The dynamics of electron-induced reactions in condensed trifluoroiodomethane (CF3I) were studied under ultrahigh vacuum conditions. Seven CF3I radiolysis products (C2F6, C2F5I, C2F3I, CF2I2, C2F4I2, CFI3 and C2F3I3) were identified using temperature-programmed desorption experiments conducted after irradiation with 4 eV electrons. Although C2F6 formation at energies above 4 eV is ascribed to electron-induced electronic excitation followed by prompt dissociation of the C–I bond to form radicals that dimerize, the formation of the other six radiolysis products at low sub-ionization incident electron energies is attributed to dissociative electron attachment (DEA) because of the observed resonance peaks in the radiolysis product yields as functions of incident electron energy (~2 to ~ 7 eV). All seven CF3I electron-induced reaction products were also identified following irradiation with 500 eV electrons. While dissociative electron attachment and/or electron impact excitation may play an important role in the high-energy radiation-induced synthesis of the high-yield product C2F6, a dramatic enhancement of up to ~ 2 × 104 in product yield per electron at 500 eV relative to that at 4 eV for some products suggests, however, that DEA is not the dominant mechanism for the high-energy radiation-induced synthesis of those products.
The goal of this review is to discuss post-irradiation analysis of low-energy (<50eV) electron-induced processes in nanoscale thin films. Because electron-induced surface reactions in monolayer adsorbates have been extensively reviewed, we will instead focus on low-energy electron-induced reactions in multilayer adsorbates. The latter studies, involving nanoscale thin films, serve to elucidate the pivotal role that the low-energy electron-induced reactions play in high-energy radiation-induced chemical reactions in condensed matter. Although electron-stimulated desorption (ESD) experiments conducted during irradiation have yielded vital information relevant to primary or initial electron-induced processes, we wish to demonstrate in this review that analyzing the products following low-energy electron irradiation can provide new insights into radiation chemistry. This review presents studies of electron-induced reactions in nanoscale films of molecular species such as oxygen, nitrogen, trifluoride, water, alkanes, alcohols, aldehydes, ketones, carboxylic acids, nitriles, halocarbons, alkane and phenyl thiols, thiophenes, ferrocene, amino acids, nucleotides, and DNA using post-irradiation techniques such as temperature-programmed desorption (TPD), reflection-absorption infrared spectroscopy (RAIRS), X-ray photoelectron spectroscopy (XPS), high-resolution electron energy loss spectroscopy (HREELS), gel electrophoresis, and microarray fluorescence. Post-irradiation temperature-programmed desorption, in particular, has been shown to be useful in identifying labile radiolysis products as demonstrated by the first identification of methoxymethanol as a reaction product of methanol radiolysis. Results of post-irradiation studies have been used not only to identify radiolysis products, but also to determine the dynamics of electron-induced reactions. For example, studies of the radiolysis yield as a function of incident electron energy have shown that dissociative electron attachment plays an important role in the electron-induced single strand breaks in DNA leading to mutagenic damage. Studies such as these not only provide insight into the fundamentals of electron-molecule interactions in the condensed phase butalso may provide information valuable to (a) furthering cost-efficient destruction of hazardous chemicals, (b) understanding the electron-induced decomposition of feed gases used in the plasma processing of semiconductor devices, (c) clarifying the role, if any, of low-energy electrons, produced by cosmic rays, contributing to the formation of the ozone hole by interacting with halocarbons and producing Cl atoms, (d) illuminating the dynamics of electron-induced oligomerization and/or polymerization, and (e) explicating the astrochemistry of icy grains.
