Emission Spectrum of Iodine Molecules in the Presence of an Inert Gas

Abstract

The emission spectrum of iodine (I₂) between 5000 and 2500 A. consists of a large number of diffuse bands (McLennan bands) and, especially in the presence of small amounts of an inert gas, of some apparently structureless continua¹ at 4800, 4300 and 3420 A. However, as has been shown, particularly by Elliott², the spectrum has quite a different appearance if excited in fluorescence by the Al spark lines at 1855–1863 A. in the presence of large amounts of nitrogen. The McLennan bands disappear almost entirely, the continua change into well-developed band systems at 4630–4440, 4321–4041(E) and 3450–3040 A.(D), and another well-developed band system appears at 2730–2520 A.( F). From a vibrational analysis of system D, Elliott was able to confirm an earlier interpretation of this system as an electronic transition from an upper state ¹Σ⁺_u to the ground state ¹Σ⁺_g of I₂ (with vibrational quantum numbers v″ = 13—29). He also set up a formula for system F, but his v″-assignment is uncertain, and indeed entirely incorrect according to our analysis. He was unable to interpret the other two systems.