Photodissociation of negative ions formed in CO2 and CO2/O2 mixtures

Abstract

A new tandem quadrupole mass spectrometer, developed explicitly for photodissociation experiments, has been used to study the photodissociation reactions of CO3− and CO4− ions formed in CO2 and CO2/O2 mixtures at pressures between 0.05 and 1.4 torr. These measurements have been accomplished at six discrete photon energies between 2.1–4.1 eV using the intense lines from a 200 W Hg lamp with a small monochromater, and more detailed measurements were carried out between 1.8–2.1 eV using a flash-lamp pumped tunable dye laser. The apparent photodissociation cross section for the reaction CO3−→O−+CO2 was measured as a function of ion source pressure at several wavelengths. The results are interpreted as indicating that CO3− ions produced by the three-body association reaction, O−+2CO2→CO3−+CO2 are initially formed in an electronically excited state and are rather slowly relaxed to the ground state by further collisions with CO2. The measured relaxation rate constant is approximately 5×10−14 cm3/sec.