Triply differential photoelectron studies of non-Franck–Condon behavior in the photoionization of acetylene

Abstract

Vibrational branching ratios and photoelectron angular distributions for alternative vibrational levels of C2H2+X 2Πu have been measured in the range 13 eV⩽hν⩽25 eV using synchrotron radiation. Below hν∼16 eV, these data exhibit strong non-Franck–Condon effects, namely, wavelength-dependent vibrational branching ratios and vibrational-state-dependent photoelectron asymmetry parameters. Moreover, enhanced excitation of bending modes of the ion is observed below hν∼16 eV, in addition to the C–C stretch mode, which is the only mode readily observed in photoelectron spectra of C2H2 at shorter wavelengths, e.g., at the He I (21.2 eV) resonance line. The non-Franck–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.