Interferences in electron emission from O2 by 30 MeV Oq+ impact

M. WINKWORTH; P.D. FAINSTEIN; M.E. GALASSI; J. BARAN; B.S. DASSANAYAKE; S. DAS; T. ELKAFRAWY; D. CASSIDY; A. KAYANI; .A. TANIS

Cluj-Napoca, Rumania

Conferencia; 4th Conference on Elementary Processes in Atomic Systems (CEPAS 2008); 2008

In recent years there has been much interest in the study of interference phenomena associated with electron emission from H2 colliding with fast ions [1,2]. Among them, H2 collisions with 1-5-MeV H+ revealed primary Young-type interferences due to coherent emission from the identical atomic centers as well as secondary interferences caused by intramolecular scattering [2]. The primary interferences showed a strong dependence on the electron observation angle, while the higher frequency secondary structures did not. More recently, interferences for 1-5-MeV H+ + N2 revealed apparently only secondary oscillations [3]. The present work extends the experimental studies to 30 MeV O5,8+ impact with O2. The 30 MeV Oq+ ion beam was obtained using the tandem Van de Graaff accelerator at Western Michigan University. After collimation, the ion beam was directed into the scattering chamber onto an O2 target supplied by a gas jet with a flow rate set to maintain single collision conditions. The energy of electrons emitted from the target were analyzed with a parallel-plate analyzer equipped with a channel electron multiplier for angles of 30º, 60° and 90º with respect to the incident beam and ejected electron energies of 15 - 400 eV. The measured molecular cross sections were divided by theoretical molecular O2 cross sections calculated using one-center wave functions. Plotted as a function of the outgoing electron velocity, the resulting ratios were fit with the sinusoidal function f(k) shown in the Fig. 1. The oscillatory structures are suggestive of secondary interferences as evidenced by their independence on the observation angle and the fact that the oscillations do not appear to be damped with increasing electron velocity, similar to the structures observed for H+ + N2 [3]. No evidence for primary interferences is seen, as was also the case for N2. On the contrary, the oscillation interval for O2 is
k ~ 4 a.u., while for N2 a value of
k ~ 2 a.u. was found. Results for O8+ + O2 were similar to those for O5+ and also gave
k ~ 4 a.u. The reason for the difference in
k for O2 and N2 is presently being explored and may be due to the higher projectile charge and lower collision velocity in the case of 30 MeV O5,8+ + O2, compared to 1-5 MeV H+ + N2 [3], resulting in a perturbation strength Z/v ~ 1. Keywords: electron interferences, coherent electron emission, molecular ionization References [1] N. Stolterfoht et al., Phys. Rev. Lett. 87 (2001) 023201. [2] J.A. Tanis and S. Hossain, Nucl. Instrum. Meth. Res. B 261 (2007) 226. [3] J.L. Baran et al., J. Phys.: Conf. Ser. 58 (2007) 215.