Ab-initio cluster calculations of hole transport and activator excitation in CsI:Tl and CsI:Na

We describe methods for using Gausian94 (an ab-initio molecular orbital program) to compute energies and electron distributions that describe (1) the formation and transport of holes in the crystal CsI and (2) the electronic nature of the lowest excited states of CsI, CsI:TI and CsI:Na. To model these processes, the Schrodinger equation was solved at the Hartree-Fock level (in some cases with Muller-Plesset second-order electron correlation corrections) for clusters ranging in size from of 12 Cs and 12 I atoms to 16 Cs and 29 I atoms in an electrostatic field generated by approximately 5800 point charges. The calculations show that(l) a relaxed hole in CsI is shared equally by two I atoms (classic V_k center) bound by an energy of 0.85 eV; (2) the energy barrier for the motion of the hole is 0.15 eV and the transition state consists of the hole on a single I atom; (3) the lowest excited state of Cs₁₂I₁₂ is a V_k center plus a diffuse electron in the Rydberg states of the other ions (an exciton); (4) the lowest excited state of the cluster NaCs₁₃I ₁₄ is an exciton whose energy is lowered by the presence of the Na atom; and (5) the lowest excited state of the cluster TICs₁₃ I₁₄ is an excited Tl atom with all Cs and I electrons in their ground states. We believe that it is now computationally possible to model hole transport and electronic excitation in a variety of compounds to guide the search for new scintillators