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Probing molecules on a surface by Cs+ reactive ion scattering: identification of C2Hx (x≤4) hydrocarbons
Applied Surface Science 203±204 (2003) 842±846
Probing molecules on a surface by Cs reactive ion scattering: identi®cation of C2Hx (x 4) hydrocarbons H. Kanga,*, C.W. Leeb, C.H. Hwangb, C.M. Kimc a
School of Chemistry, Seoul National University, Kwanak-ku, Shinrim-dong, Seoul 151-742, South Korea Department of Chemistry, Pohang University of Science and Technology, Pohang 790-784, South Korea c Department of Chemistry, Kyungpook National University, Taegu 702-701, South Korea
b
Abstract We studied molecular species appearing in the reactions of ethylene on a Pt(1 1 1) surface by the technique of Cs reactive ion scattering (Cs RIS). Dehydrogenation reaction of ethylene was examined for a surface temperature range of 100±800 K, and the RIS result veri®ed the well-known sequence of forming di-s-bonded ethylene (±CH2±CH2±), ethylidyne (BC±CH3), CH, and then surface carbons, as the temperature increased. In particular, the intermediate species in the conversion of ethylene to ethylidyne was closely investigated, which showed the presence of an ethylidene intermediate (=CH±CH3). In a study of H/D exchange reactions between surface C2D4 and H, we successfully identi®ed the ethylenes in which several deuterium atoms were substituted by hydrogen C2 D4 x Hx ; x 0 4, and quantitatively determined their relative populations. These examples demonstrate the ability of the Cs RIS method to identify small hydrocarbons and their isotopeexchanged species on surfaces. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Low-energy ion scattering; Ethylene; Platinum; Chemisorption; Isotope; Heterogeneous catalysis; Surface analysis
1. Introduction One of the major concerns in surface analysis is to identify molecules on surfaces. While the existing surface science techniques are powerful for studying atoms and simple molecules on surfaces, it is still a dif®cult problem to properly identify and characterize molecules of a complex structure. The need for molecular characterization is expected to become stronger, as the trend of surface science research and application moves from the study of simple molecules to more complex structures. At the beginning of the 21st *
Corresponding author. Tel.: 82-2-875-7471; fax: 82-2-889-1568. E-mail address: [email protected] (H. Kang).
century, we are already seeing the booming of research on complex molecules on surfaces such as self-assembled monolayers and functionalized molecules. An interesting hybrid of ion-surface scattering and SIMS techniques has been recently developed, called hyperthermal Cs reactive ion scattering (Cs RIS) [1]. The technique has been demonstrated [1] for its ability to do surface molecular analysis. The Cs RIS process is illustrated schematically in Fig. 1, by showing the trajectory of a hyperthermal Cs ion colliding with a surface adsorbed with a molecule. A simple analogy of the process may be the action of a ®sherman snatching a ®sh from water with a harpoon. A low-energy (5±100 eV) Cs ion beam collides onto a surface to induce desorption of adspecies X. The
0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 0 8 1 9 - X
H. Kang et al. / Applied Surface Science 203±204 (2003) 842±846
Fig. 1. Simpli®ed illustration of the Cs RIS trajectory leading to CsX formation. Collision of hyperthermal Cs causes desorption of neutral X from the surface. The desorbed X forms a transient CsX complex via ion±molecule attraction forces. Stabilization of the CsX complex is achieved through interaction between the complex and the surface (depicted by the dotted line).
