In fact, the bands of a compound in the vapor phase will be narrower than those of the same compound in the liquid phase for that very reason (1). The weaker the chemical interaction, the narrower the band will be. Raman spectra of compounds in the liquid, or vapor, phase consist of narrow bands whose widths depend upon the degree of chemical interaction between the molecules. That is, you learned about the normal vibrational modes of discrete molecules-as opposed to solid-state materials-and their Raman or infrared activity based upon molecular spectroscopic selection rules. If you are a chemist, you most likely learned vibrational spectroscopy as molecular spectroscopy. Wave vector, reciprocal space, and the Brillouin zone are explained with respect to Raman spectroscopy of solids. The purpose or goal of this installment of “Molecular Spectroscopy Workbench” is to help readers understand the underlying physics that affect the Raman spectra of crystalline and amorphous solids. The Raman spectra of crystalline and amorphous solids of the same chemical composition can be significantly different primarily because of the presence or absence of spatial order and longrange translational symmetry, respectively.
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