Radical-Neutral Chemical Reactions Studied at Low Temperature with VUV Synchrotron Photoionization Mass Spectrometry

A pulsed Laval nozzle apparatus employing tunable VUV synchrotron photoionization and quadrupole mass spectrometry for the study of radical-neutral chemical reactions of importance for modeling the atmosphere of Titan and the outer planets is described. The apparatus enables the study of low-temperature kinetics and isomer-resolved product branching of highly reactive radicals with unsaturated hydrocarbons reactions. The low-temperature branching ratio for the reaction of the ethynyl radical (C2H) with allene (C3H4) has been measured for the first time at 79 K. This reaction is found to yield 1,4-pentadiyne as the major reaction product (50 +/- 10%), followed by ethynylallene (28 +/- 10%) and methyldiacetylene (22 +/- 10%) via H-atom elimination from the initially formed C5H5 adduct. The derived branching ratios can be directly used to predict the chemical evolution of Titan's atmosphere.