MOMENTUM TRANSFER IN H AND H + COLLISION WITH H2 TARGETS AT INTERMEDIATE ENERGIES

ALESSI, MARIANA; FREGENAL, DANIEL; BERNARDI, GUILLERMO; FOCKE, PEDRO; WOLFF, WANIA; DE BARROS, ANA; DE CASTRO FARIA, NELSON; SUÁREZ, SERGIO

Freiburg, Alemania

Conferencia; XXV International Conference on Photonic, Electronic and Atomic Collisions; 2007

We investigated proton emission from H2 dissociation after H and H+ impact for energies ranging from 15 to 200 keV. Following previous measurements we detected the proton emission for several angles respect to the beam direction and compared both the forward and backward emissions. We observed for H+ as well as for H that the forward emission is shifted towards higher emission energies respect to the backward emission. This energy asymmetry appears for both projectiles, H or H+, when forward and backward proton emissions are compared. A systematic difference of the order of 1eV is found and it decreases as a function of the projectile energy. In backward emission, the average energy corresponding to H+ impact is higher. Different mechanisms can in principle be responsible for all these effects. However, for the forward-backward differences, the only possible explanation is the momentum transfer to the center of mass of the molecule just before the excitation to a dissociative state. Therefore, as the average energy differences between forward-backward emissions is higher for incident H than for H+, the momentum transfer to the center of mass due to H must be higher than the one due to H+. In order to estimate the limits of the momentum transfer to the center of mass of the molecule, we first look at the maximum value corresponding to the elastic collision between two particles without structures. In that case, the recoil energy is about 90% of the projectile energy. This value goes to zero for very large impact parameter. Estimates in the case of electron capture by the projectile gives a negligible value. Finally, the potential created by H is of short range, but the one created by H+ is of the Coulomb type. This fact may explain the difference between H and H+, because only small impact parameters enter in the recoil average energy of H. For H+ the average includes all impact parameter from zero to infinity.