Room-temperature adsorption of three model organosulfur compounds, dimethyl disulfide (CH3SSCH3),methanethiol (CH3SH), and dimethyl sulfide (CH3SCH3), on Si(100) has been investigated using Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), temperature-programmed desorption (TPD), and density functional theory (DFT) calculations. AES measurements show that each of these organosulfur compounds has a characteristic surface saturation coverage and reaction probability with Si(100). TPD studies indicate that methyl groups of these organosulfur compounds remain intact upon adsorption on Si(100). DFT calculations, in conjunction with our experimental results, provide insight into the most likely chemisorption channel for each organosulfur compound.
3. A. Srivastava, and R.M. Osgood, Jr. State-resolved dynamics of 248 nm methyl-iodide fragmentation on GaAs (110). Journal of Chemical Physics, 119: 10298-10306, 2003.
The 248 nm initiated dissociation dynamics of methyl iodide physisorbed on GaAs ~110! is investigated using 211 REMPI detection of the CH3 umbrella mode. The velocity and vibrational distributions of the photofragment are dependent on the adsorbate coverage and surface termination. Two translational energy distributions are measured for the CH3 fragment at high coverage, i.e., 25 ML, with maxima at 1 and 1.9 eV. These maxima are within 0.5-0.7 eV of those obtained in direct dissociation of gas-phase CH3I. At 1 ML one distribution is measured with its maxima at 0.3 eV. A modification of the surface with chemisorbed iodine reduces the yield of the CH3 at 1 ML but not at 25 ML. The dissociation dynamics observed at 1 ML is attributed to bond scission following electron attachment while that at 25 ML to direct photodissociation. At 3 ML coverage both the 1 and 25 ML components are observed. The measured fragment vibrational distribution at 1 ML has a 4 times lower v951/v950 ratio compared with that at 25 ML. Differences in the translational and vibrational distributions for the two dissociation mechanisms are attributed to the available energy and molecular geometry of photoexcited CH3I.
4. K.T. Rim, J.P. Fitts, T. Muller, K. Adib, N. Camillone III, R.M. Osgood,Jr., S.A. Joyce, and G.W. Flynn CCl/sub4/ chemistry on the reduced selvedge of a alpha-Fe/sub2/O/sub3/(0 0 0 1) surface: a scanning tunneling microscopy study. Surface Science, 541: 59-75, 2003.
Scanning tunneling microscopy (STM) and low energy electron diffraction (LEED) were used to study the degradation of CCl/sub4/ on the reduced selvedge of a natural single crystal alpha-Fe/sub2/O/sub3/(0 0 0 1) surface in ultrahigh vacuum. Before exposure to CCl/sub4/, STM images indicate that approximately 85% of the reduced surface exhibits a Fe/sub3/O/sub4/(1 1 1) 2 × 2 termination, while the remaining 15% is terminated by 1 × 1 and superstructure phases. Images obtained after room temperature dosing with CCl/sub4/ and subsequent flashing to 600 K reveal that chlorine atoms are adsorbed only on surface regions with the Fe/sub3/O/sub4/(1 1 1) 2 × 2 termination, not on 1 × 1 and superstructure regions. Chlorine atoms from dissociative adsorption of CCl/sub4/ are observed to occupy two distinct positions located atop lattice protrusions and in threefold oxygen vacancy sites. However, in companion chemical labeling experiments, chlorine atoms provided by room temperature, dissociative Cl/sub2/ adsorption on this surface are found to occupy sites atop lattice protrusions exclusively. The clear dissimilarity in STM feature shape and brightness at the two distinct chlorine adsorption sites arising from CCl/sub4/ dissociation as well as the results of the Cl/sub2/ chemical labeling experiments are best explained via reactions on a Fe/sub3/O/sub4/(1 1 1) 2 × 2 selvedge terminated by a 1/4 monolayer of tetrahedrally coordinated iron atoms. On this surface, adsorption atop an iron atom occurs for both the CCl/sub4/ and Cl/sub2/ dissociative reactions. A second adsorption site, assigned as binding to second layer iron atoms left exposed following surface oxygen atom abstraction resulting in the formation of phosgene (COCl/sub2/), only appears in the case of reaction with CCl/sub4/. The reaction mechanism and active site requirements for CCl/sub4/ degradation on iron oxide surfaces are discussed in light of this evidence and in the context of our previously reported results from Auger electron spectroscopy (AES), LEED, temperature-programmed desorption (TPD), and X-ray photoelectron spectroscopy studies.
