Direct evidence of the interference effect in the electron emission spectra from ionization of molecular hydrogen in collisions with bare C and F ions at relatively low collision energies is presented. Oscillations due to the interference are deduced by comparing the measured double differential cross sections of the electrons emitted from molecular hydrogen to those emitted from atomic hydrogen, rather than using the calculated cross sections for H as in a previous report. We believe these experimental data provide stronger support for the evidence of the interference effect. We show that it is not only a feature of very high energy collisions, but also a feature to be observed in relatively lower energy collisions.

We use the forward-backward angular asymmetry in the electron emission cross sections in fast ion impact ionization of H 2 as a probe of the inversion symmetric coherence in homonuclear diatomic molecules. The electron energy dependence of the asymmetry parameter for H 2 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. Angular distribution of various types of radiations ͑particles and photons͒ 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 ͓1-4͔ in the electron emission spectrum. The electron emission spectrum from the simplest diatomic molecule H 2 manifests yet another important aspect of interference ͓5͔ in ion-atom ionization besides the wellknown mechanisms such as soft collision, two-center effect and binary encounter ͓1-4,6-8͔. Since the two indistinguishable H atoms in the H 2 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 H 2 may be viewed as a natural coherent system which is similar to Young's double slit interference phenomenon ͓5͔. 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 ͑DDCS͒ 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 ͓9͔ and photoionization ͓5͔, very recently the evidence of Young-type interference was found in the fast-ion collisions with H 2 ͓10-12͔. 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 below͒ does not allow one to observe the oscillation directly. The oscillations, thereby, were observed in the DDCS ratios ͑H 2 -to-2H͒ 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 ͓4,12͔ in such experiment with H. Theoretical DDCS for atomic, or effective atomic H have also been employed ͓10,11͔ in the absence of an atomic H target. In such cases, the shapes of the oscillations are sensitive to the atomic parame...

No abstract

No abstract

We report the energy and angular distribution of the electron emission from an RNA base molecule uracil in collisions with 3.5-MeV/u bare C ions. The absolute double differential cross sections (DDCS) are measured for emission energy between a few to 600 eV. The angular distributions are compared to those obtained for the O2 molecule in the same experiment. The single differential cross sections (SDCS) are also deduced. The energy and angular distributions of the DDCS and SDCS are compared with the state-of-the-art quantum-mechanical models based on continuum distorted wave-eikonal initial state (CDW-EIS) and correct boundary first Born (CB1) approximations which use a suitable molecular wave function for uracil. The models, however, give substantial deviations from the observed energy and angular distributions of the DDCS as well as SDCS. The CDW-EIS calculations are closer to the data compared to the CB1. In the case of uracil a large difference in the forward-backward emission of electrons was observed in comparison to that in collisions with an oxygen molecule