Collisions of low energy polyatomic ions with self-assembled monolayer surfaces: Surface-induced dissociation and ion/surface reactions

Donald Edward Riederer, Purdue University

Abstract

The collisions of low energy (20-100 eV) polyatomic ions with organic surfaces were studied using self-assembled monolayers on gold as model targets. Two processes of interest are ion dissociation and reactions in which atoms and groups are incorporated into the ionic projectile. Chemical sputtering and projectile deposition are also discussed. When an ion collides with a solid surface a fraction of its kinetic energy is converted to ion internal energy, resulting in dissociation. This method of ion activation, termed surface-induced dissociation (SID), is a new and promising technique for characterizing ions by tandem mass spectrometry. Of great importance is the efficiency of translational to ion internal energy (T $\to$ V) conversion (maximum is desired), and the degree of neutralization that occurs (minimum is desired). These parameters were investigated for a number of different self-assembled monolayer surfaces having different functionality (alkyl, perdeuterated alkyl, perfluorinated alkyl, hydroxy, carboxy, cyano, amino, alkenyl, and ferrocenyl). It was found that the fluorinated surface provided the largest degree of T $\to$ V conversion ($\approx$20%) owing to the larger mass of the surface atoms/groups, and also minimized neutralization owing to its high ionization energy. More detailed experiments probed the mechanism of ion activation, ie. excitation of the projectile via electronic transitions vs. direct vibrational excitation, using projectile ions known to give characteristic fragments which depend on the mode of excitation. The partitioning of the projectile's initial kinetic energy into ion internal energy, surface internal energy, and post collision kinetic energy was investigated by making simultaneous mass and kinetic energy measurements on the scattered ions. The reactions of many projectile ions were studied as a function of the target surface and it is proposed that the reactions fall into two categories. One type of reaction appears to be initiated by electron transfer from the target to the projectile and is consistent with the behavior of hydrocarbon ions interacting with hydrocarbon surfaces. A second type of reaction occurs without charge transfer and is observed when transition metal ions collide with fluorinated surfaces.

Degree

Ph.D.

Advisors

Cooks, Purdue University.

Subject Area

Analytical chemistry|Chemistry

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