In crystals, Raman scattering originates from the vibration of the lattice, which is described in quantum mechanics as phonons. Then we can say Raman scattering in solid is the photons scattered by phonons.
In ZnO and GaN, there are Raman modes like E1(LO), E1(TO), A1(LO) and A1(TO), which can be recognized as the scattering by longitudinal-optic phonons or transverse-optic phonons. But what about the E2 mode? Is it the scattering by some phonon without a name, or is there a different scattering mechanism for E2 mode?
maybe, this could help you:
Article The effect of n- and p-type doping on coherent phonons in GaN
Look at Figure 1 of the paper mentioned above by Martin Hetzl. It shows the vibrational patterns of the different modes. This explains what is meant by the labels A1, B1, E1, and E2. Formally, all of them are optic modes, because first-order Raman scattering means an interaction with Gamma-point phonons (i.e., having very long wavelength), and the acoustic modes have frequency zero at Gamma. The B1 modes are Raman-inactive (at least in first-order Raman scattering).
There is no different scattering mechanism, just the vibrational pattern is different. And because of this pattern only some of the modes have the special property of being an LO or TO mode. This special property lies in the associated dipole moment, which vanishes for the E2 and B1 modes, but is nonzero for the A1 and E1 modes, so these latter modes are also infrared-active.
Thank you so much, Mr.Hetzl and Mr.Wagner. The link and the detailed explaination are of great help.
Jan-Martin Wagner please tell me that how can i identify the types of modes from graphs
Muhammad Saddique Akbar Khan, which "graphs" do you mean, specifically? Can you provide an example?
can i identify the types of modes from graph???i am student of GaN .is there any techniques which help me to identify the LO or TO modes from graph
Jan-Martin Wagner my topic is GaN .Gallium nitride. i am confusing that how you identify from the peaks of graph that this modes is LO or TO??? is there any specific frequency /wave no or other identification of such modes???
i am very thankful to you
There is both: The phonon frequencies (wavenumbers) of GaN are already known, and the selection rules (referring to the polarization direction of incoming and outgoing beams used in Raman scattering and their relative orientation to the crystal axes) for the different mode symmetries are also known. All that you find in the relevant literature (text books, monographs, journal articles).
Jan-Martin Wagner thanks .A1(TO), E1(TO), E2(high) in raman.
. you define such types of modes?
so many types of modes exist. i am searching the identification by particular values of frequency
" A1(TO), E1(TO), E2(high) in raman. you define such types of modes?"
Well: These modes are defined by the symmetry of their atomic displacements (this is what the labels refer to), which is related to the crystal structure of GaN -- namely, wurtzite. Since an elementary wurtzite cell contains 4 atoms, there are 12 phonon modes; as usual, 3 of them are acoustic modes (2 transveral and 1 longitudinal), so there are 9 optic modes (but not all of them are optically active).
"so many types of modes exist"
Make yourself familiar with them! Look again at Fig. 1 of the paper provided by Martin Hetzl (first contribution in the present Q&A thread). Look how their pattern of atomic displacements is related to the hexagonal wurtzite symmetry: Due to that symmetry, there is a basal plane and a symmetry axis perpendicular to that plane. Make sure you understand what these 9 optic modes are in detail; can you count them through, using Fig. 1? Do you understand, from the symmetry of their atomic displacement (and from the way Raman scattering works), why the B1-type modes are not Raman active?
" i am searching the identification by particular values of frequency"
Go ahead, you will find the frequency values in the literature -- e.g. in the paper provided by Martin Hetzl.
On this page, click on "Show previous answers" (at the top of this thread); then scroll up to the relevant answer given by Martin Hetzl where he provides the link to this paper.
Or, open the profile of Martin Hetzl (www.researchgate.net/profile/Martin_Hetzl), choose "Research", scroll a little down and click on "Answers" in the left column; the very first is the one you're interested in.
Or, use the RG search field to look directly for the paper; it is entitled "The effect of n- and p-type doping on coherent phonons in GaN".
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