The UV absorption spectrum and geometry of the HS2 radical

The ultraviolet absorption spectrum of the transient HS₂ radical produced in the flash photolysis of H₂S₂ was recorded. The spectrum observed in the λ = 3070–3800 Å region is assigned to the òA′ ← X²A″ electronic transition, and three progressions to the SS stretching (ν′₁), SSz.sbnd;H bending (ν′₂) and SH stretching (ν′₃) vibrations of the ²A′ upper electronic state. Open-shell CNDO/2 calculations were carried out to optimize the ground state (²A″) molecular geometry. For comparison, similar calculations were also performed on the HO₂ radical. HS₂ is bent in the ground state and nearly linear in the (²A′) and (⁴A″) excited states.