09 January 2020 8 10K Report

Hello,

I am trying to extract the Huang-Rhys factor S from the progression of LO phonon overtones in the resonance Raman spectrum of a material. In the configuration coordinate model, we consider the ground and excited electronic states as two harmonic oscillators displaced along a normal-mode coordinate. The exciton-phonon coupling is related to the total displacement, denoted as Delta (see for example Phys. Rev. B 42, 17, (1990), pp 11123-11132). In the given reference, the Huang-Rhys factor is defined as the square of the displacement S=Delta^2. However, in a different publication (J. Chem. Phys 98 (11), 1993), the definition S=1/2*Delta^2 is used. I have found multiple examples of each definition being used.

I believe that the definition should trace back to the original paper by Huang and Rhys ( Proc. Roy. Soc. A, 1950, 204, 406), but I am finding it difficult to draw a parallel between that and more recent publications. Can anyone please explain why the factor 1/2 is sometimes introduced, with no reference to the possibility of omitting it? Thank you in advance.

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