desorbed X is picked up by the same incoming Cs ion to form a CsX cluster, which is detected by a mass spectrometer to reveal the mass of X. Note that this process is different from that of Cs SIMS which utilizes sputtered Cs ions to make CsX clusters. The RIS method has several unique features for detecting molecules on surfaces. First, it probes the desorbed neutrals (X) by measuring the scattered ions (CsX). In RIS experiment CsX is detected with a mass spectrometer ionizer switched off and, therefore, the measured product distribution is not interfered by the molecular cracking associated with post-ionization. Second, CsX is formed only when X is instantaneously desorbed upon Cs collision. Because the reactive scattering occurs in an ultrafast timescale (<10 12 s) [1], any species desorbed in a delayed time, for instance, the desorption products due to beaminduced secondary reactions, will be left behind the scattered Cs and not be detected. In this study, we used Cs RIS to analyze the various molecular species appeared in the dehydrogenation reaction of ethylene on a Pt(1 1 1) surface. Ethylene dehydrogenation reaction on a Pt(1 1 1) surface is one of the model reactions of heterogeneous catalysis, and its major reaction scheme is quite well established from extensive studies [2]. For brief introduction of the reaction, ethylene physisorbs as a molecule on a Pt(1 1 1) surface at temperature below 240 K, and is dehydrogenated to ethylidyne (PtBC± CH3) in the temperature range of 240±310 K. Above 450 K, ethylidyne is decomposed to C2H, CH, and
843
then eventually to surface carbons. Ethylene and ethylidyne species on Pt(1 1 1) have been clearly identi®ed by various experimental techniques, but the path of ethylene dehydrogenation to ethylidyne, taking place at 240±310 K, has not been well established. Several intermediates were proposed for this path, including surface ethyl (Pt±CH2±CH3), vinyl (Pt±CH=CH2), and ethylidene species (Pt=CH± CH3). We also examined the ethylenes in which several hydrogen atoms are substituted by deuterium on Pt(1 1 1). The H/D exchanged hydrocarbon species often provide a clue for understanding reaction mechanisms of surface reactions. These species, however, are very dif®cult to reliably determine their relative populations if a molecule undergoes multiple H/D substitution on a surface. These H/D substituted molecules are similar in their vibrational spectroscopic features. SIMS data can be quite confusing as the adsorbed molecules are fragmented by energetic ion beams. In the case of desorption methods, the desorbed molecules undergo fragmentation inside an ionizer of a mass spectrometer, rendering it dif®cult to do quantitative analysis. 2. Experimental We carried out the experiment in an angle-resolved RIS instrument [3]. The apparatus consists of a UHV chamber with a base vacuum of 5 10 11 Torr, a lowenergy Cs ion gun mounted on a rotatable stage, a quadrupole mass spectrometer (QMS) detector, an Auger electron spectrometer (AES), and a sputter ion gun. The Cs beam from the gun had an intensity of 1±10 nA cm 2 at energy of 5±100 eV. A Pt(1 1 1) single crystal was mounted on a heating-and-cooling stage of a vacuum sample manipulator and was located in an electric ®eld-free scattering region. The Pt sample was cleaned by repeated cycles of oxygen treatment, Ar ion sputtering, and annealing in UHV. After sample cleaning, ethylene gas was dosed on the surface at the desired reaction temperatures. Adsorbed molecules and the resulting surface products were probed by the RIS technique. After one cycle of measurement, a clean Pt surface was regenerated by heating the sample in O2 gas to remove the surface carbons left from ethylene reaction, and by ¯ash-annealing in UHV to evaporate Cs deposits.
