Vacuum-Ultraviolet Photolysis of Acetylene in Inert Matrices. Spectroscopic Study of the Species C2

Abstract

The vacuum‐ultraviolet photolysis of acetylene isolated in Ar, Ne, and N₂ matrices at 4° and at 14°K is shown to lead to the appearance of a number of visible‐ultraviolet absorption bands which may be assigned to the species C₂, as well as to an 1848 cm⁻¹ infrared absorption assigned, with the aid of isotopic substitution studies, to the free radical HC₂. The (0, 0) band of the Mulliken system of C₂ is observed with great intensity at 2382 Å (Ar matrix) or at 2323 Å (Ne matrix) and the (2, 0), (3, 0), and (4, 0) bands of the Phillips system of C₂ are observed in an Ar matrix at 8700, 7675, and 6880 Å, respectively, indicating that the x ¹Σ_g⁺ state of C₂ is the ground state of this species not only in the gas phase but also in the matrix. Features assigned by previous workers to the Swan transition of triplet C₂ appear at 5206 and 4725 Å. A third member of the progression, not previously observed, appears at 4334 Å. Experiments utilizing C₂D₂ and C₂H₂ (58% ¹³C) support the assignment of these features to C₂. The 5206‐Å band system decreases in intensity and finally disappears when the sample is subjected to radiation of wavelength near 2500 Å. Problems associated with the assignment of these features are discussed, and a possible mechanism for their photolytic destruction is suggested.