Desorption induced by keV molecular and cluster projectiles

Abstract

A new experimental method has been developed for studying negative secondary ion (SI) emission from solid surfaces bombarded by polyatomic primary ions of 5 to 30 keV. The method is based on the time-of-flight (TOF) analysis of primary ions which are produced by either ²⁵²Cf fission fragment induced desorption or by extraction from a liquid metal ion source, and then accelerated into a field free region. The primary ions included organic monomer, dimer, and fragment ions of coronene and phenylalanine, (CsI)(subscript n) C⁵⁺ cluster ions, and Au(subscript n)⁺cluster ions. Secondary electrons, emitted from a target surface upon primary ion impact, are used to identify which primary ion has hit the surface. An event-by-event coincidence counting technique allows several secondary ion TOF spectra, correlated to several different primary ions, to be acquired simultaneously. Negative SI yields from organic (phenylalanine and dinitrostilbene), CsI, and Au surfaces have been measured for a number of different mono- and polyatomic primary ions. The results show, for example, yields ranging from 1 to 10% for phenylalanine (M-H)⁻. 1 to 10% for I⁻ and 1 to 5% for Au⁻, with Cs₂⁺ and Cs₃I₂⁺ clusters as projectiles. Yields for the same surfaces using Cs⁺ primary ions are much less than 1%, indicating that SI yields are enhanced with clusters. A yield enhancement occurs when the SI yield per atom of a polyatomic projectile is greater than the SI yield of its monoatomic equivalent, at the same velocity. Thus, a (M-H)⁻ yield increase of a factor of 50, when phenylalanine is bombarded with Cs₃I₂⁺ instead of Cs⁺, represents a yield enhancement factor of 10. For the projectiles and samples studied, it was observed that the heavier the mass of the constituents of a projectile, the larger the enhancement effects, and that the largest yield enhancements (with CsI and Au(subscript n) projectiles) occur for the organic target, phenylalanine. One possible explanation for the larger enhancements with organics, namely a thermal spike process, appears unlikely. Experiments with high and low melting point isomers of dinitrostilbene, bombarded with Cs₂I⁺ and Cs⁺ projectiles, showed larger Cs₂⁺ yield enhancements for the high melting point isomer.