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

摘要

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 (C_2H) with allene (C_3H_4) 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 C_5H_5 adduct. The derived branching ratios can be directly used to predict the chemical evolution of Titan's atmosphere.