Crossed-beam universal-detection reactive scattering of radical beams characterized by laser-induced-fluorescence: the case of C₂ and CN

Abstract

Continuous supersonic beams of dicarbon (C₂) and cyano (CN) radicals have been generated by a high-pressure radio-frequency discharge beam source starting from dilute mixtures in rare gases of suitable precursor molecules. Their internal quantum state distributions have been characterized by laser-induced-fluorescence (LIF) in a new crossed molecular beam-laser apparatus. These supersonic beams have been used to study the reactive scattering of C₂ and CN radicals with unsaturated hydrocarbons. This paper reports here on the C₂ and CN radical beam characterization by LIF and on dynamics studies of the reactions CN + C₂H₂ (acetylene) and CN + CH₃CCH (methylacetylene) by the crossed molecular beam scattering technique with universal mass spectrometric detection and time-of-flight analysis. The role of CN rovibrational excitation on the dynamics of the CN + C₂H₂ reaction is discussed with reference to previous dynamics and kinetics studies. These reactions are of interest in the chemistry of planetary atmospheres (Titan) and the interstellar medium as well as in combustion.

Keywords:

• reaction dynamics
• laser induced fluorescence
• crossed molecular beam technique
• radical beams
• C₂ and CN radicals
• reactions of CN with unsaturated hydrocarbons

Acknowledgements

It is our greatest pleasure to dedicate this article to Professor Richard N. Zare on his seventieth birthday. His contributions to Molecular Reaction and Photon-Induced Dynamics via pioneering and state-of-the-art experiments have been a source of inspiration for many researchers in the field. We acknowledge financial support from the Italian MIUR (Ministero Istruzione Università Ricerca) under projects PRIN (2007-H9S8SW). This work has also been supported in the initial stages by the European Union Marie-Curie human resources and mobility programme, including post-doctoral fellowships for K. M. Hickson and X. Wang, under contract MCRTN-CT-2004-512302, Molecular Universe, and through the Coordination Action 001637 (Europlanet). R. Petrucci thanks FSE (Fondo Sociale Europeo)- Regione Umbria–Ministero del Lavoro e delle Politiche Sociali for a fellowship. We thank P. Sharkey, A. Paladini, R. Cireasa, M. Pasquini and R. Piani for their contribution in setting up the LIF system and E.B. Jochnowitz for help in the spectra simulations of C₂. SDLP also acknowledges support from the French Agence Nationale de Recherche, Programme Blanc ‘Cold reactions of neutral species, CRNS’. Finally, PC and SDLP acknowledge the Galileo Programme 2008/2009 between Perugia and Rennes. The Perugia, Rennes and Bordeaux authors acknowledge the COST Action CM0805 ‘The Chemical Cosmos Understanding chemistry in astronomical environments’.