EPR in Selenium

Abstract

EPR studies at X‐band frequencies have been carried out between 300° and 77°K on amorphous and crystalline selenium. Dominating the spectra of the amorphous material were a large number of paramagnetic centers ∼10¹⁹/cc which contribute to an unusually broad and inhomogeneous line of width, ΔH_{p p}∼3Kg and g=2.3 to 2.8±0.01. The linewidths and g values were very sensitive to heat treatments. This broad line is attributed to electrons at chain ends in a highly disordered region of the solid. A different species was also present in much lower concentrations ∼10¹³/cc, as observed by a sharp resonance with peak‐to‐peak linewidth, ΔH_{p p}∼6 G and g=2.0039±0.0006, which is close to the g value of 2.0023 for a free electron. Heat treatments altered the magnitude of the signal in an erratic manner and up to ∼10¹⁶/cc spins were observed with no changes in ΔH_{p p} or g value. The identification is rather uncertain. Theoretical considerations of the g value and linewidth favor a center associated with an impurity (i.e., oxygen) at the end of the selenium chain but have shown that a chain end interacting with a neighboring chain cannot be ruled out. Experimental results tending to minimize the role of oxygen were that no conclusive differences were observed when samples were heat treated in air or vacuum or prepared by evaporation, and the appearance of a virtually identical resonance when single crystals of hexagonal selenium were damaged (mechanically and by electron bombardment). No resonances were found in pure ``as grown'' monoclinic or hexagonal selenium. This result was unexpected for the latter material, and chemisorption of oxygen on the surface is thought to be the explanation.