The Acetylene S₀ Surface: From Dispersed Fluorescence Spectra to Polyads to Dynamics

Abstract

Abstract Dispersed Fluorescence spectra of acetylene have been recorded and unzipped, through a fortuitous coincidence of FC active modes and active anharmonic resonances, there by pictorially revealing the key factors controlling the vibrational dynamics on the acetylene S 0 surface. When energy is placed into very high excitation of the trans ‐bend and moderate excitation of the CC stretch, three approximately conserved quantities survive, i.e., no trace of the expected total breakdown of vibrational constants of motion due to energetic access to the vinylidene isomer is observed for vibrational energies up to at least 16400 cm −1 . Unzipping the DF spectrum reveals the FC intensity profiles for the acetylene à ↔ X̃ system with two FC active modes. Also, the unzipped DF spectrum clearly demonstrates that IVR increases within a polyad by increasing trans ‐bead, v 4 , and by decreasing CC stretch, v 2 . Increasing IVR with increasing v 4 or decreasing v 2 , are both cases where the first few resonance tiers play the dominant role in controlling IVR for the initial states prepared in these experiments, very high excitation in the trans ‐bend and moderate excitation in the CC stretch.