Partial Pressure of Se2 and Optical Density of Selenium Vapor in the Visible and Ultraviolet

Abstract

The optical density of selenium vapor D_λ has been measured between 1900 Å and 2.0 μ at T₀=860°, 700°, 500°, 400°, and 330°C and for total pressures (determined by the temperature of a liquid selenium appendage) between 7×10⁻⁵ and 1.0 atm. Previously observed vibronic bands of Se₂ are seen and have a maximum value of D_λ near 3405 Å. Between 1900 and 2700 Å, D_λ is not linearly dependent upon the partial pressure of Se₂, p₂. The densities in this range bear a constant ratio to one another for fixed T₀ over as many as two orders of magnitude in D₂₁₀₀ and appear to be due to a single species, Se_n, n>2. Assuming D₃₄₀₅ is the sum of partial densities due to Se₂ and Se_n, each of which obeys Beers law, and that D₂₁₀₀ is proportional to p_n then p₂=r(D₃₄₀₅—kD₂₁₀₀)/L, where L is the optical path length. The value of k, which is the ratio D₃₄₀₅/D₂₁₀₀ for pure Se_n, is chosen to be 0.18 so that log D₂₁₀₀ varies linearly with log(D₃₄₀₅−0.18D₂₁₀₀). The Beers‐law constant r is obtained from measurements at sufficiently low total pressures that the vapor is all Se₂ in the optical cell and from published total‐vapor‐pressure measurements. The values of p₂ obtained agree to within 8% or better with those calculated from the Se₂–Se₄–Se₆–Se₈ equilibrium constants of Illarionov and Lapina and are significantly lower than those calculated from the standard tabulation of Stull and Sinke. For λ≥5000 Å, D_λ varies as the square of p₂ and appears to be due to pressure broadening of the Se₂ transition. The number of atoms in Se_n is 4 at T₀=860°C and 5 at lower temperatures. The latter number is interpreted as an average arising from the near equality of the partial pressures of Se₄ and Se₆ over the range of our measurements of D₂₁₀₀.