Introduction
Photoabsorption of simple polyatomic molecules in vacuum ultraviolet (VUV) region induces a variety of dynamical processes characteristic of highly excited electronic states caused by the interactions among Rydberg and valence excited states. A tunable VUV source generated by four-wave mixing in a nonlinear gas medium has been regarded as an ideal light source to investigate dynamical phenomena occurring in the highly excited states. In most cases, molecules undergo fast dissociation to form atomic and molecular fragments in their ground or electronically excited states. When fragments are formed in a photoemissive state, the VUV-laser excitation spectrum recorded by monitoring the photoemission corresponds with the high-resolution absorption spectrum, which is called here a photofragment emission yield spectrum.
In the present study, in order to investigate predissociation processes of acetylene (C2H2) in the vibronic levels in the D(1πu →3dσg; 1Πu), E(valence; 1A), and F(1πu →3dπg; 1Σu+) states located in the VUV (135−130 nm) region, their photofragment emission yield spectra were recorded under jet-cooled condition by monitoring the fluorescence emitted from the C2H(B 2A') photofragment using a tunable VUV laser with ~0.3 cm−1 resolution.
The dissociation processes from the vibrational levels in the D and F states having an excitation in the anti-symmetric C−H stretch were investigated by IR-VUV double resonance excitation scheme, in which acetylene is pre-excited into the ν3 (3.03 µm) and ν1+ν3 (1.52 µm) levels in the electronic ground state followed by the VUV excitation. [normal modes of C2H2]