We have investigated reactions in condensed CF2Cl2 induced by electrons of subexcitation energies under ultrahigh vacuum conditions. The yields of the CF2Cl2 radiolysis products (C2F4Cl2, C2F3Cl3, and C2F2Cl4) were determined as functions of electron energy (Ei) (~1 to ~4.5 eV) and electron dose (ﬂuence) (1.5 × 1016 e/cm2 to 2.2 × 1016 e/cm2) by postirradiation temperature-programmed desorption. In general, we ascribe the formation of these radiolysis products to reactions of •CFCl2 and •CF2Cl radicals, which are generated by dissociative electron attachment (DEA) to CF2Cl2. Consistent with condensed-phase experiments, which report the electron stimulated desorption of F- ions via transient negative ion formation at incident electron energies near 4 eV, the yield of products C2F2Cl4 and C2F3Cl3 derived from reactions of •CFCl2 show maxima near ~3eV. The production of C2F3Cl3 and C2F4Cl2 at electron energies as low as 2 eV indicates that •CF2Cl is generated; however, the absence of a clear resonance maximum in the C2F4Cl2 yield function indicates that the production of •CF2Cl radicals via DEA is not the sole or dominant pathway for the formation of C2F4Cl2. Indeed, the variation of radiolysis product yield with electron dose suggests that the C2F4Cl2 signal may additionally be generated by reactions of CF2Cl2 with the diﬂuorocarbene radical (:CF2), which we propose is also formed by DEA to CF2Cl2. Although numerous studies have revealed chemical damage induced by DEA, our results represent one of the ﬁrst studies to unambiguously demonstrate (via a clear signature of resonance) chemical synthesis induced by this process.
A recently constructed TOF mass spectrometer was used to measure the electron-stimulated desorption of anions from thin ﬁlms of vapor-deposited CF2Cl2 in the 0–20 eV energy range. In addition to the previously observed signals of F− and Cl−, the anions CF−, F2−, CF2−, ClF−, and Cl2− are detected. Measurements as a function of both incident current and CF2Cl2 coverage on a Kr substrate indicate that all desorbed ions are the result of unimolecular dissociation, initiated by the impact of a single electron. Both dissociative electron attachment (DEA) and dipolar dissociation (DD) processes are found to contribute to the desorption yield, but to an extent that is anion dependent. The DEA and DD derived yields of the anions demonstrate distinctly different dependencies on molecule–metal distance, as has been observed in other molecular solids. As previously reported, prolonged bombardment of the CF2Cl2 ﬁlm leads to the production of Cl2 molecules as evidenced by an enhanced desorption signal of Cl− at incident electron energies near 5 eV. We have used this effect used to measure the relative cross-section for production of Cl2 as a function of energy and observe a threshold energy for this process of 7 eV.
The low-energy electron-induced chemistry of condensed CCl4 has been investigated with use of post-irradiation temperature-programmed desorption. These experiments were partly motivated by the growing interest in developing facile and economical methods for decomposing CCl4 and other halocarbons which through widespread use in industry have become potent environmental pollutants. The experimental procedure involves low-energy (50 eV) electron irradiation (fluence < x 1016 electrons cm-2) of nanoscale thin films (5 monolayers) of CCl4 grown at 100 K on a Mo(110) single crystal under ultrahigh vacuum (UHV) conditions.Results of post-irradiation temperature-programmed desorption experiments were used to identify C2Cl4, C2Cl6, C3Cl6, C4Cl6, and C5Clx as low-energy electron-induced reaction products of condensed CCl4. Two longer chain chlorocarbon oligomers with six or more carbons were also detected but not identified. Although low-energy electron-induced oligomerization reactions have been previously reported for molecules such as thiophene and cyclopropane, the results presented herein represent the first study to specifically identify the products of such reactions, demonstrating the utility of post-irradiation temperature-programmed desorption experiments to study the radiation chemistry of condensed matter.
We report on the first direct investigation of the low-energy elecgtron-induced production of neutral species from the chlorofluorocarbon CF2Cl2, commonly known as Freon-12 or CFC-12. Our experiments were motivated by a newly proposed hypothesis, which suggests that low-energy electrons produced by cosmic rays, in addition to UV-vis photons from the sun, interact with chloroflurocarbons to produce chlorine atoms that subsequently destroy ozone in the Antarctic. Our experimental procedure involves low-energy (5-100 eV) electron irradiation of nanoscale thin films (~10 A thickness) of CF2Cl2 grown at 100 K on a molybdenum single crystal in an ultrahigh vacuum chamber (p ~ 1 x 10-10 Torr). Post-irradiation temperature-programmed desorption experiments were used to identify C2F4Cl2, C2F3Cl3, C2F2Cl4, C2F3Cl, C2F2Cl2, and C2F4 as electron-induced radiolysis products of CF2Cl2. In contrast to previous studies of photon-induced dissociation, our studies of electron-induced dissociation demonstrate facile C-F bond cleavage in CF2Cl2. This finding may have implications for understanding the partitioning of Cl and F among source, sink, and reservoir gases in the stratosphere.
