Ab initio simulation of hole diffusion in CsI

 

Eugene Heifets and William A. Goddard III

Material and Process Simulation Center, Beckman Institute (139-74), California Institute of Technology, Pasadena, Ca 91125

 

Stephen Derenzo and M. J. Weber

Lawrence Berkeley National Laboratory, University Of California, Berkeley, Ca 94720

 

The ab initio embedded cluster approach was applied to study diffusion of self-trapped holes in CsI. Using the valence Los-Alamos basis set and effective core potentials, Quantum chemical calculations of a small cluster (Cs8I3) were done at Hartree-Fock (HF) and MP2 levels. With HF diffusion barrier for a hole jump 0.12eV looks too small, if we shall take into account that repolarization of the crystal during the hole jump would drop barrier by ~0.1eV. This indicating that HF is not adequate for treating such processes. With MP2, we obtained a barrier of 0.27 eV, which compares to the experimental value of 0.13eV. Account for the crystal repolarization will lead to a value quite close to experiment.

 

We estimated the absorption spectrum for a self-trapped hole using single excitation Configuration Interaction (CIS) technique. To obtain converged results, we found it necessary to include at least 4 iodine ions along the hole axis in the calculated cluster. Atomic orbitals of two additional anions mix with the hole wave function in excited states and cause a shift of excitation energies and appearance of additional ultraviolet absorption band.

 

This work was supported by Lawrence Berkeley National Laboratory and by NSF Chemistry.

 

 

 

 

 

 

 

Excitation energies for Vk-center (self-trapped hole ) in CsI, calculated by CIS method in relaxed cluster I4Cs12

There are 6 allowed transitions, but only 2 s transitions have large enough oscillator forces to be visible in a absorption spectrum.

#

Excitation Energies

Osc.forces

polarization

1,2

1.7345 eV 714.82 nm

f=0.0001

p x, p y

3,4

2.4105 eV 514.35 nm

f=0.0000

p x, p y

5

2.6662 eV 465.02 nm

f=0.5725

s

6

4.4737 eV 277.14 nm

f=0.0000

s

7

4.4875 eV 276.28 nm

f=0.0322

s

8,9

4.5013 eV 275.44 nm

f=0.0000

p x, p y

10,11

4.5021 eV 275.39 nm

f=0.0005

p x, p y

 

 

 

 

 

 


For slides of the full talk, see below

The scheme of scintillation process PDF

The Embedded Cluster Model (current implementation) PDF

The Surface Representation of the Electrostatic Embedding Potential (SCREEP) Method PDF

I3Cs8 cluster Postscript

The scheme of self-trapped hole in a barrier and in an equilibrium positions PDF

Differential density map of self-trapped hole in an equilibrium position Postscript

Adiabatic profiles for self-trapped hole jump in CsI (I3Cs8 cluster) PDF

Changes in positions of I ions and of the center of charge during STH jump PDF

I4Cs12 cluster Postscript

Excitation energies for Vk-center (self-trapped hole) in CsI PDF