Molecular Modulation Mass Spectrometry Kinetic Study of the ClO Free Radical

Abstract

In complex photochemical reactions the effective mechanism may be different at low and at high light intensities, since the ratio of radicals to reactants may approach unity at high light intensities and may be 10−4 or less at low light intensities (such as sunlight). The method of flash photolysis has been highly successful in measuring the spectra and kinetics of free radicals in systems of high light intensities. In this laboratory a method named “molecular modulation spectrometry” has been developed, which measures the spectra and lifetimes of free‐radicals in lowintensity photochemical systems. Previous versions have utilized infrared and ultraviolet spectrometry. In this article the method is extended to ion‐counting mass spectrometry. The molecular‐modulation mass spectrometer was used to re‐investigate the kinetics of the CIO radical at low pressure and at low light intensities. Studies by flash photolysis have shown that the CIO radical decays by simple, second‐order kinetics This study confirms the previous result found in low intensity photolysis of CI2 and O2 that the decay process is complex and dependent on inert gas The change in rate law between high and low light intensities and trends exhibited by the parameters, k′ and k″, show that we still do not have a complete understanding of the mechanism of this reaction.