ASYMMETRY PARAMETERS FOR CO2 AROUND THE C-K AND O-K IONIZATION EDGES FROM THE ANISOTROPY OF THE ION DISTRIBUTIONS

Asymmetry parameter (β) values have been determined from the C+, O+, and CO+ signals in the time-of-flight (TOF) mass spectra and the CO+-O+ signal in the photoion-photoion coincidence (PIPICO) spectra of CO2 excited and/or ionized with monochromatic synchrotron radiation around the C K and O K ionization edges using a rotatable time-of-flight mass spectrometer. Positive β values of 0.4 were found for the C+ ions at the C 1s-12πu* and O 1s-12πu* resonances, respectively, and negative values between -0.1 and -0.4 were obtained for the other TOF and PIPICO signals at the same photon energies. The difference between these values and the expected value of -1 results from bending the molecule in the excited state before or during its ionic fragmentation. An O-C-O bond angle of 124°was found for CO2 in the C 1s-12πu* state from the β value for the O+-CO+ PIPICO signal. The β values were close to the isotropic value of zero at all other photon energies for the C+ ions. β values for the O+ and CO+ signals in the TOF spectra and the O+-CO+ signals in the PIPICO spectra were found to be similar. Slightly negative β values were found below the C K edge, rising to a maximum value of 0.9 at the σu* shape resonance at 312.2 eV. Strongly positive β values were found just below the O K edge and were assigned to a transition of an O 1s electron to an s-type Rydberg orbital coupled with a shape resonance in the σg ionization channel. Two maxima were observed in the β values above the O K edge and assigned to shape resonances in the σg and σu continuum channels. The lower energy resonance in the σg channel was found at 542.5 eV and the position of the higher energy shape resonance in the σu channel was found to be 560 eV. The importance of using mass-resolved ion spectra to determine the β parameters for molecules which undergo a change in geometry upon excitation of a core electron is highlighted by a comparison of these results with the symmetry-resolved ion yield spectra of CO2. © 1995 The American Physical Society.