Effects of electronic stopping power on fast-ion-induced secondary ion emission from methanol microdroplets

Abstract

The formation processes of secondary ions in liquid materials were studied for methanol microdroplets bombarded by carbon ions with incident energies of 0.4–4.0 MeV, where the corresponding electronic stopping power ranged 300–800 eV nm−1. Positive and negative secondary ions including molecular fragments, methanol clusters, and reaction products were investigated, and each ion yield was examined as a function of electronic stopping power Se. We observed different Se-dependence on the emission yields between positive and negative ions. For positive cluster ions [(CH3OH)n + H]+ (n = 2−10), the yield nonlinearly increases and follows the power-law Seα with α = 3. For negative secondary ions, the value of α varies according to secondary ion species or ion mass: α ≈ 0 for fragments with small mass (CH−, CH2−, and OH−), α = 0.5–1.5 for reaction products with medium mass(C2−, C2H−, C2HO−, and C2H5O−), and α = 1.2−1.5 for clusters with large mass [(CH3OH)n – H]− (n = 1−25). The latter finding implies that the value of α is a quantity related to the electronic energy density depending on the distance from the ion trajectory.