Dissociation dynamics of K-shell excited diatomic molecules

Behavior of fragment ions from N2 and O2 induced by irradiation of monochromatized synchrotron radiation has been studied near the N-K and the O-K edges, respectively, using a time-of-flight (TOF) mass spectrometer, which is operated in a photoelectron-photoion coincidence (PEPICO) mode and a photoion-photoion coincidence (PIPICO) mode. It was found that a singly charged molecular ion is only produced at an energy corresponding to the transition of a 1s electron to π* orbital, and yield spectra of singly charged and doubly charged atomic ions have a profile similar to a photoabsorption spectrum. The yield of N22+ was a few percent of the total ion yield in the N-K edge region, but O22+ was hardly generated in the O-K edge region. A simulation calculation method was used for obtaining kinetic energy distributions of fragment ions from PEPICO and PIPICO spectra. The obtained distributions are compared with those calculated from Auger electron spectra. In order to clarify the symmetry of K-shell excited states of N2, O2, and NO and to determine the energy position of a σ-shape resonance, angular distributions of fragment ions from the molecules excited by linearly polarized soft X-rays have been measured in the region of the N-K and the O-K edges. It was found that the asymmetry parameter (β) characterizing these distributions derived from the TOF spectra is close to -1 at the photon energy which corresponds to the resonance transition of 1s → π* in all the molecules. At the σ-shape resonance, the β parameter becomes positive. The β parameters for N2 and NO in transitions to the Rydberg orbitals vary oscillatorily due to the symmetry of the corresponding transitions. The β parameter for N2 is compared with that calculated from the multiple-scattering theory. (J.P.N.) 90 refs