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H. Kang et al. / Applied Surface Science 203±204 (2003) 842±846
The RIS measurement was performed in two ways. First, we scanned the QMS to obtain the mass spectrum of RIS products (CsX), from which we identi®ed molecules on the surface. Second, the QMS was ®xed to several pre-selected CsX peaks, and their intensity variation was monitored as a function of time while the sample temperature was increased. This mode of experiment monitored the temperature-dependent reaction kinetics in real-time. Study of RIS products as a function of Cs beam energy provided additional information about the surface species being examined. The scattering geometry was specular, with both the Cs beam and the QMS detector at 458 to the target surface, unless speci®ed otherwise. 3. Results and discussion We examined the surface species produced during ethylene dehydrogenation, by taking RIS mass spectra at temperatures between 100 and 800 K. Fig. 2 shows evolution of the RIS spectra taken as the reaction proceeds with temperature increase. Initial adsorption of C2H4 at 220 K produced a CsC2 H4 signal (spectrum (a)), con®rming that ethylene existed in a molecular state at the temperature. An extra peak due to physisorption of background water (CsH2O) is also observed. On increasing the temperature to 280 K (spectrum (b)), a CsCH3 peak appeared with a concomitant decrease of CsC2 H4 peak. At 320 K, only the CsCH3 peak appears (spectrum (c)). The peak is interpreted to be due to ethylidyne, produced by collisional breakage of the C±CH3 bond and by pickup of the methyl group. Ethylidyne is strongly bound to a Pt surface through a metal±carbon triple bond, and thus it is reasonable that the collision induce the C± CH3 bond cleavage instead of intact desorption of the CCH3 unit. Further increase of the temperature to 520 K (spectrum (d)) gave rise to CsC and CsCD signals (C2D4 was adsorbed instead of C2H4 in this experiment). In spectrum (e), taken at 770 K, only the CsC peak remained. The results are consistent with the knowledge that ethylidyne undergoes gradual decomposition at high temperature to C2H, CH, and then eventually to Cn species. It can be seen that these RIS spectra are consistent with the dehydrogenation path of ethylene on Pt(1 1 1) established in previous studies.
Fig. 2. Mass spectra of RIS products at several intermediate temperatures during ethylene decomposition on Pt(1 1 1). (a) An ethylene-adsorbed Pt surface was prepared by exposing 0.5 L of C2H4 at 220 K. The surface temperature was then raised to 280 K (b), 320 K (c), 520 K (d), and 770 K (e). D-substituted ethylene (C2D4) was adsorbed on surfaces (d) and (e). Cs impact energy was 30 eV in (a)±(c) and was increased to 50 eV in (d) and to 100 eV in (e). A 458/458 specular scattering geometry was used.
Spectrum (b) corresponds to the region in which the intermediate from ethylene to ethylidyne is formed, and it shows CsCH3 and CsC2 H4 peaks. This observation suggests that the intermediate is ethylidene, but denies the possibility of vinyl and ethyl species. Ethylidene can produce CsC2 H4 by molecular desorption and CsCH3 by C±C fragmentation, the latter process being analogous to the CsCH3 production from ethylidyne. This interpretation was supported by a control experiment that detected CsC2 H5 upon ethyl group adsorption on Pt(1 1 1) and studies of RIS products as functions of Cs impact energy, reaction time, and temperature [4]. Fig. 3 shows the result of a real-time kinetic study for CsCH3 and CsC2 H4 signals, obtained while the reaction temperature was varied from 220 to 570 K. In Fig. 3(a), the strong CsC2 H4 intensity below 260 K is due to molecular ethylene adsorption, and ethylene is transformed to ethylidene in the region of 270±290 K
H. Kang et al. / Applied Surface Science 203±204 (2003) 842±846
Fig. 3. Variation of CsC2 H4 (a) and CsCH3 (b) intensities with a temperature increase from 220 to 570 K. The vertical scales indicate the relative intensities of the two signals. The surface was initially adsorbed with 0.5 L of C2H4 at 220 K. The curves were obtained by real-time monitoring of the signals during the temperature increase. The temperature scan rate was 0.5 K/s below 400 K and 2 K/s above 400 K. Cs beam energy was 30 eV with 458/458 specular geometry.
where the CsC2 H4 intensity decreases rapidly. In Fig. 3(b), a small intensity CsCH3 signal at 270± 290 K originates from ethylidene, and the strong signal above 290 K is due to stable ethylidyne species at these temperatures. Mass spectrometric detection of Cs RIS products makes the technique particularly useful for identifying isotopically substituted molecules. We examined the H/D exchange reactions between C2D4 and H on a Pt(1 1 1) surface. Fig. 4 shows typical mass spectra of the Cs RIS products produced from a Pt surface coadsorbed with H and C2D4 at two different temperatures: (a) 200 K at which the isotope exchange is prohibited, and (b) 270 K at which the isotope exchange is facile. Spectrum (a) veri®es molecular adsorption of ethylene without H/D exchange. Spectrum (b) shows the presence of various H/D exchange products (C2D3H, C2D2H2, C2DH3, and C2H4) at 270 K. Note that the intensity distribution in spectrum (b) truly represents the relative amounts of the corresponding H/D substituted ethylenes on the surface. In the absence of the ionizer function of the mass spectrometer, the possibility of molecule cracking and
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Fig. 4. Mass spectra of RIS products from a Pt(1 1 1) surface dosed with 0.5 L of H2 and 0.5 L of C2D4 at 200 K. The sample temperature was 200 K (a) and 270 K (b).
