Spin-orbit Splitting and Lifetime Broadening in the A²Δ Electronic State of l-C₅H

Optical absorption bands at ～18772 and ～18807 cm^-1, previously assigned to A²Δ-X2Π electronic origin band transitions of the linear carbon-chain radicals C₅H and C₅D, respec-tively, have been reinvestigated. The spectra have been recorded in direct absorption apply-ing cavity ring-down spectroscopy to a supersonically expanding acetylene/helium plasma. The improved spectra allow deducing a l-C₅H upper state spin-orbit coupling constant A'=-0.7(3) cm^-1 and a A²Δ lifetime of 1.6±0.3 ps.