Triply differential photoelectron studies of nonhyphen;Franckndash;Condon behavior in the photoionization of acetylene

摘要

Vibrational branching ratios and photoelectron angular distributions for alternative vibrational levels ofC2H2+Xthinsp;2Pgr;uhave been measured in the range13thinsp;eVles;hngr;les;25thinsp;eVusing synchrotron radiation. Belowhngr;sim;16thinsp;eV,these data exhibit strong nonhyphen;Franckndash;Condon effects, namely, wavelengthhyphen;dependent vibrational branching ratios and vibrationalhyphen;statehyphen;dependent photoelectron asymmetry parameters. Moreover, enhanced excitation of bending modes of the ion is observed belowhngr;sim;16thinsp;eV,in addition to the Cndash;C stretch mode, which is the only mode readily observed in photoelectron spectra ofC2H2at shorter wavelengths, e.g., at theHethinsp;Ithinsp;(21.2thinsp;eV)resonance line. The nonhyphen;Franckndash;Condon behavior is attributed to resonant photoionization processes, whose identification is discussed in the framework of several recent theoretical and experimental studies on acetylene and related molecules.