isotope scrambling in a mass spectrometer is eliminated. Also, these substituted ethylenes should exhibit almost the same RIS sensitivity owing to their identical geometric structures. Considering that quantitative determination of multiple H/D exchange products is a dif®cult problem in surface analysis, Cs RIS has unique ability for this application. 4. Conclusion This study showed that the Cs RIS technique can identify various molecular species that appeared along the dehydrogenation reaction of ethylene on Pt(1 1 1). Kinetic study and detection of ethylidene intermediate revealed the mechanistic path of the reaction. Also, the relative populations of H/D substituted ethylenes on the surface have been quantitatively determined. Cs RIS detects neutral species desorbed from a surface by CsX formation, which gives a great advantage for surface molecular analysis compared to the techniques that require post-ionization of molecules or secondary particle ionization.
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H. Kang et al. / Applied Surface Science 203±204 (2003) 842±846
Acknowledgements This work was supported by the BK21 program from the Ministry of Education and from Korea Science and Engineering Foundation (C.M.K.). References [1] H. Kang, M.C. Yang, K.D. Kim, K.Y. Kim, Int. J. Mass Spectrom. Ion Process 174 (1998) 143.
[2] N. Sheppard, Ann. Rev. Phys. Chem. 39 (1988) 589. [3] S.-J. Han, C.-W. Lee, C.-H. Hwang, K.-H. Lee, M.C. Yang, H. Kang, Bull. Korean Chem. Soc. 22 (2001) 883. [4] C.-H. Hwang, C.-W. Lee, H. Kang, C.M. Kim, Surf. Sci. 490 (2001) 144.
Probing molecules on a surface by Cs reactive ion scattering: identi®cation of C2Hx (x 4) hydrocarbons H. Kanga,*, C.W. Leeb, C.H. Hwangb, C.M. Kimc a
School of Chemistry, Seoul National University, Kwanak-ku, Shinrim-dong, Seoul 151-742, South Korea Department of Chemistry, Pohang University of Science and Technology, Pohang 790-784, South Korea c Department of Chemistry, Kyungpook National University, Taegu 702-701, South Korea
b
Abstract We studied molecular species appearing in the reactions of ethylene on a Pt(1 1 1) surface by the technique of Cs reactive ion scattering (Cs RIS). Dehydrogenation reaction of ethylene was examined for a surface temperature range of 100±800 K, and the RIS result veri®ed the well-known sequence of forming di-s-bonded ethylene (±CH2±CH2±), ethylidyne (BC±CH3), CH, and then surface carbons, as the temperature increased. In particular, the intermediate species in the conversion of ethylene to ethylidyne was closely investigated, which showed the presence of an ethylidene intermediate (=CH±CH3). In a study of H/D exchange reactions between surface C2D4 and H, we successfully identi®ed the ethylenes in which several deuterium atoms were substituted by hydrogen C2 D4 x Hx ; x 0 4, and quantitatively determined their relative populations. These examples demonstrate the ability of the Cs RIS method to identify small hydrocarbons and their isotopeexchanged species on surfaces. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Low-energy ion scattering; Ethylene; Platinum; Chemisorption; Isotope; Heterogeneous catalysis; Surface analysis
1. Introduction One of the major concerns in surface analysis is to identify molecules on surfaces. While the existing surface science techniques are powerful for studying atoms and simple molecules on surfaces, it is still a dif®cult problem to properly identify and characterize molecules of a complex structure. The need for molecular characterization is expected to become stronger, as the trend of surface science research and application moves from the study of simple molecules to more complex structures. At the beginning of the 21st *
Corresponding author. Tel.: 82-2-875-7471; fax: 82-2-889-1568. E-mail address: [email protected] (H. Kang).
