Multiphoton dissociation dynamics of molecular oxygen O2 via two-photon resonant Rydberg states in the UV region.

The photodissociation and photoionization of O2 and the subsequent photodissociation of O2+ in the wavelength region of 200 to 240 nm are reported using resonance enhanced multiphoton ionization (REMPI) and velocity map imaging detection. A series of two-photon allowed Rydberg states with principle quantum number n = 3-11 converging to the ground electronic state of O2+X2Πg are used as doorway states to reach the region of superexcited states of O2 in the three-photon energy range of 15.8-18.6 eV. A detailed analysis of the kinetic energy release and anisotropy parameters of photofragments extracted from velocity map images reveals competition between neutral dissociation and autoionization and leads to the identification of different O+ formation channels. Moreover, the measurement of anisotropy parameters for each channel gives additional information on the symmetry of electronic states involved in the absorption process. Formation followed by the dissociation of vibrationally excited O2+ is the strongest channel over the full wavelength range studied. Ground and vibrationally excited O2+(X2Πg, a4Πu, A2Πu) are formed and dissociated to ionic products via one and two-photon processes. Neutral dissociation to form electronically excited atoms is important at the longer wavelengths studied and becomes noticeably less important at shorter wavelengths. These results agree with and expand on a previous study from our lab of O+ formation at a single (2 + 1) REMPI wavelength, and the results obtained in this study are found to complement our study of the electronically analogous counterpart S2, where most of the S+ ions arise from electronically excited S* atoms. The results of this study will also be of use in the pixel-to-velocity calibration of any velocity map imaging apparatus in the wide ultraviolet wavelength regions. Because O2 is a common reactant or product in many molecular dynamics studies, knowledge of its ionization/dissociation pathways at commonly used wavelengths should also be useful in avoiding signal overlap problems.