SCANNING FORCE MICROSCOPY STUDIES OF SURFACE-DEFECTS INDUCED BY INCIDENT ENERGETIC MACROMOLECULAR IONS

Scanning force microscopy (SFM) has been employed to study morphologic al changes associated with single-ion-impacts on SiO2, highly oriented pyrolytic graphite (HOPG), and single-crystals of the amino acid L-va line. Massive biomolecular ions are generated using an electrospray io nization source and a quadrupole mass spectrometer. The target is bias ed for post-acceleration and resulting impact velocities are around 50 lan/s. Impacts of molecular ions with masses between 5.7 kDa (bovine insulin) and 66.3 kDa (bovine albumin) are studied. Surfaces are probe d with SFM operated in both the continuous contact (repulsive force) m ode and an intermittent contact mode which eliminates damaging lateral forces during scanning. The use of different SFM modes allows an asse ssment to be made of the role of artefacts in the obtained images. Sur face topological features due to individual ion impacts were identifie d and had the appearance of low hillocks on SiO2 and HOPG, and shallow craters on L-valine. Results of a study of defect size versus inciden t projectile molecular mass are presented for approximately constant i mpact velocity. Slow but energetic massive polyatomic ions are expecte d to create regions of high energy volume density leading to cratering . Hillock formation may reflect a premature end to the crater formatio n process due to rapid diffusion of deposited energy into the bulk.