Analysis of Low-Lying Gerade Rydberg States of Acetylene Using Two-Photon Resonance Fluorescence Excitation Spectroscopy

The np gerade Rydberg states of acetylene were analyzed using two-photon resonance fluorescence excitation spectroscopy in the 72 000–93 000 cm–1 energy region. The npπ1Σg+ and npπ1Δg Rydberg series (n = 3–5) were identified in the fluorescence excitation spectrum measured by monitoring the C2 d3Πg–a3Πu Swan system. Some vibronic bands were assigned to the npπ1Δg–X1Σg+ transition on the basis of rotational analysis. The 5pσ1Πg state was observed, which is the first such observation in an npσ1Πg series. Rotational analysis of the 5pσ1Πg–X1Σg+ transition showed e/f-symmetry dependent predissociation of acetylene in the 5pσ1Πg state. The 000 band of the deuterated acetylene (C2D2) 4pπ1Σg+–X1Σg+ transition exhibits an atypical structure, which was satisfactorily reproduced by a simple model of quantum interference between the discrete and quasi-continuum states. The predissociative lifetimes of the npπgerade Rydberg states were estimated from the spectral profiles. The predissociation mechanism of acetylen...