X-Ray Photoemission Study of Orthorhombic Selenium; a New Allotrope of Crystalline Selenium

In order to study the molecular species of a new allotrope of crystalline selenium (orthorhombic Se), X-ray photoelectron spectroscopy (XPS) has been applied to it for the first time. A comparison of the XPS spectrum of orthorhombic Se with those of rhombohedral and monoclinic Se consisting of Se6 and Se8 ring molecules respectively almost excluded the possibility that orthorhombic Se may consist of both Se6 and Se8 ring molecules. The CNDO molecular orbital calculations of the electronic structure for several Se7 ring molecules of various structures suggest that orthorhombic Se is composed of a chair-type Se7 ring molecule.