IONIZING AND NONIONIZING DECAYS OF SUPEREXCITED ACETYLENE MOLECULES IN THE EXTREME-ULTRAVIOLET REGION

Absolute measurements of the photoionization and photodissociation cross sections of C2H2 have been made using continuum monochromatized synchrotron radiation in the 53-93 nm region. The absolute photoabsorption cross section and photoionization quantum yield of C2H2 have also been measured. The excitation spectra of C2(d 3-PI-g --> a 3-PI-u), C2(C 1-PI-g --> A 1-PI-u), CH(A 2-DELTA --> X 2-PI), and H(Lyman-alpha) fluorescence have also been obtained. The obtained results have been compared with theoretical calculations. An unresolved problem of the spectral interpretation concentrated on the sigma* and pi* shape resonances has been clarified by the straightforward demonstration of the photoionization quantum yield. The ionizing and nonionizing decay processes of the superexcited C2H2 molecules have been discussed in view of the strong competition of autoionization and neutral dissociation. An overlapping nature of Rydberg states with the shape resonance is found to be important.