The exciton-phonon coupling in semiconductor and molecular crystals have been studied a lot. This coupling leads certainly to the modifications of the exciton electronic structure. It must also modify the phonons appreciably. It all depends on the system under consideration and the strength of this coupling. There have been many theoretical calculations on this effect. The literature is vast but you can look at some of the papers I found. I also found a lecture note of A. Marini of the university of Rome. Here is the link:

http://www.fisica.uniroma2.it/~marini

Here are references of some papers:

PRB 23, 2731 (1981)

PRB 32, 6437 (1985)

PRB 55, 7593 (1997)

PRB 56, 7491 (!997)

PRL 103, 058301 (2009)

You can also look at the references cited in these papers.

there is an old book by Davydov that you may also consult:

A. Davydov, Theory of Molecular Excitons (McGraw-Hill, New York, 1962).

@Darag: What I have given in above only some general references about phonon-exciton coupling. But to understand the structure you observed in your photoluminance one must know the details of the sample you are looking at. Is the phonon structure of your substance known? Are there any ab-initio phonon calculations? Do you know the phonon dispersion and also the phonon density of states from neutron scattering? What type of excitons do you expect in your samples? Localized Frenkel excitons or Mott-Wannier delocalized excitons? Is your sample a semiconductor with a small band gap or is it a wide-gap semiconductor or is it a molecular crystal? All these details are relevant to the interpretation of your photoluminance spectra.

Which phonons do you think are likely to couple with the exciton? Acoustic or optic?

Dear Tapan,

Thank you very much for your reply and for the references. I am looking at the direct transition in CuAlO2. There has been much theoretical studies of the direct transition all of which seem to agree that this occurs at the L-point gap and is ~3.5 eV. In this material, given that the free exciton PL is observed at room temperature the binding energy of exciton of the PL is strong. Furthermore it has been reported that the conduction (in some temperature ranges) is due to polaron hopping. I am therefore working from the basis that the exciton is a highly localised Frenkel type. Both emissions observed show a fine structure, as mentioned the periodicity of which I cannot identify with the phonon dispersion calculations reported (Pellicer-Porres, J.; Martinez-Garcia, D.; Segura, A.; Rodriguez-Hernandez, P.; Munoz, A.; Chervin, J. C.; Garro, N.; Kim, D. Physical Review B 2006, 74 (18) ) I haven’t found yet any neutron scattering experiments which confirm the calculated dispersion. If the fine structure is due to phonon coupling at the L point, the closest modes are the A1g and A2g modes. But at present I have no feeling as to why these modes should play an important role other than their frequency with respect to what I am observing. I look forward to reading the suggested references and hopefully will gain some more insight into what process may be at work.

Thank you

Tapan has offered very helpful information. I would like to add something based on my own experience. Usually one does not need to consider the phonon frequency change due to the coupling (of course it does as higher order effects), but a change in the atomic configuration between the excited and ground state (in the so-called configuration coordinate model). For a highly localized exciton, you could get a phonon replica with its frequency not matching any special K point of BZ. It depends on both the phonon density of states, electronic wavefunction, and symmetry. These two papers offer some general background and one example: Zhang et al., PRB 47, 6330 (1993); J. Lumin. 85, 247 (2000). Hope this help.

Dear Yong,

thank you for your assistance and for the papers you have highlighted. I think they may prove to be very helpful in understanding the problem.

Thanks again

Daragh