Molecular ion imaging and dynamic secondary-ion mass spectrometry of organic compounds

Abstract

An ion microscope equipped with a resistive anode encoder imaging system has been used to acquire molecular secondary ion images, with lateral resolution on the order of 1 .mu.m, from several quaternary ammonium salts, an amino acid, and a polynuclear aromatic hydrocarbon which were deposited onto copper transmission electron microscope grids. All images were generated by using the secondary ion signal of the parent molecular species. The variation of parent and fragment molecular ion signals with primary ion dose indicates that, for many bulk organic compounds, bombardment-induced fragmentation of parent molecules saturates at primary ion doses of (1-8) .times. 1014 ions/cm2. Subsequent ion impacts cause little further accumulation of damage in the sample, and intact parent molecular ions are sputtered even after prolonged ion bombardment (i.e. primary ion doses > 1 .times. 1016 ions/cm2). This saturation process allows molecular images to be obtained at high primary ion doses and allows depth profiles to be obtained from simple molecular solid/metal test structures.