Why are the Raman Spectra of Crystalline and Amorphous Solids Different?

[@gravitywavesRMS]

Thank you for the hepful talk! Writing up a portion of my dissertation, I thought "do I really understand the selection rule for Raman spectroscopy?". I found your whitepaper that cited your video and its been very illuminating.

[@dtuschel]

I am glad that you found my Spectroscopy publication on this subject along with this companion video to be helpful in writing your dissertation.

[@amribrahim7850]

Thanks so much for the deep elaboration

[@PaulGaurav]

Thank you for the class Dr. David

[@datle7225]

Thank you, very helpful.

[@francobisturi2994]

Thank your very much for the excellent talk. I have one question: why the phonon angle is exactly half of the scattered photon? Thanks again.

[@dtuschel]

Your question is best answered by the explanation that I give beginning at 36:25 in the video with attention to the vector diagram of the incident and scattered wave vectors and the phonon. (The diagram on the PowerPoint slide shows only the 90 degree configuration.) Note that as theta, the angle between the incident and scattered (detected) wave vectors changes, the angle between q and the incident wave vector changes by 1/2 theta. And so as one samples at different angles relative to the incident laser beam, one samples phonons propagating at different 1/2 theta angles through the material. The angle between q and ki will always be 1/2 that between ks and ki.

[@francobisturi2994]

Thank you very much, now I got it. For a deeper dive into the topic, what textbook do you suggest I should buy? The "Vibrational Spectroscopy of Solids" by Sherwood? Thanks.

[@dtuschel]

Yes, I think that Vibrational Spectroscopy of Solids (ISBN: 0-521-08482-2, Cambridge University Press, 1972) by P.M.A. Sherwood is the best book on this topic.

[@МарияАнурова-я7н]

David, thanks a lot for your videos! It's a great helpfull for me :)

[@dtuschel]

Thank you for your encouraging words. I hope that these videos help you and others develop their craft in Raman spectroscopy and imaging.

[@live4Cha]

excellent! very helpful

[@harshkumar-hp7cn]

Can the longitudinal-transverse mode splitting increase the number of Raman modes for a particular space group, such as the tetragonal phase of barium titanate, which belongs to the P4mm space group and has 12 optical modes allowed. Now, If the modes further break down into LO and TO, will the number of modes increase, (say 20 or more) ?

[@dtuschel]

When working with single crystals, the number and observation of Raman active modes will be determined by the crystal point group to which the material belongs, the particular face of the crystal illuminated, the polarization of the incident beam, and the direction and polarization of light collection. Furthermore, long range electrostatic forces within the crystal can lead to longitudinal-transverse splitting in the spectra as you have noted. Whether or not this happens is dependent upon the crystal's chemical composition and the relative strengths of the short and long range electrostatic forces of the material. For an in depth treatment of this subject and your questions, I recommend to you the book titled Vibrational Spectroscopy of Solids by P.M.A. Sherwood (Cambridge at the University Press, 1972, ISBN: 0-521-08482-2). In particular, see pages 109-115.

[@harshkumar-hp7cn]

Thank you so much. Unfortunately, I am unable to find pdf of the book. Can you please tell where I can find the soft copy of book. I will be grateful.

[@harshkumar-hp7cn]

Dear sir, If possible, please tell any alternative book as I am unable to find this

[@dtuschel]

You should be able to find this book in a university scientific library. Also, the book is available for purchase at Alibris (www.alibris.com) and Amazon; I just checked both websites for their availability. I don't know of a better alternative book for your questions and this particular subject.

[@DW-iq8lt]

thank you for the video. But what is the physical meaning (besides the mathematical definition you mentioned) of the Brillouin zone?

[@dtuschel]

I appreciate your desire to understand the "physical meaning" of reciprocal space or the Brillouin zone. Our physical senses allow us to grasp the meaning of normal space and dimensions because we live in them. Therefore, we don't have much difficulty associating wavelength in units of nanometers or Angstroms with normal space or crystal dimensions. The phonon as a traveling wave is defined by the wave vector k in units of reciprocal length, typically cm-1. So we find it convenient mathematically to operate in reciprocal space or the Brillouin zone when dealing with phonons and wave vectors. However, we don't live in or sense reciprocal space. Therefore, our physical experience makes it very difficult to apprehend or understand the "physical meaning" of reciprocal space or the Brillouin zone. I think the best that you can do is to understand the mathematics of it.

[@DW-iq8lt]

@@dtuschel Thank you, David. I attended your talk at Metrology Symposium 2019 - Stanford Nanofabrication Facility as well. That was a great talk too.

[@ayoobyousif4818]

thank a lot

[@NicholasWingComposer]

Interesting. Does the Brillouin Zone encompass the center of gravity and the core of the electromagnetic field of the crystal cell structure? Please disregard the following question if it is off base: Theoretically, if one could create the conditions or extreme state of matter to prohibit the target structure from vibrating would it then become impossible to achieve a Raman backscatter?