We use the forward-backward angular asymmetry in the electron emission cross sections in fast ion impact ionization of H2 as a probe of the inversion symmetric coherence in homonuclear diatomic molecules. The electron energy dependence of the asymmetry parameter for H2 exhibits oscillatory structure due to Young-type interference in contrast to atomic targets such as He. The asymmetry parameter technique provides a selfnormalized method to reveal the interference oscillation independent of theoretical models and complementary measurements on atomic H target. DOI: 10.1103/PhysRevA.74.060701 PACS numbers: 34.50.Fa, 34.50.Gb Angular distribution of various types of radiations particles and photons is known to be quite sensitive to various effects associated with different physical processes in atomic, nuclear, plasma physics and other branches of physics. In fast ion-atom ionization, the long range Coulomb interaction of the final state electrons with the target and the projectile ions influences the evolution of the electron wave function and thereby the angular distribution of electron emission. Such two-center effect is known to cause a large forward-backward asymmetry 1‐4 in the electron emission spectrum. The electron emission spectrum from the simplest diatomic molecule H2 manifests yet another important aspect of interference 5 in ion-atom ionization besides the wellknown mechanisms such as soft collision, two-center effect and binary encounter 1‐4,6‐8. Since the two indistinguishable H atoms in the H2 molecule may be considered as the coherent emission sources of phase coupled electrons in a large impact parameter collision, their contributions add coherently and an interference effect should be observed. Therefore, the electron emission from H2 may be viewed as a natural coherent system which is similar to Young’s double slit interference phenomenon 5. We demonstrate here that the additional mechanism of Young-type interference plays a major role in the angular asymmetry of electron double differential cross section DDCS and asymmetry parameter itself would be a sensitive test to study the interference for a diatomic molecular target. Following the initial theoretical studies on the interference effect in electron scattering 9 and photoionization 5, very recently the evidence of Young-type interference was found in the fast-ion collisions with H2 10‐12. Ideally one would have expected an oscillation in the DDCS spectrum due to interference. But a steep fall of the DDCS by about four or five orders of magnitude see below does not allow one to observe the oscillation directly. The oscillations, thereby, were observed in the DDCS ratios H2-to-2H which was explained due to the interference. However, the experiments using H are rare due to the experimental constraint and oscillations in the DDCS ratios were observed 4,12 in such experiment with H. Theoretical DDCS for atomic, or effective atomic H have also been employed 10,11 in the absence of an atomic H target. In such cases, the shapes of the oscillations are sensitive to the atomic parameters such as the effective atomic number Zeff which is model dependent. So far oscillations have not been observed based on the H2 DDCS data only. In this work we present a method based on the analysis of the asymmetry parameter which is independent of normalization procedures and permits therefore to observe the interference effect directly in the H2 data only. By eliminating the need for the data on atomic H is an important step forward in the study of this
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Dec 4, 2020
Burak Can Cerik + 1
Open Access
