AVS 59th Annual International Symposium and Exhibition
    Applied Surface Science Thursday Sessions
       Session AS-ThM

Paper AS-ThM12
Electrospray Droplet Impact/SIMS: Some Insights into the Collisional Events

Thursday, November 1, 2012, 11:40 am, Room 20

Session: Applications of Large Cluster Ion Beams
Presenter: K. Hiraoka, University of Yamanashi, Japan
Authors: K. Hiraoka, University of Yamanashi, Japan
Y. Sakai, University of Yamanashi, Japan
S. Ninomiya, University of Yamanashi, Japan
R. Takaishi, University of Yamanashi, Japan
Correspondent: Click to Email
Electrospray droplet impact (EDI) uses the water droplets with charge number of 60-300 and masses of 6.2Χ105-1.6Χ107 u. The charged droplets impact the sample surface with the velocity of about 12 km/s. Because this value is higher than sound velocities of solids, the supersonic collision takes place at the moment of collision. The supersonic collision is followed by the enormous pressure build-up at the interface and electronic excitation for the species near the colliding interface will follow. The observation of the ionic products by mass spectrometry and the surface analysis by XPS are regarded as the study of the ionization/desorption processes induced by the supersonic collision. For the sample of self-assembled monolayers, only the molecular ions of organic layer but no adducts with gold atoms were observed. In addition, no gold cluster ions but only Au+ was observed after Au surface was exposed. This indicates no ablation but atomic- or molecular-level etching takes place in EDI. For all the organic and inorganic samples investigated, no modification of components on the surface was observed after EDI irradiation as far as XPS measurements are concerned. Non-selective etching means that all the elements of the samples are desorbed with the same probabilities. These results suggest that the collisional events taking place in EDI is highly non-thermal. The water clusters do not penetrate into the sample but they are reflected backward in very short time, maybe in ps. The high ionization efficiency for EDI (useful yield: ~10%) may be explained by the curve crossing mechanism. The sample of AgF was only one example found so far that suffered from surface modification. Due to the very reactive nature of F atoms, the surface was enriched by Ag during EDI irradiation. The CF3- and CF3COO- ions observed at the start of irradiation decreased and were taken over by [AgF3OH]- and [CH3COOAg + CH3COO]-. The enrichment of silver on the AgF sample (i.e., reactive loss of fluorine) is evident. The ions CF3- and CF3COO- originated from CH3COOH used for the electrospray of 1 M aqueous CH3COOH solution. Supersonic collision and followed chemical reactions are complicated and the mechanism of desorption/ionization in EDI remains to be elucidated.