Electron stimulated desorption of neutral molecular fragments is used to study degradation of ordered organic thin films under low-energy (0–18 eV) electron impact, and total electron doses ranging between 180–550 μC/cm2. Different saturated linear thiols HS(CH2)nX (n=2 or 15, and X=CH3 or COOH) are adsorbed from solution onto a gold surface to produce a self-assembled monolayer (SAM). Here, we present yield function measurements for electron stimulated desorption of moities such as H2, CH3, CH3CH2, CH3CH2CH2, CO, and CO2 from such thin chemisorbed films. For CH3-terminated SAMs, neutral fragment desorption thresholds lie between 5–7 eV, whereas for COOH-terminated SAMs, desorption thresholds as low as 0.2 and 3–5 eV are observed. The results suggest that the incident electrons interact with functional groups localized at the film–vacuum interface, which then leads to predominantly methyl group C–H, and C–COOH bond cleavage. In addition to nonresonant degradation mechanisms, which vary monotonically from threshold with increasing incident electron energy, structures in the neutral fragment desorption yield functions are related to resonant electron attachment. Particularly for Au–S(CH2)15COOH monolayers, this mechanism leads to a desorption peak of CO fragments at incident electron energies near 1.0 eV.

Low-energy electron impact (E i ) 0-17 eV) on organic monolayers chemically bound to Au substrates is shown to induce the desorption of H 2 . The threshold and the maximum yields for this excitation/reaction/ desorption channel are observed at E i ) 7.0 ( 0.5 eV and E i ) 10.0 ( 0.5 eV, respectively, for each of the n-alkane monolayers examined (Au-S-(CH 2 ) n CH 3 , n ) 3, 7, 11, and 15). From the dependence of the H 2 yields on the incident electron energy in the E i ) 6-12 eV regime, which closely resemble the H -desorption yield previously reported for physisorbed alkanes, we propose that most of the H 2 production originates from the dissociative electron attachment to the n-alkane film constituents; H 2 formation may also proceed by direct excitation of the hydrocarbons to the dissociative S 1 state. The desorption of H 2 from chemisorbed Au-S-CH 2 C 6 H 5 is first observed at E i ∼ 7.5 ( 0.5 eV, with a broad maximum desorption yield extending from E i ) 12-17 eV. The sensitivity of the desorption yields to the film constituents suggests that H 2 formation proceeds by molecule-specific channels and suggests that chemisorbing volatile species to conductive substrates may be useful in the study of electron-induced reactions of adsorbates at ambient or elevated temperatures. It also indicates that the use of electron beams to prepare reactive surfaces will require detailed characterization studies and highly selective excitation mechanisms to avoid undesirable decomposition channels. IntroductionElectron-induced processes in physisorbed systems are increasingly being used to initiate reactions, 1 modify surfaces, 2 simulate photolysis processes, 3 and generate reactive intermediates in well-defined states. In particular, low-energy processes involving electronic states with energies below the ionization energy of the adsorbates have been shown to provide a methodology for the selective control of surface events, due to the energetic dependence of the induced processes. The latter is often a sensitive function of the electron-molecule interaction potential, which in turn is determined by the electronic state, the conformation, and the local environment of the individual adsorbates. Electron-stimulated desorption (ESD) of neutral and anionic fragments can provide detailed information on the dissociation and desorption dynamics of weakly interacting species (usually present in cryogenic matrices or physisorbed onto metal surfaces). Although several new atomic and molecular phenomena have been detected and characterized by such desorption studies, 4-6 the dissociative channels and dynamics are often closely related to the analogous dissociation experiments conducted in the gas phase. This enables the surface and condensed phase experiments to focus on the often subtle influences present in these high-density environments. By definition, such studies are restricted to temperatures at which the physisorbed species are condensed onto the substrate or into the host matrix; for most systems, this implies sample tempera...

The impact of low-energy (1-30 eV) electrons on self-assembled monolayers of heterogeneous oligonucleotides chemisorbed on a gold surface has been investigated by mass spectrometry of desorbed neutral species in an attempt to understand the consequences of secondary electron damage in a short sequence of a DNA single strand. We demonstrate that the most intense observable neutral species (CN, OCN and/or H(2)NCN) desorbed from Cy(6)-Th(3) and Cy(6)-(BrdU)(3) oligos are related to primary fragmentation of the bases induced by electron impact. The dependence of the neutral species desorption on electron energy shows typical signatures of dissociative electron attachment initiated by the formation of shape- and core-excited resonances (i.e. single-electron and two-electron- one-hole transitory anions, respectively). Substitution of dTh by BrdU increases the production of neutral fragments by as much as a factor of about 3 for the entire electron energy range. When the distribution of secondary electrons along radiation tracks in H(2)O is taken into account, we show that the probability for electron damage to heterogeneous oligonucleotides is enhanced by a factor of 2.5-3 for electron energies below 20 eV for both sensitized and unsensitized strands.

Radiation-induced damage to homo-oligonucleotides is investigated by electron-stimulated desorption of neutral fragments from chemisorbed organic films. Six and 12 mers of cytidine phosphate (poly dCs) and thymidine phosphate (poly dTs) are chemisorbed from various solutions onto a crystalline gold substrate by a thiol modification at the 3' end and are irradiated under ultra-high vacuum conditions with 5-25 eV electrons. The mass selected neutral desorption yields consist mainly of fragments of the DNA bases, i.e. CN and OCN (and/or H2NCN for poly dCs) from both poly dCs and poly dTs, indicating that the electrons interact specifically via fragmentation of the aromatic ring of either of the bases. Other heavier fragments are also detected such as H3CC-CO from poly dTs. The yields generally possess a threshold near 5 eV and a broad maximum around 12-13 eV incident electron energy. Dissociative electron attachment as well as electronically excited neutral or cation states are believed to be responsible for the various desorption yields. The latter yields are consistently larger for oligos chemisorbed from water and acetone solutions, compared to methanol solution. The invariance of the fragment yield intensities with oligo length suggests that the molecules are likely to adsorb almost parallel to the surface.

Low-energy (0.5-30 eV) electron irradiation of the thymine-and halogen-substituted nonamers Au/Cy* 6 T 3 and Au/Cy* 6 (FU,BrU,IU) 3 , chemisorbed onto a gold surface, leads to the desorption of neutral CN and OCN from fragmentation of the DNA base ring. No neutral halogen-containing species are detected. The cross sections at 15 eV for CN and OCN desorbing from the substituted oligomers bromouracil, fluorouracil, iodoracil, and thymine are measured to be 7.5-, 4.5-, (2.5-3.0)-, and (3.0-3.5) × 10 -17 cm 2 , respectively; the difference between T and BrU is about 2-to 3-fold over the investigated energy range. The decrease of neutral species produced, in the sequence BrU > FU > IU ≈ T, is observed over the entire 0.5-30 eV range. Unimolecular dissociation of resonant and nonresonant electron-induced uracil-like radicals is likely to be responsible for the formation/desorption of these neutral species. Above 20 eV incident electron energy, the initial process essentially involves nonresonant interactions, whereas within the 5-20 eV energy range, core-excited resonances are implicated. At lower energy, the neutral fragments, which are observed only for bromouracil-substituted oligonucleotides, are induced via a shape resonance. The behavior of the magnitude of the CN and OCN yields with incident electron energy, along with empirical threshold calculations for neutral fragment formation and considerations of transient anion dissociation dynamics, led us to propose possible mechanisms for pyrimidine ring fragmentation by slow electron impact.