Chemistry of Cyclopropane on Pt(111):  Thermal, Electron, and Photon Activation

Abstract

The thermal, electron, and photon-induced chemistry of cyclopropane, c-C3H6, adsorbed on Pt(111) at 100 K has been studied. The thermal chemistry is simple. Adsorption is saturable, no multilayer accumulates, and thermal desorption exhibits only c-C3H6 with one peak at 144 K. High-resolution electron energy loss spectra results indicate that c-C3H6 adsorbs with the plane of the ring tilted away from the surface normal. The electron-induced chemistry is more complex. Irradiation with 50 eV electrons activates dissociation of adsorbed c-C3H6 at 100 K, and there are thermally activated reactions during subsequent temperature-programmed desorption. The total cross-section for destruction of c-C3H6 is 8.2 (±0.2) × 10-17 cm2. For low electron fluences, <1016 cm-2, the chemistry involved is describable in terms of C−C bond breaking to open the ring and form metallacyclic species, some of which promptly lose H to form allylic (C3H5) moieties. In subsequent thermally activated reaction-limited processes, propylene, methane, ethylene, and hydrogen desorb. Only carbon remains above 800 K. Activation with 6.4 eV (193 nm) photons is also effective, but the cross section for loss of c-C3H6 is nearly 4 orders of magnitude lower than that for electron irradiation.

Reactions in condensed formic acid (HCOOH) induced by low energy (<20 eV) electrons

“…Reactions in condensed material induced by low energy electrons (o20 eV) have been the subject of many investigations within the last years. Among these, we only mention ozone synthesis in an oxygen film , 21 CO formation in condensed acetone, 22 activation and transformation of adsorbed cyclopropane [23][24][25] or Cl 2 formation from a C 2 F 4 Cl 2 film. 26,27 Extended exposure of the FA sample to the electron beam indicates formation of CO 2 (and possibly H 2 O) at the surface as can be seen from the evolution of the desorption features and signatures in thermal desorption spectra of the irradiated sample.…”

Synthesis of Cl2 induced by low energy (0–18 eV) electron impact to condensed 1,2-C2F4Cl2 molecules

“…Cl À plus the corresponding radical. 7,8 In the framework of electron driven reactions in condensed material we mention here (in addition to the abovementioned syntheses of ozone 1 and CO 2 ) activation of adsorbed cyclopropane and transformation into propene, [9][10][11] reactive scattering by ionic fragments, 12 hydrogen abstraction from amorphous water ice, 13 bond selective dissociation of alkanethiol-based self assembled monolayers, 14 DNA strand breaks 15 or desorption of neutrals from chemisorbed biological molecular systems. 16 Electron stimulated reactions involving cations include desorption of H 2 CO + and H 3 O + from a hydrogen-containing CO 2 matrix.…”

The role of low-energy (≤ 20 eV) electrons in astrochemistry

“…our experiments simulate these interactions. and photons [23,[25][26][27][28]. Nevertheless, even these very limited investi-462 gations have involved electrons/photons whose energies exceed the…”