Vacuum–ultraviolet photodissociation of C2H2 via Rydberg states: a study of the fluorescent pathways

The visible fluorescence produced by vacuum–ultraviolet photodissociation of acetylene, particularly through the Rydberg states, has been studied in a gas-flow system using synchrotron radiation as a light source between 154 and 60 nm and an original light collection device. Excitation of the Rydberg states below the first IP reveals only the C2H product by its ÖX̃ continuum emission spectrum between 330 and 900 nm. Evolution in the emission spectral profile as a function of excitation wavelength has been observed, indicative of specific internal distribution favoring the bending levels of the à state of C2H. A new assignment of the Rydberg series converging to the first excited state à 2Ag of the cation is proposed on the basis of recent experimental and theoretical characterization of this state and leads to an approximate value of the C–C stretching frequency of the cation (ν2≈1500 cm−1).