Speakers
Description
The photodetachment of the negative ion of oxygen, although a seemingly simple process, is the subject of a long lasting discrepancy between theory and experiment. On the experimental side, the reference values for the photodetachment cross section are those of Smith and Branscomb et al. [1, 2], which have been widely used to put other relative measurements on an absolute scale. On the theoretical side, computing the photodetachment cross section for an open-shell ion remains a challenging task. However, even the latest ab-initio calculation significantly differs from the experiments, both in shape and magnitude [3].
We present a joint experimental and theoretical study of the one-photon detachment of O−. The experiment was performed using the animated-crossed-beam method [4], which relies on repeatedly sweeping the laser beam across the ion beam while recording the ion current, the laser power and the number of neutral atoms produced. The cross section is then direclty retrieved from these easily measurable quantities, without unnecessary assumptions on the interaction volume. The theoretical work is twofold. The residual oxygen atom is described by a configuration interaction (CI) expansion that is voluntarily restricted in order to keep the photoionization calculations simple, yet including enough correlation to reproduce the electron affinity of the O−(1s22s22p52Po) initial state and the polarizability of the O(1s22s22p43P) ground state with sufficient precision. The CI description is then used to perform ab initio, non-perturbative R-Matrix Floquet calculations.
The present theoretical and experimental results agree within error bars. They lie some 15% lower than the extensive calculation of Zatsarinny and Bartschat [3]. They are also significantly higher than the previous measurements, the difference reaching 20% above 2.2 eV, where we do not reproduce the plateau previously observed. A long-standing discrepancy between theory and experiment is thus resolved. The present values may also question other photodetachment cross sections that were calibrated using the value of Smith, e. g., for the C− ion [5].
References
[1] S. J. Smith, Proceedings of the 4th International Conference on Ionization Phenomena in Gases (ICPIG) (1959).
[2] L. M. Branscomb, S. J. Smith and G. Tisone, J. Chem. Phys. 43, 2906 (1965).
[3] O. Zatsarinny and K. Bartschat, Phys. Rev. A 73, 022714 (2006).
[4] M. Génévriez and X. Urbain, Phys. Rev. A 91, 033403 (2015).
[5] M. L. Seman and L. M. Branscomb, Phys. Rev. 258, 1602 (1962).