5. K.Adib, G.Totir, J.P. Fitts, T. Miller, G.W. Flynn, S.A. Joyce and R.M. Osgood, Jr. Chemistry of CCl/sub4/ on Fe3O4(111)-(2x2) Surfaces in the Presence of Adsorbed D2O. Surface Science, 537: 191-204, 2003.
Temperature programmed desorption (TPD) was used to study surface reactions of Fe3O4(1 1 1)-(2 × 2) sequentially exposed, at ~100 K, to vapor-phase D2O and CCl4. Previous TPD and XPS results have indicated that in the absence of D2O, CCl4 dissociatively adsorbs on Fe3O4(1 1 1) producing chemisorbed Cl and CCl2. Subsequent heating of the surface results in abstraction of lattice iron and oxygen atoms and causes them to desorb as FeCl2 and OCCl2, respectively. This study shows that when this Fe3O4 surface is exposed only to D2O, TPD measures a rich surface chemistry with multiple desorption events extending as high as ~800 K, indicating dissociative adsorption of D2O on the Fe3O4(1 1 1) surface. After sequential exposure to D2O and then CCl4, the production of FeCl2 and OCCl2 from adsorbed CCl4 is suppressed, indicating that D2O fragments block the surface reactive sites.
6. K. Adib, D.R. Mullins, G.Totir, N.Camillone III, J.P. Fitts, K.T. Rim, G.W. Flynn, R.M. Osgood, Jr. Dissociative adsorption of CCl/sub4/ on the Fe/sub3/O/sub4/(111)-(2x2)selvedge of Alpha Fe/sub2/O/sub3/(0001)Surface Science, 524 (1-3): 113-128, 2003
The surface reactions of CCl/sub4/ with the Fe/sub3/O/sub4/(111)-(2x2) selvedge of naturally occuring Alpha Fe/sub2/O/sub3/(0001) single-crystals have been investigated using synchrotron X-ray photoelectron spectrocopy (XPS) and temperature-programmed desorption (TPD. CCl/sub4/ was found to dissociate on the Fe/sub3/O/sub4/ surface at 100 K producing chemisorbed Cl and adsorbed CCl/sub2/. TPD shows that the large majority of the dissociatively adsorbed CCl/sub2/ fragments extract latice oxygen and desorb as phosgene at >275 K. However, the XPSs spectra show no evidence for the formation of surface-bound phosgene, at 100 K, indicating that its formation involves two steps. The first step, dissociation, is spontaneous at 100 K, whereas the second, oxygen atom abstraction to form phosgene, requires thermal excitation. Cl chemisorption yielded two seperate species, the mono-and dichloride terminations of surface iron sites. The identification of these two surface terminations is based on the coverage dependence and the surface temperature history of their Cl2p/sub2/3/ peak intensity. For example, heating to >450 K allows the monochloride to transform into iron dichloride, indication Cl adatom mobility at these temperatures.
7. Nicholas Camillone III, Kaveh Adib, Khalid A. Khan, Dan Mocuta, and Richard M. Osgood, Jr. Dimethyl Sulfide Formation from Adsorbed Methanethiol: Surface Trapping of UV-Generated Reaction Intermediates. J. Phys. Chem B, 106 (48): 12491-12498, 2002
We present studies of the photoinduced surface chemistry of methanethiol adsorbed on GaAs(110). UV irradiation of these molecules, which are intact at ~90 K, induces scission of the S-H bond, leaving the CH3S fragment trapped at the surface. The measured cross section at 193 nm for this photoinduced process is ~3.6 × 10/sup-20/ cm/sup2/. Postirradiation heating of the surface-bound products of the methanethiol photoreaction results in the evolution of dimethyl sulfide with the same desorption kinetics as observed for the strictly thermal production of dimethyl sulfide from dissociatively adsorbed methyl disulfide. Thus, UV irradiation of CH/sub3/SH/GaAs(110) followed by thermal activation of the surface-trapped intermediates is shown to provide a new route to partial desulfurization of CH/sub3/SH.