The surface-induced and electron-induced chemistry of trifluoroiodomethane (CF3I), a potential replacement for chlorofluorocarbons (CFCs) and chlorofluorobromocarbons (halons), were investigated under ultrahigh vacuum conditions (p ~ 1 x 10-10 Torr) on Mo(110). Results of temperature-programmed desorption (TPD) experiments indicate that dissociative adsorption of CF3I leads only to nonselective decomposition on Mo- (110), in contrast to reactions of CF3I on other metal surfaces. Desorption of CF3 radicals and atomic iodine was detected mass spectrometrically during low-energy (10-100 eV) electron irradiation of four monolayer thick films of CF3I condensed at 100 K. Results of postirradiation temperature-programmed desorption experiments were used to identify CF2I2, C2F5I, C2F6, C2F4I2, and CFI3 as electron-induced reaction products of CF3I. Except for CFI3, all of these electron-induced reaction products of CF3I have been previously identified in ç-radiolysis studies, supporting our earlier claim that temperature-programmed desorption experiments conducted following low-energy electron irradiation of multilayer thin films provide an effective method to investigate the effects of high-energy radiation, including radical-radical reactions.
Recent advances in spot-welding technology such as high frequency direct current inverter welders provide an improved and reproducible method to spot-weld difficult junctions. The importance of removing the oxide layers on metal surfaces, accurately delivering the weld pulse profile, and controlling the force applied to the materials during the welding process are discussed in the context of resistance spot-welding a molybdenum crystal to a tantalum loop and attaching a tungsten–rhenium thermocouple to the crystal. ©2001 American Institute of Physics.
HyperChem 5. Coleman, William F.; Arumainayagam, Christopher R.. Dep. Chem., Wellesley Coll., Wellesley, MA, USA. J. Chem. Educ. (1998), 75(4), 416. CODEN: JCEDA8 ISSN: 0021-9584. Journal; Book Review written in English. AN 1998:186958 CAPLUS (Copyright 2001 ACS)
Formaldehyde(CH2O) reaction on Mo(110) is studied with temp. programmed reaction and IR reflectance absorbance spectroscopy. We present preliminary results which demonstrate the evolution of gas-phase ethylene from the formaldehyde reaction, to the best of our knowledge the first example of carbon-carbon bond formation on clean Mo(110). This reaction is proposed to proceed via an ethylene dialkoxide intermediate, analogous to that formed during reaction of ethylene glycol on Mo(110). Other reactions include hydrogenation of CH2O to form a methoxy intermediate which subsequently undergoes C-O bond scission to evolve gas-phase Me radicals at apprx. 600 K.
The photoactivation of chemisorbed O2 in the presence of chemisorbed CO on Pt(111) has been investigated for uv light in the range 3.87–4.77 eV (260–320 nm). Three photoprocesses first-order in O2 coverage have been separated and for the first time the cross sections for each are reported. The dominant process is O2 photodissociation (Qdiss=4.0±0.1×10–21 cm2). The second most probable process is photodesorption (Qdes=2.2±0.1×10–21 cm2). The least probable process is photoreaction with chemisorbed CO (Qrxn=0.35±0.03×10–21 cm2). Previous studies of Qrxn have reported cross sections as high as 5×10–17 cm2.
The reactions of ethylene glycol on Mo(110) were studied using temp.-programmed reaction, IR reflection absorption, and X-ray photoelectron spectroscopies. The major reaction pathway is double C-O bond scission to evolve gas-phase ethylene at 350 and 390 K. Both X-ray photoelectron and IR spectra demonstrate the existence of two surface intermediates, a bidentate (-OCH2CH2O-) and a monodentate (-OCH2CH2OH) species, at satn. coverage of ethylene glycol. We demonstrate that all ethylene glycol in the mixed overlayer of mono- and bidentate species reacts via a bidentate surface intermediate. Furthermore, in contrast to previous studies on other surfaces, the dialkoxide ethylene glycol intermediate is shown to be more reactive than similar monoalkoxides on Mo(110). Finally, anal. of the IR spectra demonstrates that the bidentate species adsorbs with C2 (or lower) symmetry at 300 K.
