Photofragment action spectroscopy of the acetylene molecule in the vacuum ultraviolet

The photodissociation dynamics of acetylene to hydrogen atoms, C2H2 → C2H + H and C2D2 → C2D + D, with C2H and C2D in the ground X̃ 2Σ+ and/or excited electronic à 2Π 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.