Stark effect and electroabsorption of CTEs

An externally applied static electric field F shifts the exciton levels and these shifts are reflected in the absorption spectra.56 In the case of CTEs the shift is linear in the field  

In these expressions we neglected the corrections quadratic in the field F. For the Frenkel exciton, which has no permanent dipole moment, such quadratic contributions are the lowest-order ones and the energy reads 

The dielectric constant in the frequency region of the CT transition can be expressed by the formula 

for CTEs the leading effect of a static electric field on e(w) is seen to be quadratic in the field and proportional to the second derivative of e(w, 0). 

For the interpretation of experiments it is important to know the corrections to the real and imaginary parts of the refractive index, n and k, respectively, where k is measured as a function of frequency and gives the absorption spectrum. These corrections can be found easily from the relations e = (n + in)2. Thus, the corrections to n and k, which we denote as 5n and Sk, read 

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