Visible emission from the vibrationally hot C2H radical following vacuum-ultraviolet photolysis of acetylene: Experiment and theory

Abstract

Photolysis of acetylene has been performed by vacuum-ultraviolet excitation with the synchrotron radiation via the Rydberg states converging to the first ionization potential ͑IP͒ at 11.4 eV. Only the visible fluorescence of the ethynyl radical was observed in the à 2 ⌸-X 2 ⌺ ϩ system. Excitation of several Rydberg states of acetylene over a large energy range between 9 and 11.4 eV allowed us to observe for the first time the evolution of this continuum with increasing Rydberg excitation. Intensity calculations based on accurate ab initio potential energy surfaces of C 2 H were performed by using a one-dimensional model accounting for the large-amplitude motion of the H atom around the CC bond and for the overall rotation of the radical. These calculations successfully reproduce the observed visible continuum ͑maximum at 500 nm and blue side cutoff at 400 nm͒ and bring new information on the distribution of the internal energy deposited in the fragment. For most excited Rydberg states, predissociation occurs in a rather low time scale, leaving the C 2 H fragment in the à state, vibrationally hot, mostly with significant excitation in the bending mode around the isomerization barrier.

Effect of Noncovalent Interactions on Vibronic Transitions: An Experimental and Theoretical Study of the C2H⋅⋅⋅CO2 Complex

“…[14,15] This speciesp lays important roles in combustion and astrochemistry; [16][17][18][19] thus, its intermoleculari nteractions are of ac lear interest. The C 2 Hr adical has been extensively characterized by electronic, vibrational, and EPR spectroscopies; [14,[20][21][22][23][24][25][26] however,i ntermolecular complexes of this species are unknown.…”

Dissociative Excitation of Acetylene Induced by Electron Impact: Excitation-emission Cross-sections

“…Acetylene is the smallest hydrocarbon with triple bond between carbon atoms. From the spectroscopic point of view acetylene is interesting for its abundance in the space (Boyé et al 2002, Tanabashi et al 2007, Lacy et al 1989, Brook et al 1996, Tucker et al 1974. The molecule has also relevance to different plasma technologies (Childs et al 1992, Smith et al 2001, Somé et al 1995.…”

A sensitivity study of the neutral-neutral reactions C + C$_{\sf 3}$ and C + C$_{\sf 5}$ in cold dense interstellar clouds

“…The value at 15 K if only vibrational stabilization is allowed is a much lower 1 × 10 −13 cm 3 s −1 , which serves as a reasonable lower limit. The value obtained with the ν |ν = δ ν ,ν assumption is certainly an extreme upper limit, because the diagonal FranckCondon overlaps are certainly lower than unity, the electronic transition moment, assumed to be independent of energy, may well decrease with increasing energy for certain states (Boyé et al 2002), and the internal conversion from the ground X 3 Σ coefficient is not facile, but a reasonable guess is that they contribute to produce a factor of ≈0.25, leading to a reduced value at 15 K of 8 × 10 −13 cm 3 s −1 . We adopt as an error limit an uncertainty of ±75%, allthough this error does not quite cover the extreme maximum and minimum values discussed above.…”