Spectroscopy and Predissociation of Acetylene in the np Gerade Rydberg States

The n p gerade Rydberg states of acetylene were studied with various spectroscopic techniques to measure one-color resonant multiphoton ionization (REMPI) spectra, two-color REMPI mass spectra, fluorescence spectra, and fluorescence excitation spectra in the two-photon energy region of 72 000−92 000 cm -1 . The rotational analysis was successfully performed for one of the upper levels, which was assigned to the 4p 1 Δ g Rydberg state. All of the gerade Rydberg states prepared under the two-photon resonance conditions were found to be predissociative, and the two-photon preparation was accompanied by the fluorescence of the C 2 d 3 Π g → a 3 Π u Swan system. In the fluorescence excitation spectra measured by monitoring the C 2 Swan system, higher members of the n p 1 and 1 Δ g Rydberg series ( n ≥ 4) were observed and assigned for the first time. The predissociative lifetimes of the n p gerade Rydberg states are estimated from the spectral bandwidths. The mechanisms of the predissociation of acetylene in the Rydberg states and the formation of C 2 in the d 3 Π g state are discussed.