicted. With intense excitation at 77°K in a narrow spectral range near 1960 A, which corresponds to excitation only to the upper excited state, the predicted emission has been found. The existence and characteristics of the new emission provide confirmation of the model, particularly of the quantitative effect of spin-orbit interaction.