century, we are already seeing the booming of research on complex molecules on surfaces such as self-assembled monolayers and functionalized molecules. An interesting hybrid of ion-surface scattering and SIMS techniques has been recently developed, called hyperthermal Cs reactive ion scattering (Cs RIS) [1]. The technique has been demonstrated [1] for its ability to do surface molecular analysis. The Cs RIS process is illustrated schematically in Fig. 1, by showing the trajectory of a hyperthermal Cs ion colliding with a surface adsorbed with a molecule. A simple analogy of the process may be the action of a ®sherman snatching a ®sh from water with a harpoon. A low-energy (5±100 eV) Cs ion beam collides onto a surface to induce desorption of adspecies X. The
0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 0 8 1 9 - X
H. Kang et al. / Applied Surface Science 203±204 (2003) 842±846
Fig. 1. Simpli®ed illustration of the Cs RIS trajectory leading to CsX formation. Collision of hyperthermal Cs causes desorption of neutral X from the surface. The desorbed X forms a transient CsX complex via ion±molecule attraction forces. Stabilization of the CsX complex is achieved through interaction between the complex and the surface (depicted by the dotted line).
desorbed X is picked up by the same incoming Cs ion to form a CsX cluster, which is detected by a mass spectrometer to reveal the mass of X. Note that this process is different from that of Cs SIMS which utilizes sputtered Cs ions to make CsX clusters. The RIS method has several unique features for detecting molecules on surfaces. First, it probes the desorbed neutrals (X) by measuring the scattered ions (CsX). In RIS experiment CsX is detected with a mass spectrometer ionizer switched off and, therefore, the measured product distribution is not interfered by the molecular cracking associated with post-ionization. Second, CsX is formed only when X is instantaneously desorbed upon Cs collision. Because the reactive scattering occurs in an ultrafast timescale (<10 12 s) [1], any species desorbed in a delayed time, for instance, the desorption products due to beaminduced secondary reactions, will be left behind the scattered Cs and not be detected. In this study, we used Cs RIS to analyze the various molecular species appeared in the dehydrogenation reaction of ethylene on a Pt(1 1 1) surface. Ethylene dehydrogenation reaction on a Pt(1 1 1) surface is one of the model reactions of heterogeneous catalysis, and its major reaction scheme is quite well established from extensive studies [2]. For brief introduction of the reaction, ethylene physisorbs as a molecule on a Pt(1 1 1) surface at temperature below 240 K, and is dehydrogenated to ethylidyne (PtBC± CH3) in the temperature range of 240±310 K. Above 450 K, ethylidyne is decomposed to C2H, CH, and
843
then eventually to surface carbons. Ethylene and ethylidyne species on Pt(1 1 1) have been clearly identi®ed by various experimental techniques, but the path of ethylene dehydrogenation to ethylidyne, taking place at 240±310 K, has not been well established. Several intermediates were proposed for this path, including surface ethyl (Pt±CH2±CH3), vinyl (Pt±CH=CH2), and ethylidene species (Pt=CH± CH3). We also examined the ethylenes in which several hydrogen atoms are substituted by deuterium on Pt(1 1 1). The H/D exchanged hydrocarbon species often provide a clue for understanding reaction mechanisms of surface reactions. These species, however, are very dif®cult to reliably determine their relative populations if a molecule undergoes multiple H/D substitution on a surface. These H/D substituted molecules are similar in their vibrational spectroscopic features. SIMS data can be quite confusing as the adsorbed molecules are fragmented by energetic ion beams. In the case of desorption methods, the desorbed molecules undergo fragmentation inside an ionizer of a mass spectrometer, rendering it dif®cult to do quantitative analysis. 2. Experimental We carried out the experiment in an angle-resolved RIS instrument [3]. The apparatus consists of a UHV chamber with a base vacuum of 5 10 11 Torr, a lowenergy Cs ion gun mounted on a rotatable stage, a quadrupole mass spectrometer (QMS) detector, an Auger electron spectrometer (AES), and a sputter ion gun. The Cs beam from the gun had an intensity of 1±10 nA cm 2 at energy of 5±100 eV. A Pt(1 1 1) single crystal was mounted on a heating-and-cooling stage of a vacuum sample manipulator and was located in an electric ®eld-free scattering region. The Pt sample was cleaned by repeated cycles of oxygen treatment, Ar ion sputtering, and annealing in UHV. After sample cleaning, ethylene gas was dosed on the surface at the desired reaction temperatures. Adsorbed molecules and the resulting surface products were probed by the RIS technique. After one cycle of measurement, a clean Pt surface was regenerated by heating the sample in O2 gas to remove the surface carbons left from ethylene reaction, and by ¯ash-annealing in UHV to evaporate Cs deposits.