8. Nicholas Camillone III, Kaveh Adib, Jeffrey P. Fitts, Kwang, T. Rim, George W. Flynn, S. A. Joyce and Richard M. Osgood. Surface termination dependence of the reactivity of single crystal hematite with CCl/sub4/. Surface Science, 511 (1-3): 267-282, 2002
We describe ultrahigh vacuum Auger electron spectrometric measurements of the uptake of chlorine following the room temperature exposure of single crystal hematite, -Fe/sub2/O/sub3/, to CCl/sub4/. We compare the surface chemistry of two specific surface phases formed on the basal plane of -Fe/sub2/O/sub3/: the Fe/sub3/O/sub4/(1 1 1)-(2×2) "selvedge" and the -Fe/sub2/O/sub3//Fe1-xO "biphase." For Fe/sub3/O/sub4/(1 1 1)-(2×2) an estimated saturation level of Cl of ~75% of a monolayer is readily attained. Carbon uptake is well below that expected for simple stoichiometric dissociative chemisorption, consistent with desorption of organic products during the surface reaction. Low energy electron diffraction measurements suggest that, dependent upon preparation procedures, at least two types of -Fe/sub2/O/sub3//Fe1-xO biphase structures can be formed. Surprisingly, upon exposure to CCl/sub4/, Cl uptake does not occur on either of these biphase surfaces, despite the fact that these surfaces are thought to have the same surface concentrations of iron and oxygen as Fe/sub3/O/sub4/(1 1 1). The dramatic difference between the reactivity of the Fe/sub3/O/sub4/ and biphase surfaces suggests that the active site for the dissociative adsorption of CCl/sub4/on Fe/sub3/O/sub4/(1 1 1)-(2×2) comprises both an iron cation and an oxygen anion with a surface-normal-oriented dangling bond that is uncapped by iron cations. Electron stimulated and thermal desorption of Cl from the saturated Fe/sub3/O/sub4/(1 1 1)-(2×2) selvedge is also reported.
9. Abneesh Srivastavaa and Richard M. Osgood, Jr Photoreaction dynamics of CH/sub3/I multilayers on GaAs(1 1 0): REMPI probing of the CH/sub3/ umbrella mode. Chemical Physics Letters, 355 (3-4):371-377,2002
The UV photodissociation dynamics of CH/sub3/I multilayers on GaAs(1 1 0) are studied using 2 + 1 resonant enhanced multiphoton ionization (REMPI) detection of the CH/sub3/v/sub2/ umbrella vibrational mode. The vibrational distribution formed appears much colder than the distribution resulting from gas-phase photodissociation of CH/sub3/I. Measurements of the surface reaction cross-section versus photon exposure and time-of-flight (TOF) studies of the CH/sub3/ translational energy suggest that the colder temperature is attributable to dissociative electron attachment (DEA) by photoemitted substrate electrons.
10. K. Adib, N. Camillone III, 1, J. P. Fitts, K. T. Rim, G. W. Flynn, S. A. Joyce, and R. M. Osgood Jr. CCl/sub4/ chemistry on the magnetite selvedge of single-crystal hematite: competitive surface reactions. Surface Science, 497 (1-3): 127-138, 2002
Temperature programmed reaction/desorption (TPR/D) studies were undertaken to characterize the surface chemistry which occurs between CCl/sub4/ and the Fe/sub3/O/sub4/ (1 1 1) selvedge of single crystal -Fe/sub2/O/sub3/ (0 0 0 1). Six separate desorption events are clearly observed and four desorbing species are identified: CCl/sub4/, OCCl/sub2/, C/sub2/Cl/sub4/ and FeCl/sub2/. It is proposed that OCCl/sub2/, CCl/sub4/ and C/sub2/Cl/sub4/ are produced in reactions involving the same precursor, CC/sub2/. Three reaction paths compete for the CCl/sub2/ precursor: oxygen atom abstraction (for OCCl2), molecular recombinative desorption (for CCl/sub4/) and associative desorption (for C/sub2/Cl/sub4/). During the TPR/D temperature ramp, the branching ratio is observed to depend upon temperature and the availability of reactive sites. The data are consistent with a rich site-dependent chemistry.
11. X.J. Shen, H. Kwak, A.M. Radojevic, S. Smadici, D. Mocuta, and R. M. Osgood, Jr. Momentum-resolved excited-electron lifetimes on stepped cu(775). Chemical Physics Letters, 351 (1-2):1-8, 2002.
The lifetime of n=1 image-state electrons on stepped on Cu(775) has be en measured as a function of their translational momentum, k/sub ||/. At the ter race normal this lifetime is equal to that for flat Cu(111), i.e. tau =18+or-2 f s. For motion parallel to the step orientation, the lifetime decreases symmetric ally about k/sub ||/=0 and the absolute values are in close agreement with that predicted by a recent many-body theory. Contrasting data for motion perpendicula r to the step edges is also presented
12. Nicholas Camillone III, K.A. Khan, J.A. Yarmoff, and R.M. Osgood, Jr., Surface-reconstruction-switched adsorbate photofragmentation dynamics. Physical Review Letters, 87 (5):056101/1-4, 2001.
