Double Photoionization of Molecular Nitrogen

We have investigated the double photoionization process of molecular nitrogen using monochromatized synchrotron radiation and ion time-of-flight spectrometry. Although the doubly charged N$_{2}$ parent ions have the same mass-to-charge ratio as the singly charged atomic-nitrogen ions, both species exhibit a different width in the ion time-of-flight spectrum because the molecular ions do not move (except for the thermal motion) in contrast to the atomic fragment ions. Due to the break-up motion of the atomic nitrogen ions, the corresponding peak appears as a broad line in the spectrum whereas the molecular ions show up as a narrow peak. The different peak widths allowed us to separate both processes and enabled us to derive the molecular double-to-single photoionization ratio of N$_{2}$ over a large photon energy range.