844
H. Kang et al. / Applied Surface Science 203±204 (2003) 842±846
The RIS measurement was performed in two ways. First, we scanned the QMS to obtain the mass spectrum of RIS products (CsX), from which we identi®ed molecules on the surface. Second, the QMS was ®xed to several pre-selected CsX peaks, and their intensity variation was monitored as a function of time while the sample temperature was increased. This mode of experiment monitored the temperature-dependent reaction kinetics in real-time. Study of RIS products as a function of Cs beam energy provided additional information about the surface species being examined. The scattering geometry was specular, with both the Cs beam and the QMS detector at 458 to the target surface, unless speci®ed otherwise. 3. Results and discussion We examined the surface species produced during ethylene dehydrogenation, by taking RIS mass spectra at temperatures between 100 and 800 K. Fig. 2 shows evolution of the RIS spectra taken as the reaction proceeds with temperature increase. Initial adsorption of C2H4 at 220 K produced a CsC2 H4 signal (spectrum (a)), con®rming that ethylene existed in a molecular state at the temperature. An extra peak due to physisorption of background water (CsH2O) is also observed. On increasing the temperature to 280 K (spectrum (b)), a CsCH3 peak appeared with a concomitant decrease of CsC2 H4 peak. At 320 K, only the CsCH3 peak appears (spectrum (c)). The peak is interpreted to be due to ethylidyne, produced by collisional breakage of the C±CH3 bond and by pickup of the methyl group. Ethylidyne is strongly bound to a Pt surface through a metal±carbon triple bond, and thus it is reasonable that the collision induce the C± CH3 bond cleavage instead of intact desorption of the CCH3 unit. Further increase of the temperature to 520 K (spectrum (d)) gave rise to CsC and CsCD signals (C2D4 was adsorbed instead of C2H4 in this experiment). In spectrum (e), taken at 770 K, only the CsC peak remained. The results are consistent with the knowledge that ethylidyne undergoes gradual decomposition at high temperature to C2H, CH, and then eventually to Cn species. It can be seen that these RIS spectra are consistent with the dehydrogenation path of ethylene on Pt(1 1 1) established in previous studies.
Fig. 2. Mass spectra of RIS products at several intermediate temperatures during ethylene decomposition on Pt(1 1 1). (a) An ethylene-adsorbed Pt surface was prepared by exposing 0.5 L of C2H4 at 220 K. The surface temperature was then raised to 280 K (b), 320 K (c), 520 K (d), and 770 K (e). D-substituted ethylene (C2D4) was adsorbed on surfaces (d) and (e). Cs impact energy was 30 eV in (a)±(c) and was increased to 50 eV in (d) and to 100 eV in (e). A 458/458 specular scattering geometry was used.