Fourier transform IR reflection absorption spectroscopy (FT-IRAS) was used to probe the non-dissociative adsorption of N2 on an atomically clean Pt(111) single crystal. In contradiction to a previous IRAS study of nitrogen adsorption on a Pt(111) foil at 120 K, no nitrogen IR (IR) band was obsd. on a fully annealed Pt(111) surface at 90 K. Following Ar+ ion bombardment, adsorption of nitrogen at 90 K produces an intense IR band at .apprx.2222cm-1 attributed to the N-N stretching mode of mol. nitrogen adsorbed on defect sites produced by ion bombardment. Annealing the Ar+ ion sputtered surface to a temp. above .apprx.750 K completely suppresses the adsorption of nitrogen at 90 K. Based on these and other results, it is postulated that nitrogen adsorbs at 90 K mainly on monovacancies on platinum. It is suggested that this specific adsorption occurs by sigma donation from nitrogen to the base of monovacancy sites which possess a low d-electron d. compared to surface Pt atoms.
The exposure of multilayers of an adsorbate to low energy (£55 eV) electrons under ultrahigh vacuum (UHV) conditions followed by temp.-programmed desorption (TPD) is shown to be an effective method to identify radiolysis products. In conjunction with isothermal expts., postirradn. TPD expts. were used to identify eight previously known radiolysis products of methanol. The utility of the method is demonstrated by the identification of a previously unknown methanol radiolysis product: methoxymethanol (CH3OCH2OH). Moreover, this technique allows study of the dependence on initial electron energy, providing addnl. insight into the phys. processes underlying radiation chem.
Supersonic mol. beam techniques and temp. programmed desorption (TPD) were used to study the adsorption dynamics of propane onto clean and propane-covered Pt(111). The propane sticking probability was measured directly as a function of incident translational energy, Ei, incident angle, qi, and propane coverage, qcov, at a surface temp. of 95 K Under these exptl. conditions, propane adsorbs molecularly onto Pt(111) terrace sites. Non-normal energy scaling is obsd. at all propane coverages indicating the importance of parallel momentum in the adsorption process. At all incident translational energies and angles studied, the sticking probability on a propane covered surface, S(qcov), increases with increasing propane coverage. Upon satn. of the Pt(111) terrace sites, spontaneous desorption is obsd. in the direct adsorption probability expts.
Recent progress in the application of supersonic mol. beam techniques to the study of gas-surface interfacial phenomena is reviewed. 336 Refs.
The trapping dynamics of Xe on Pt(1110 was probed as a function of Xe coverage with supersonic mol.-beam techniques. Adsorption probabilities were measured directly at a surface temp. of 95 K at coverages ranging from zero to monolayer satn. at incident translational energies between 6 and 63 kJ/mol and incident angles between 0 and 60°. In apparent agreement with the predictions of the original P. Kisliuk (1957) model, the adsorption probability at the lowest incident translational energy (6 kJ/mol) remains almost const. with coverage up to near monolayer satn. However, in contradiction to the original Kisliuk model, at higher incident translational energies, the trapping probability increases nearly linearly with Xe coverage up to near monolayer coverage. For example, the trapping probability increases from 0.06 to 0.42 for an incident translational energy of 63 kJ/mol at normal incidence as the coverage is increased from zero to satn. monolayer coverage. This behavior can be explained adequately by a model that incorporates enhanced trapping onto the monolayer compared to the clean surface, a property of the model that is confirmed directly by expts. presented herein. The angular dependence of the adsorption probability shows progressive deviation from normal energy scaling with increasing Xe surface coverage, proving that the degree to which parallel momentum participates in the adsorption process increases with adsorbate coverage. The initial trapping probability of Xe onto the monolayer is independent of incident angle indicating total-energy scaling. The above findings are qual. identical to the authors' previous results for the mol. adsorption of ethane on the same surface, suggesting that these phenomena occur, in general, for weak mol. adsorption regardless of mol. shape and internal degrees of freedom, at least for small mols.
A review with 26 refs. of the authors' work on adsorption of methane, ethane, propane, and Xe on Pt(111). General trends for the dependence of adsorption probability on incident translational energy, incident angle, surface temp., and adsorbate coverage are described.
