Scattering and Coherence Phenomena in the Photoionization of Small Molecules [article]

Daniel Rolles, Uwe Becker, Technische Universität Berlin, Technische Universität Berlin
2005
Scattering, interference, and coherence phenomena are discussed for the inner-shell photoionization of the two showcase molecules carbon monoxide and nitrogen. The inner-shell photoionization process is studied using high resolution angle-resolved photoelectron time-of-flight spectroscopy as well as angle-resolved photoelectron--photoion coincidence spectroscopy (ARPEPICO). In the latter technique, the spatial orientation of free gas-phase molecules is determined from the fragment-ion momenta
more » ... gment-ion momenta measured in an ion time-of-flight spectrometer equipped with a position sensitive anode. In the heteronuclear CO molecule, the scattering of the carbon core photoelectron on the neighboring oxygen atom leads to photoelectron diffraction, which, for the first time, is studied over a wide energy range in a free gas-phase molecule. The measurement reveals a different diffraction behavior of the forward and backward scattering channels and provides the means for a direct determination of the molecular structure of a free molecule via photoelectron diffraction. It also provides direct evidence for the multiple scattering character of the so-called shape resonance, which is a prominent feature in the photoionization of small molecules. In the homonuclear nitrogen molecule, the inversion symmetry of the molecule and the resulting non-locality of the core electrons lead to coherent photoelectron emission from two identical atomic sites. The two-slit nature of the emission process is demonstrated by characteristic interference patterns in the molecule frame photoelectron angular distribution of the 1 sigma_g; and 1 sigma_u core states. The transition to the symmetry-broken system of localized electrons is studied by comparing different isotope substituted species of the N2 molecule. Isotope substitution is shown to lead to a partial localization of the non-local core hole, exhibited by a parity mixing of the outgoing photoelectron wave, if the substitution breaks the inversion symmetry of the molecule. This is the first experimental [...]
doi:10.14279/depositonce-1085 fatcat:4x6aq45edvdvxfehefiac63nrq