Spectrum (b) corresponds to the region in which the intermediate from ethylene to ethylidyne is formed, and it shows CsCH3 and CsC2 H4 peaks. This observation suggests that the intermediate is ethylidene, but denies the possibility of vinyl and ethyl species. Ethylidene can produce CsC2 H4 by molecular desorption and CsCH3 by C±C fragmentation, the latter process being analogous to the CsCH3 production from ethylidyne. This interpretation was supported by a control experiment that detected CsC2 H5 upon ethyl group adsorption on Pt(1 1 1) and studies of RIS products as functions of Cs impact energy, reaction time, and temperature [4]. Fig. 3 shows the result of a real-time kinetic study for CsCH3 and CsC2 H4 signals, obtained while the reaction temperature was varied from 220 to 570 K. In Fig. 3(a), the strong CsC2 H4 intensity below 260 K is due to molecular ethylene adsorption, and ethylene is transformed to ethylidene in the region of 270±290 K
H. Kang et al. / Applied Surface Science 203±204 (2003) 842±846
Fig. 3. Variation of CsC2 H4 (a) and CsCH3 (b) intensities with a temperature increase from 220 to 570 K. The vertical scales indicate the relative intensities of the two signals. The surface was initially adsorbed with 0.5 L of C2H4 at 220 K. The curves were obtained by real-time monitoring of the signals during the temperature increase. The temperature scan rate was 0.5 K/s below 400 K and 2 K/s above 400 K. Cs beam energy was 30 eV with 458/458 specular geometry.
where the CsC2 H4 intensity decreases rapidly. In Fig. 3(b), a small intensity CsCH3 signal at 270± 290 K originates from ethylidene, and the strong signal above 290 K is due to stable ethylidyne species at these temperatures. Mass spectrometric detection of Cs RIS products makes the technique particularly useful for identifying isotopically substituted molecules. We examined the H/D exchange reactions between C2D4 and H on a Pt(1 1 1) surface. Fig. 4 shows typical mass spectra of the Cs RIS products produced from a Pt surface coadsorbed with H and C2D4 at two different temperatures: (a) 200 K at which the isotope exchange is prohibited, and (b) 270 K at which the isotope exchange is facile. Spectrum (a) veri®es molecular adsorption of ethylene without H/D exchange. Spectrum (b) shows the presence of various H/D exchange products (C2D3H, C2D2H2, C2DH3, and C2H4) at 270 K. Note that the intensity distribution in spectrum (b) truly represents the relative amounts of the corresponding H/D substituted ethylenes on the surface. In the absence of the ionizer function of the mass spectrometer, the possibility of molecule cracking and
845
Fig. 4. Mass spectra of RIS products from a Pt(1 1 1) surface dosed with 0.5 L of H2 and 0.5 L of C2D4 at 200 K. The sample temperature was 200 K (a) and 270 K (b).
isotope scrambling in a mass spectrometer is eliminated. Also, these substituted ethylenes should exhibit almost the same RIS sensitivity owing to their identical geometric structures. Considering that quantitative determination of multiple H/D exchange products is a dif®cult problem in surface analysis, Cs RIS has unique ability for this application. 4. Conclusion This study showed that the Cs RIS technique can identify various molecular species that appeared along the dehydrogenation reaction of ethylene on Pt(1 1 1). Kinetic study and detection of ethylidene intermediate revealed the mechanistic path of the reaction. Also, the relative populations of H/D substituted ethylenes on the surface have been quantitatively determined. Cs RIS detects neutral species desorbed from a surface by CsX formation, which gives a great advantage for surface molecular analysis compared to the techniques that require post-ionization of molecules or secondary particle ionization.
846
H. Kang et al. / Applied Surface Science 203±204 (2003) 842±846
Acknowledgements This work was supported by the BK21 program from the Ministry of Education and from Korea Science and Engineering Foundation (C.M.K.). References [1] H. Kang, M.C. Yang, K.D. Kim, K.Y. Kim, Int. J. Mass Spectrom. Ion Process 174 (1998) 143.
[2] N. Sheppard, Ann. Rev. Phys. Chem. 39 (1988) 589. [3] S.-J. Han, C.-W. Lee, C.-H. Hwang, K.-H. Lee, M.C. Yang, H. Kang, Bull. Korean Chem. Soc. 22 (2001) 883. [4] C.-H. Hwang, C.-W. Lee, H. Kang, C.M. Kim, Surf. Sci. 490 (2001) 144.