The dynamics of associative adsorption of ethane on Pt(111) as a function of ethane coverage was probed with supersonic mol. beam techniques. Adsorption probabilities were measured directly at coverages ranging from 0 to monolayer satn. at incident translational energies between 10 and 40 kJ/mol and incident angles between 0 and 60° at a surface temp. of 95 K. In contrast to the predictions of the original P. Kisliuk (1957) model, at all incident translational energies and incident angles the adsorption probability increases with ethane coverage up to near monolayer coverage. This behavior can be fitted quite adequately by a model that incorporates enhanced adsorption onto the covered surface compared to the clean surface, utilizing the exptl. value of the adsorption probability onto the satd. monolayer. The angular dependence of the adsorption probability shows progressive deviation from normal energy scaling with increasing surface coverage, suggesting that the effective corrugation of the gas surface interaction potential increases with adsorbate coverage.
The mol. interaction of C2H6 with Pt(111) was studied by using supersonic mol. beam techniques and temp.-programmed desorption (TPD). Ethane adsorbs molecularly on the terrace sites of Pt(111) at 95 K with a satn. coverage of .apprx.0.3 monolayer and desorbs at .apprx.132 K during TPD. Isothermal desorption expts. suggest that C2H6 desorbs from the second layer with first-order desorption kinetics. Dissocn. of molecularly adsorbed C2H6 is negligible upon subsequent heating. The trapping probability of C2H6 on the clean surface at normal incidence decreases from .apprx.0.91 to 0.13 as the incident translational energy (ET) increases from 6 to 40 kJ/mol at a surface temp. of 95 K in apparent semiquant. agreement with a modified hard-cube model. However, over a range of incident angles, qi, the initial trapping probability scales with Et(cos2qi) demonstrating the participation of momentum parallel to the surface in the trapping process and the necessity of more sophisticated theories, such as 3-dimensional trajectory calcns., to describe adequately trapping over a wide range of incident translational energies and incident angles. At all incident translational energies studied, the trapping probability of C2H6 at a surface temp. of 95 K increases continuously with C2H6 coverage up to monolayer satn., indicating that C6H6 traps more efficiently onto adsorbed C2H6 than onto a clean surface. This behavior is expected since the mass of adsorbed C2H6 is significantly less than that of a Pt atom, leading to increased energy transfer upon impact with the adsorbed species. Based on this principle, extrinsic precursor states are expected for mol. adsorption on all surfaces except at translational energies that preclude trapping into the second layer. Together with mol. beam expts. of C2H6 dissocn. on Pt(111) performed in the authors' lab., the exptl. results indicate that dissociative C2H6 adsorption on Pt(111) proceeds more effectively via direct collisional activation at high incident translational energies rather than via a precursor-medicated mechanism at low incident translational energies.
Ethane trapping in a secondary-layer state on Pt(111) was studied with supersonic mol. beam techniques in order to investigate the dynamics of extrinsic precursor adsorption. Initial trapping probabilities of ethane on an ethane covered Pt(111) surface were measured directly as a function of incident translational energy and incident angle at a surface temp. of 95 K. At all incident translational energies and angles, the initial trapping probability into the second-layer state is higher than on a clean surface. In addn., the initial trapping probability into the second layer decreases less with incident translational energy than the initial trapping probability onto the clean surface. In contrast to previous findings for non-dissociative weak adsorption on clean surfaces showing the initial trapping probability to increase with incident angle, the initial trapping probability into the second layer is independent of incident angle indicating "total" energy scaling. A dynamical corrugation of the adsorbed layer is postulated to rationalize this strong deviation from the "normal" energy scaling implicit in one-dimensional theories of trapping.
The dynamics of Xe trapping on Pt(111) was studied using supersonic at. beam techniques. Initial trapping probabilities (S) were measured directly as a function of incident translational energy (ET) and angle of incidence (qi) at a surface temp. (Ts) of 95 K. The initial trapping probability decreases smoothly with increasing ET cosqi, rather than ET cos2qi, suggesting the participation of parallel momentum in the trapping process. Accordingly, the measured initial trapping probability falls off more slowly with increasing incident translational energy than predicted by one-dimensional theories. This finding is in near agreement with previous mean translational energy measurements for Xe desorbing near the Pt(111) surface normal, assuming detailed balance applies. Three-dimensional stochastic classical trajectory calcns. presented also exhibit the importance of tangential momentum in trapping and satisfactorily reproduce the exptl. initial trapping probabilities.
