Elucidation of C2 and CN formation mechanisms in laser-induced plasmas through correlation analysis of carbon isotopic ratio

Laser ablation molecular isotopic spectrometry (LAMIS) was recently reported for rapid isotopic analysis by measuring molecular emission from laser-induced plasmas at atmospheric pressure. With C-13-labeled benzoic acid as a model sample, this research utilized the LAMIS approach to clarify the formation mechanisms of C-2 and CN molecules during laser ablation of organic materials. Because the isotopic ratios in the molecular bands could deviate from statistical distribution depending on their formation pathways, the dominant mechanism can be identified through a comparison of the experimental observed isotopic patterns in the molecular emission with the theoretical statistical pattern. For C-2 formation, the experimental (CC)-C-12-C-12/(CC)-C-13-C-12 ratios not only support a recombination mechanism through atomic carbon at early delay time but also indicate the presence of other operating mechanisms as the plasma evolves; it is proposed that some of the C-2 molecules are released directly from the aromatic ring of the sample as molecular fragments. In contrast, the temporal profiles in the C-12/C-13 ratios derived from CN emission exhibited opposite behavior with those derived from C-2 emission, which unambiguously refutes mechanisms that require C-2 as a precursor for CN formation; CN formation likely involves atomic carbon or species with a single carbon atom. (C) 2014 Elsevier B.V. All rights reserved.