Raman and Infrared Spectra of Rhombohedral Selenium

Raman and infrared spectra of rhombohedral selenium have been measured and the fundamental bands observed in these spectra are assigned by analogy to the S 6 ring molecule. Calculation of the normal mode vibration for the Se 6 ring using a modified Urey-Bradley force field confirms this assignment. Anomalous softening of the A 1g stretching mode relative to other modes and to the corresponding A 1 mode in the Se 8 ring was observed. This phenomenon is explained by the idea of interference of inter-molecular interaction with the intra-molecular bonding. Comparison of the observed spectra of rhombohedral selenium with those of other allotropes (α-monoclinic, trigonal, and amorphous selenium) suggests that Se 6 ring molecules may exist in amorphous selenium.