Vicinage effect for hydrogen clusters in Si3N4 and SiO2
Abstract
We have measured the evolution with depth z of the vicinage effect and Coulomb explosion for H2+ and H3+ molecular ions traversing thick, uniform amorphous targets of Si3N4 and SiO2. High depth resolution is achieved by scanning the energy per nucleus of the incoming molecules Eb around the energy ER corresponding to narrow resonances in the cross sections of 18O(p,α)15N at 151 keV and in 15N(p,αγ)12C at 429 keV. The corresponding reaction yield Y(Eb) is, to first order, inversely proportional to the stopping force Smol on the molecular fragments, giving a direct image of the evolution of the vicinage effect and Coulomb explosion with z. At the target surface, the vicinage effect enhances the stopping force on the molecules by a factor χ>1. As they penetrate the matter, the distance between the molecule nuclei increases, as a consequence of Coulomb explosion and multiple scattering lateral displacement, and the vicinage effect decreases with depth z. The experiment yields information on the screened Coulomb repulsive potential via the repulsion velocity Vlim observed at large z, where the distances Rij between the molecule nuclei are large compared to the adiabatic cutoff Rad (no vicinage effect). We show that the first order view, that Y(Eb) ∝Smol-1, is profoundly modified by the energy fluctuations induced by the Coulomb explosion and lateral multiple scattering. Our observations are satisfactorily reproduced when modeled using a dynamic screening radius rs in the range Rad (for H3+) to 2Rad (for H2+) for the Coulomb explosion and the evolution with Rij of χ given by the dielectric model for the interaction of the exploding molecules with the target electron gas.
- Publication:
-
Physical Review A
- Pub Date:
- April 2012
- DOI:
- 10.1103/PhysRevA.85.042901
- Bibcode:
- 2012PhRvA..85d2901L
- Keywords:
-
- 34.50.Bw;
- 79.77.+g;
- Energy loss and stopping power