The magnetic circular dichroism spectrum of acetylene

The magnetic circular dichroism (MCD) spectrum of gas phase acetylene has been studied in the spectral region 320-140 nm. No MCD signal was detected in the region of the first transition. The observation of irregular MCD bands beginning at 185 nm confirm assignment of this latter band as the 0-0 band of the second transition. The discrete and relatively narrow bands in the region 155-140 nm exhibit pure A-term character, lending unambiguous confirmation to the assignment of ¹Π_u for the state. The observed absorption spectrum in this region is interpreted as consisting of two non-interacting systems, one involving the ¹Π_u Rydberg state and the other, a broad continuous background, cannot be assigned. It is however suggested that it may be due to the ¹Σ_u⁺ state of the π → π^* excited configuration.