Dissociative chemisorption of methane on clean Pt(111) was studied with a supersonic mol. beam. Initial dissociative sticking probabilities from 0.01 to 0.19 were measured directly with incident total beam energies from 68 to 95 kJ/mol, surface temps. from 500 to 1250 K, and angles of incidence from 0° to 45° measured from the surface normal. The nozzle temp. and stagnation pressure were both fixed so that the effect of translational energy at a fixed incident vibrational energy could be probed. The initial dissociative sticking probability of methane on clean Pt(111) equalled 0.06 ± 0.02 and was independent of surface temp. between 500 and 1250 K for a fixed normal incident kinetic energy of 68 kJ/mol, implying that dissocn. proceeded via direct collisional activation rather than via trapping or precursor-mediated processes in this energy range. The initial dissociative sticking probability of methane on clean Pt(111) increased exponentially with increasing normal kinetic energy. The barrier height for C-H bond rupture by kinetic energy is 121 kJ/mol. The exponential dependence is consistent with a model for dissociative methane adsorption that involves quantum mech. tunneling of a hydrogen atom through a one-dimensional, parabolic barrier of this height with a thickness at half height of 0.13 ± 0.01 .ANG.. Differences in the initial dissociative sticking probabilities obsd. on Pt(111) vs. Ni(111) and W(110) in studies from different labs. can be reconciled on the basis of the different vibrational energies employed, but this explanation does not account for the high reactivity on Ir(110)-(1 x 2). The activation barriers predicted by the bond order conservation theory of Shustorovich ((1986) agree closely with the barrier heights estd. from the tunneling model for Pt(111), W(110), and Ir(110) if no correction for energy dissipation to the lattice is made. The activation barriers predicted by the MO anal. of Anderson and Maloney (1988) are much lower than the barrier heights estd. from the tunneling model. The barrier thickness show qual. agreement with the C-H bond elongations in the transition state predicted by their MO theory, but do not correlate with the cryst. radii of the metal atoms or the stiffness of the metal lattices. Furthermore, the values of the tunneling parameters may be dependent on the vibrational and translational energies employed in the studies.
The dynamics of mol. CH4 adsorption on Pt(111) were probed with supersonic mol. beam techniques. Initial trapping probabilities were measured directly between 0.94 and 0.16 for incident total translational energies between 3.4 and 20.2 kJ/mol and angles of incidence (with respect to the surface normal) between 0° and 45° at a surface temp. (Ts) of 100 K. The incident CH4 mols. were rotationally and vibrationally cold. The initial trapping probability decreases with increasing incident translational energy (ET) and decreasing angle of incidence (qi) and varies smoothly with incident normal energy (En = ET cos2qi), indicating a low corrugation of the mol.-surface interaction potential. The dependence of the initial trapping probability on incident normal translational energy agrees quant. with both a modified hard cube model and the U. Leuthaeusser (1981, 1983) theory at incident normal translational energies <8 kJ/mol. At higher incident normal translational energies, the obsd. initial trapping probabilities are higher than the values predicted by both models. Energy loss mechanisms other than surface phonon excitations may account partially for this discrepancy. A rapid decrease in the apparent adsorption probability as the surface temp. approaches 140 K is caused by the competitive influence of desorption. The temp. at which the apparent adsorption probability goes to zero agrees well with the desorption temp. measure independently by temp.-programmed desorption. In accordance with the aforementioned models, the measured in-plane angular distributions suggest that the trapping probability is relatively independent of surface temp. in the range of 160 to 500 K. The relatively low intensity of CH4 found near the surface normal in the angular distributions may be explained partially by a wider than cosine angular distribution or the trapped-desorbed channel, which is consistent with the observation that the trapping probability increases with angle of incidence. Comparison of the initial trapping probability vs. normal translational energy data to previous mean translational energy measurements of CH4 mols. desorbing from Pt(111) at the surface normal suggests that detailed balance applies for the non-equil. situation involving a collimated monoenergetic mol. beam of CH4 incident on a Pt(111) surface.
G.R. Schoofs, C.R. Arumainayagam, and R.J. Madix, "Dynamics of Ethane Adsorption on and Desorption from Pt(111) Determined from Direct Sticking Probability Experiments," J. Vac. Sci. Technol. A 6(3) (1988) 882.
**Note: In this publication, Arumainayagam is misspelled as Arumaninayagam