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laser probing

Abstract:

PARTICLE-INDUCED DESORPTION OF BENZOHYDROXAMIC ACID DERIVATIVES ADSORBED ON A COPPER SURFACE STUDIED BY LASER-IONIZATION MASS SPECTROMETRY S.Wyczawska1, E. Vandeweert1, R. E. Silverans1, P. Lievens1,*, O. Blajiev2, A. Hubin2 and H. Terryn2 1 Laboratory of Solid State Physics and Magnetism, K.U.Leuven, Celestijnenlaan 200D, B 3001 Leuven, Belgium 2 Department of Metallurgy, Electrochemistry and Materials Science, Faculty of Applied Science, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium 1. INTRODUCTION Hydroxamic acids, although of particular importance in biology, medicine, and industry [ ], have remained a not very well characterized class of organic compounds. Investigating these molecules is very challenging since their adsorption and orientation on metal substrates is depending on concentration, temperature, and nature of the solvent [ ]. The surface adsorption processes of hydroxamic acids are not well known, although applications of this class of molecules adsorbed on oxide surfaces are considered promising. Benzohydroxamic acids (BHA) and derivatives have been proposed as prospective copper corrosion inhibitors with lower toxicity than other more commonly used molecules [ ]. Hydroxamic acids are strong complexing agents able to bind metallic ions such as iron, copper and some heavier metals, depending on the end group of the molecules [ ]. In this context, we will investigate how the bonding of several derivatives of benzohydroxamic acid on a copper surface is influenced by different p-substituents in the benzene ring. 2. EXPERIMENTAL METHOD We studied BHA (Figure 1) and a number of derivatives (p-Cl-BHA, p-methoxy-BHA and p-nitro-BHA) adsorbed on copper surfaces. Figure 1: Benzohydroxamic acids Copper was mechanically polished and cleaned with ethanol. In order to remove the contamination introduced by mechanical polishing, the specimens were electropolished immediately before the deposition of the organic molecules. Finally the samples were rinsed with MilliQ water, dried in a nitrogen stream, and put in the deposition solution immediately afterwards. The time of treatment was 2 hours, and the temperature 20 °C. After deposition the specimens were dipped for 2 minutes in the pure solvent, taken out, and rinsed in a stream of solvent for several seconds. Finally they were dried with nitrogen. We investigated ion-beam sputtering of these samples with ns laser ionization in combination with mass spectrometry, probing neutral molecular fragments desorbed from BHA’s adsorbed on Cu or Cu2O substrates. This experimental technique is highly sensitive and selective, and allows to measure fragmentation patterns, fragment-selective flight-time distributions, and damage profiles after cation irradiation [ ]. From the markedly different signatures in the flight-time distributions, evidence is building that several mechanisms, including prompt (ballistic) and delayed (reaction-induced) desorption of complete molecules as well as molecular fragments are competing [ ]. We use a one-color two-step resonance enhanced multiphoton ionization scheme (REMPI) to ionize the molecular fragments containing a benzene chromophore. REMPI has the advantage that low photon fluences are needed to ionize the molecular fragments, thus reducing photofragmentation. 3. EXPERIMENTAL RESULTS We investigated the desorption process of unsubstituted BHA and other derivatives (Cl-, CH3O-, NO2-) dissolved in water, methanol, or DMSO. The recorded mass spectra are markedly different for samples grown from different solvents. For BHA we could detect a benzene signal (78 amu) in all cases with comparable delay times and widths (15 µs and 9 µs respectively). For the other molecules benzene was only visible from samples dissolved in water or DMSO (p-Cl-BHA and p-methoxy-BHA), except for p-nitro-BHA where benzene could be detected only from methanol dissolved samples. Additionally we investigated sputtering of copper. From the flight time distribution we found that copper from samples dissolved in water and DMSO is sputtered later that from methanol dissolved. This may indicated that copper prepared in water and DMSO was oxidized, resulting in an easier attachment for the molecules with oxygen. The observed solvent dependent desorption behavior may be related to a solvent dependent molecule orientation [2]. The molecule dependent features may be related to differences in binding energy between the molecules and copper. 4. REFERENCES [1] M. J. Miller, Chem. Rev. 89, 1563 (1989) [2] B. Garcia, S.Ibeas, et al., Inorg. Chem. 42, 5434 (2003) [3] A. Shaban, E. Kalman, J. Telegdi, Electrochim. Acta, 43, 159 (1997) [4] O. Blajiev, A. Hubin, et al., J. Raman Spectrosc. 34, 295 (2003) [5] E. Vandeweert, J. Bastiaansen, et al., Appl. Phys. Lett. 82, 1114 (2003) [6] Z. Postawa, C.A. Meserole, et al., Nucl. Instr. Meth. in Phys. Res. B, 182, 148 (2001)