Tandem quadrupole study of laser photodissociation of CO−3

Abstract

Absolute cross sections for the production of O− by photodissociation of CO− 3 have been determined at intervals of 1 nm or less throughout the range from 400 to 635 nm using a tandem quadrupolemass spectrometer equipped with a tunable dye laser. In addtion, the dependence of the apparent photodissociation cross section on ion source pressure, laser polarization, laser intensity, ion lens voltage, and pressure of CO2 in the photon–ion interaction region has been studied at selected wavelengths. The threshold for photodissociation of ground state CO− 3 is found to be 2.258±0.008 eV in excellent agreement with a recent indirect determination of a lower limit for D(CO2–O−) of 2.27 eV, but in disagreement with earlier photodissociation results. Studies of collisional enhanced dissociation of laser excited ions indicate that only a fraction of the ions excited between threshold and 2.6 eV dissociate unimolecularly within 50 microsecond observation time. Below 2.2 eV, two photondissociation of CO− 3 is observed, and at 581 nm (2.13 eV) saturation of one of the transitions involved is observed, indicating a photoexcitation cross section of c a. 4×10−17 cm2. A model based on these observations is proposed which appears to account for the apparent disagreement between our results and earlier laser photodissociation work.