Vicinage effect on secondary-electron emission in the forward direction from amorphous carbon foils induced by swift C₂⁺ ions

For the first time the disappearance of the vicinage effect on the secondary-electron yield in the forward direction from amorphous carbon foils bombarded with 62.5 keV/uC₂⁺ ions was observed for thick foils of 61-150 μg/cm². The threshold internuclear distance where the vicinage effect disappears was determined by calculating trajectories of the fragment ions considering the Coulomb explosion in order to estimate the characteristic interaction length that causes the vicinage effect. The threshold internuclear distance is much larger than that for the energy loss, which is related to a production process of scattered electrons leading to the secondary-electron emission. Moreover, the threshold internuclear distance increases as the velocity of the ion increases. This result means that a transport or transmission process of scattered electrons is very important for the appearance of the vicinage effect.