Ion/surface reactions, surface-induced dissociation and surface modification resulting from hyperthermal collisions of OCNCO+, OCNCS+, (CH3)2SiNCO+, and (CH3)2SiNCS+ with a fluorinated self-assembled monolayer surface

Abstract Low-energy (10–90 eV) collisions of the pseudohalogen-containing ions OCNCO + (1), OCNCS + (2), (CH 3 ) 2 SiNCO + (3), and (CH 3 ) 2 SiNCS + (4) with fluorinated self-assembled (F-SAM) monolayer surfaces lead to surface-induced dissociation (SID) and to a variety of ion/surface reactions. The lowest energy fragmentation process in both OC-NCO + and OC-NCS + is C-N bond cleavage but the strength of these bonds is significantly different. They are estimated from surface-induced dissociation data taken as a function of collision energy (energy resolved mass spectra) to be 4 and 3 eV, respectively. The silyl ions, (3) and (4), preferentially fragment by Si-C bond cleavage and dissociate more readily than (1) and (2). Other SID processes also occur by simple cleavage of the various strong bonds in these ions and charge retention by the sulfur in preference to oxygen is evident in the isothiocyanate spectra. The collision energy dependence of the scattered ion spectra display the competition between elastic scattering, dissociation, reaction, and surface sputtering. The extent of sputtering increases with collision energy and is greater in (1) and (2) than it is in the silicon-containing ions (3) and (4), and the implications of this result for the ionization energy of the radicals corresponding to (1)–(4), are addressed. On the other hand, OCNCO + and OCNCS + are less reactive than their silyl counterparts, and data suggest that the observed reactions do not proceed by charge exchange but instead by a direct reaction mechanism. The reaction products in the two pairs of ions (e.g. FCO + from (1), FCS + from (2), vs. SiF + and SiH 2 F + from (3) and (4) are notably different, consistent with the differences in the SID behavior and showing that the major reactive site in (3) and (4) is the silicon atom. Comparisons between the isocyanates and isothiocyanates show that the NCS group confers much greater reactivity than the NCO group within each pair of projectile ions. Analogies are found in the ion/surface reactions of the halogens and pseudohalogens, including the apparent displacement of fluorine in the F-SAM by NCO and NCS groups suggested by the scattered ions at m/z 73 (OCNCF + ), m/z 64 (NCF + 2 ) and m/z 45 (NCF + ). Evidence that a pseudohalogen group exchanges with a fluorine from the surface is also found in the presence of FCO + and FCS + among the scattered products of collisions of (2) with the fluorocarbon surface. The collision energy dependence of these ion/surface reaction products for the OCNCO + and OCNCS + projectile ions provides evidence for dissociation at the surface followed or accompanied by bond formation. A general mechanism is proposed for the observed ion/surface reactions based on Lewis acid/base rather than redox chemistry and the occurrence of fragmentation at the surface rather than after departure. The silyl ions show ion/surface reactions which are dominated by fluorine abstraction from the surface but they also include such remarkable processes as fluorine-for-methyl substitution, which occurs with the isothiocyanate projectile ion (4). Surface modification of fluorinated self-assembled monolayer surfaces was accomplished by prolonged bombardment with low-energy OCNCS + and (CH 3 ) 2 SiNCS + ions. Evidence is provided for incorporation of methyl, silyl, and NCS groups into the modified surface, although the energetic conditions needed to cause the bond dissociations necessary for ion/surface reactions make the formation of modified surfaces especially difficult in these cases.