Ion/surface reactions, surface-induced dissociation and surface modification resulting from hyperthermal collisions of OCNCO+, OCNCS+, (CH3)(2)SiNCO+, and (CH3)(2)SiNCS+ with a fluorinated self-assembled monolayer surface
Ion/surface reactions, surface-induced dissociation and surface modification resulting from hyperthermal collisions of OCNCO+, OCNCS+, (CH3)(2)SiNCO+, and (CH3)(2)SiNCS+ with a fluorinated self-assembled monolayer surface
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DOI：
10.1016/s0168-1176(96)04502-8
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发表时间：
1997-01-01
期刊：
影响因子：
--
通讯作者：
Cooks, RG
中科院分区：
文献类型：
--
作者：
Miller, SA;Luo, H;Cooks, RG
含拟卤素离子OCNCO+（1）、OCNCS+（2）、（CH 3）（2）SiNCO+（3）和（CH 3）（2）SiNCS+（4）与氟化自组装（F-SAM）单层表面的低能（10-90 eV）碰撞导致表面诱导解离（SID）和各种离子/表面反应。在OC-NCO+和OC-NCS+中，最低能量的断裂过程是C-N键断裂，但这些键的强度显著不同。它们估计从表面诱导解离数据作为碰撞能量（能量分辨质谱）的函数，分别为4和3 eV。甲硅烷基离子（3）和（4）优先通过Si-C键断裂而断裂，并且比（1）和（2）更容易解离。其他SID过程也发生在这些离子中的各种强键的简单裂解和电荷保留的硫优先于氧是明显的异硫氰酸酯光谱。散射离子谱的碰撞能量依赖性显示了弹性散射、离解、反应和表面溅射之间的竞争。溅射的程度随着碰撞能量的增加而增加，并且在（1）和（2）中比在含硅离子（3）和（4）中更大，并且解决了该结果对于对应于（1）-（4）的自由基的电离能的影响。另一方面，OCNCO+和OCNCS+的反应活性低于其甲硅烷基对应物，数据表明观察到的反应不是通过电荷交换进行的，而是通过直接反应机制进行的。两对离子中的反应产物（例如，来自（1）的FCO+、来自（2）的FCS+，相对于来自（3）和（4）的SiF+和SiH 2F+）显著不同，这与SID行为的差异一致，并表明（3）和（4）中的主要反应位点是硅原子。异氰酸酯和异硫氰酸酯之间的比较表明，NCS基团赋予更大的反应性比NCO基团内的每对射弹离子。在卤素和拟卤素的离子/表面反应中发现了类似性，包括在m/z 73（OCNCF+）、m/z 64（NCF 2+）和m/z 45（NCF+）处的散射离子所表明的NCO和NCS基团对F-SAM中的氟的表观置换。在（2）与氟碳化合物表面碰撞的散射产物中，在FCO+和FCS+的存在下，也发现了假卤素基团与来自表面的氟交换的证据。这些离子/表面反应产物的OCNCO+和OCNCS+射弹离子的碰撞能量依赖性提供了证据，在表面的解离之后或伴随着键的形成。提出了一个一般的机制所观察到的离子/表面反应的基础上刘易斯酸/碱，而不是氧化还原化学和碎片的发生在表面，而不是离开后。甲硅烷基离子表现出离子/表面反应，这些反应主要是从表面夺取氟，但它们也包括诸如氟取代甲基的显著过程，这与异硫氰酸酯抛射离子一起发生（4）。用低能OCNCS+和（CH 3）（2）SiNCS+离子长时间轰击氟化自组装单分子膜表面，得到了甲基、甲硅烷基和NCS基团掺入改性表面的证据，尽管离子/表面反应所需的能量条件使改性表面的形成特别困难。
Low-energy (10-90 eV) collisions of the pseudohalogen-containing ions OCNCO+ (1), OCNCS+ (2), (CH3)(2)SiNCO+ (3), and (CH3)(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 SiH2F+ 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 (NCF2+) 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 (CH3)(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.