Energy distributions of hydrocarbon secondary ions from thin organic films under keV ion bombardment: Correlation between kinetic and formation energy of ions sputtered from tricosenoic acid

Abstract

The kinetic energy distributions (KED) of secondary ions sputtered from tricosenoic acid films under Ga + bombardment have been presented recently [1]. Below mass 100, the positive ion spectrum of tricosenoic acid is mainly constituted by the series of C x H y + peaks. Within each C i H y + series, the KED of these ions broadens when the number y of hydrogen atoms decreases. In this paper, we propose a phenomenological model accounting for the characteristic behavior of these ions. It is based on an initial transfer of momentum leading to the emission of an original fragment, reflecting the chemical structure of the target, and followed by a fast reorganization when its internal energy is in excess. Consequently, the ions emitted as a result of a violent collision, carrying a large excess of internal energy, will exhibit both a broad KED and a high degree of reorganization with respect to the structure of the original fragment. This interpretation highlights the effect of the chemical and molecular structure of the organic target on the emission process of the secondary molecular ions. Indeed, this structure determines the nature of the original fragments which have proved their utility for analytical purpose. These fragments lead to intense peaks in the mass spectra, allowing the recognition of the original chemical structure of the samples (fingerprint). In addition, such secondary ions resulting from a direct emission pathway, as suggested by their very narrow KED, turn out to be the best candidates for quantification.