Spectroscopy and Predissociation of Acetylene in the np Gerade Rydberg States

The np 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 ∏ g → a 3 ∏ u Swan system. In the fluorescence excitation spectra measured by monitoring the C 2 Swan system, higher members of the np 1 Σ + g and 1 Δ g Rydberg series (n ≥ 4) were observed and assigned for the first time. The predissociative lifetimes of the np 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 ∏ g state are discussed.