Photofragment action spectroscopy of the acetylene molecule in the vacuum ultraviolet

Abstract The photodissociation dynamics of acetylene to hydrogen atoms, C 2 H 2 → C 2 H + H and C 2 D 2 → C 2 D + D, with C 2 H and C 2 D in the ground X 2 Σ + and/or excited electronic A 2 Π states, has been investigated for the first time with tunable narrow-band vacuum ultraviolet laser radiation between 121 and 133 nm by employing hydrogen atom photofragment action spectroscopy. Action spectra have been taken at 2.5 cm −1 resolution with acetylene in a molecular beam adiabatically cooled internally to about 4 K. They exhibit pronounced discrete predissociative resonances of Rydberg and valence excited states resolved to homogeneous widths. From them dissociation lifetimes of individual states from 5 × 10 −14 s to longer than 2 × 10 −12 s are derived.