Mass Spectrometer Study of the Vapors from Red Phosphorus and Arsenic

Abstract

The subliming vapors from red phosphorus and arsenic have been subjected to electron bombardment and the mass spectra studied over a range of electron energies, orifice sizes, and crucible temperatures. Detailed appearance potential curves of the monomer, dimer, trimer, and tetramer ions are presented. The relative mass spectra did not change with orifice size or crucible temperature. The hypothesis of essentially complete sublimation as the tetramer is advanced and defended. Concurrent studies of the rate of sublimation under Langmuir conditions versus temperature lead to heats of activation for the sublimation process which are in satisfactory agreement with earlier weight loss studies. Influence on the mass spectrum by thermal dissociation at the mass spectrometer filament was studied by increasing the area of glowing tungsten in the ion source. Analysis of changes in the appearance potential curves so produced reveals structure in these curves not detectable directly. A search for heavy ions failed to detect P₈⁺ ions in the abundance reported earlier but detected As₆⁺ and As₈⁺ ions with an abundance about 10^—5 as great as As₄⁺.