The thermal and electron-induced chemistry of biacetyl (CH 3 COCOCH 3 ) on Ag(111) has been studied using temperature-programmed desorption (TPD), X-ray photoelectron spectroscopy, and ultraviolet photoelectron spectroscopy. No thermal decomposition of biacetyl occurs, confirming that Ag(111) is inert with respect to the breaking of C-C, CdO, and C-H bonds. There are five molecular biacetyl desorption peaks in TPDs180, 178, 174, 188, and 215 K. The peak at 180 K is attributed to monolayer adsorption, and its saturation peak area is used to scale other TPD biacetyl peaks. The peak at 188 K is assigned to multilayers and that at 215 K to desorption from defect sites. The peaks at 174 and 188 K are discussed in terms of coverage dependent reorientation and bilayers. Nonthermal excitation pathways by which the surface chemistry of biacetyl may be directed were explored by irradiating 1 ML of biacetyl with 50 eV electrons. During irradiation, CO, CH 3 , ketene (H 2 CdCdO), and C 2 H 6 desorb. After irradiation, five new post-irradiation TPD peaks appear. These are identified as H 2 at 210 K, CH 4 at 235 and 315 K, H 2 CdCdO at 240 K, and reaction-limited CH 3 COCOCH 3 at 440 K. XPS shows C (a) and O (a) remain on the surface after heating to 700 K.

Articles you may be interested inElectroninduced surface chemistry: Production and characterization of NH2 and NH species on Pt (111) Lowenergy electron diffraction study of a disordered monolayer of H2O on Pt(111) and an ordered thin film of ice grown on Pt (111) The electron-induced chemistry of one monolayer ͑1 ML͒ of biacetyl ͑CH 3 COCOCH 3 ͒ on Ag͑111͒ has been studied using temperature programmed desorption ͑TPD͒, x-ray photoelectron spectroscopy, and Auger electron spectroscopy. Monolayer biacetyl has a sharp saturable parent peak at 180 K, with a tail toward higher temperatures that is attributed to defect sites. No thermal decomposition occurs, confirming that Ag͑111͒ is inert with respect to the breaking of C-C, CvO, and C-H bonds. Nonthermal excitation pathways by which the surface chemistry of biacetyl may be directed were explored by irradiating 1 ML of biacetyl with 50 eV electrons. During irradiation, CO, CH 3 , ketene ͑H 2 OvCvO͒, and C 2 H 6 desorb. After irradiation, the parent TPD peak area drops with little change in position or shape. The initial total cross section for electron-induced decomposition of biacetyl is 8.0Ϯ0.2ϫ10 Ϫ17 cm ϩ2 . As products accumulate, this drops to 4.0Ϯ0.2ϫ10 Ϫ17 cm ϩ2 . The cross section has a threshold around 8 eV and rises smoothly with increasing energy. The ionization of biacetyl is proposed as the initiation step.

The photon-driven chemistry of biacetyl (CH3COCOCH3) on Ag(111) has been studied using temperature-programmed desorption, Auger electron spectroscopy, and time-of-flight mass spectrometry. Irradiation of both monolayer and multilayer biacetyl with 193 nm photons results in the prompt ejection of CH3 radicals that exhibit bimodal translational temperatures of 2700 ± 100 and 290 ± 20 K. Irradiation of multilayer biacetyl with 193 nm also leads to ejection of CO with translational temperatures of 1170 ± 40 and 140 ± 10 K, ketene (CH2CO) with translational temperatures of 1000 ± 60 K, and acetyl-containing fragments with translational temperatures of 320 ± 20 K. Indicative of the ejection of all reaction products from the surface during irradiation, postirradiation TPD shows loss of parent only, and C(a) and O(a) are absent from the surface after irradiation and subsequent heating to 700 K. The cross section for total loss of 1 ML biacetyl is 2.0 ± 0.3 × 10-21 cm+2 at 193 nm. Studies using 248 nm radiation, as well as radiation from a Hg arc lamp, were also undertaken.

The intensity of 10.6-^m radiation scattered from carbon plasmas has been measured as a function of incidence and collection angles (with angular resolution ^1 0 3°), and of laser mode structure. Over an incident intensity range lO^-lO 13 W cm" 2 , total reflectivity was typically ^>8%. At certain angles, reflectivity often showed 100% temporal modulation. The relevance of these results to critical-surface and stimulated-scattering phenomena is discussed.The reflection and scattering of radiation from laser-produced plasmas is a topic of active experimental and theoretical interest. Extensive measurements on a wide variety of targets have been reported, for example, at wavelengths of X = 0.694/ 1.06, 2 " 7 and 10.6 juim. 8 " 15 In most of these experiments the focusing arrangements were such that the target simultaneously encountered a wide range of angles of incidence (0). This Letter describes measurements made with angular resolution higher than those in previous work, 5 * 12 and discusses the exceptionally strong time variation of reflectivity which has been observed with use of multimode lasers and the absence of detectable modulation when a laser is operated on a single transverse axial mode.As shown in Fig. 1, a 75-J, 50-ns pulse from a plane-polarized multimode C0 2 laser of cross section 5 cmx 10 cm was focused by a 7.8-cmdiam (//4.4), 22-cm-focal-length, KC1 lens onto a solid-graphite target. Radiation backscattered into the focusing lens, and the incident radiation, were sampled by a 16% NaCl beam splitter and imaged onto photon-drag detectors PD2 and PDl. Radiation sidescattered into the remaining ~2.07r ster was collected by a spherical copper mirror and focused into a photon-drag detector PD3 lon, Back scattered photon drag PD2 cated behind the target. Variations of absorption with 6 were investigated by placing annular apertures at the laser output window (thus restricting the uncertainty in 101 to 0.65°-1.3°) while the backscattered and sidescattered signals were measured into the collection angles of the lens and mirror, respectively. Similarly, the distribution of radiation scattered into angles cp±Acp was measured (with the full //4.4 cone of radiation incident) by placing a series of annular apertures before the PD2 imaging lens, giving a resolution Acp = 0.4°-0.8°

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