Energy-resolved angular distributions of neutral fragments ejected during photoinduced electron transfer reaction of CH3Br on GaAs(100) exhibit three distinct methyl-radical ejection channels. These undergo marked changes when the termination is switched from the Ga-rich c(8*2) to the As-rich c(2*8). Our observations are consistent with a strong adsorption-site dependence of the dynamics.
13. X.J. Shen, H. Kwak, D. Mocuta, A.M. Radojevic, S. Smadici, and R.M. Osgood, Jr. Observation of a one-dimensional state on stepped cu(775). Physical Review B (Condensed Matter and Materials Physics), 63 (16):165403/1-8, 2001.
We report on using high-resolution angle-resolved two-photon photoemission to observe a one-dimensional surface state 0.27 eV below the Fermi level on a stepped Cu(111) surface. The state is dispersive along the step direction but localized perpendicular to the step orientation. The origin of the state may be explained by confinement due to the step-edge potential
14. K.A. Khan, N. Camillone III, J.A. Yarmoff, and R. M. Osgood, Jr. Modification of the surface termination of gaas(001) using photon-activated electron-transfer reactions. Surface Science, 458 (1-3):53-62, 2000.
We report the use of a photon-activated electron-transfer chemical reaction to modify the surface termination of the (001) face of single-crystal GaAs. The technique involves the extraction of surface Ga atoms as a result of reaction with bromine. The bromine is produced at the surface by photoinitiated dissociative electron attachment to methyl bromide molecules in a single monolayer physisorbed at 80 K. Subsequent to the photoinduced surface reaction, the gallium is removed by annealing to desorb a gallium halide product. We have demonstrated that variation in photon exposure and thermal treatment allows the surface reconstruction to be controllably adjusted from the Ga-rich c(8*2) to the (4*6), (3*1) and As-rich c(2*8) terminations. A comparison of the results obtained with several surface diagnostics, including low energy electron diffraction, temperature programmed desorption and energy-resolved photofragment angular distribution measurements, indicates that the most As-rich surface obtained by our technique is identical in structure to that of a control surface prepared using the standard thermal reaction technique. In principle, the use of this photon-activated reaction, and others like it, allows precise adjustment and patterning of the surface structure based on control of photon or electron exposure, molecular coverage, thermal treatment and lateral patterning of the incident photon or electron beam
15. N. Camillone III, K.A. Khan, and R. M. Osgood, Jr. The thermal chemistry of model organosulfur compounds on gallium arsenide (110). Surface Science, 453(1-3):83-102, 2000.
Organothiols and related organosulfur compounds hold promise for use as self-assembled ultra-thin electron beam resists, II-VI material growth precursors, and monolayer passivation layers. We report the results of temperature-programmed reaction and desorption (TPR/D) studies of the surface chemistry of three model organosulfur compounds, CH/sub 3/SH, (CH/sub 3/S)/sub 2/ and (CH/sub 3/)/sub 2/S, on the (110) face of gallium arsenide. Our measurements indicate that each of these species interacts strongly with the GaAs(110) surface. The first monolayer of CH/sub 3/SH desorbs at 300 K, with a desorption wave peak temperature (extrapolated to the zero coverage limit) corresponding to a desorption activation energy of 0.81 eV. This value is high compared to 110 K for the second layer, suggesting a chemical interaction between the thiol and the surface, despite the fact that the monolayer feature in the TPR/D spectra exhibits first-order kinetics, and the molecule is observed to desorb intact. The behavior of (CH/sub 3/S)/sub 2/ is markedly different; the disulfide decomposes upon adsorption (or during the TPR/D temperature ramp) and desorbs predominantly as (CH/sub 3/)/sub 2/S at 500 K, perhaps as the result of a concerted associative/dissociative desorption process that leaves sulfur at the surface. This result is also evidence that adsorbed CH/sub 3/SH and (CH/sub 3/)/sub 2/S do not assume equivalent surface-bound forms and supports the idea that the hydrogen atom of the thiol remains in close association with the adsorbed parent molecule, while the disulfide undergoes dissociative adsorption. Finally, our results indicate that (CH/sub 3/)/sub 2/S, like CH/sub 3/SH, molecularly adsorbs/desorbs. The desorption activation energy, extrapolated to zero coverage, is 0.79 eV, consistent with a strong adsorbate-substrate interaction. The similarity of this value to that for CH/sub 3/SH suggests that a specific sulfur-surface interaction dominates the desorption kinetics at low coverage. In the monolayer regime, with increasing exposure, the monolayer desorption feature in the TPR/D spectra broadens and exhibits strong asymmetry, and ultimately develops a lower temperature feature, indicating complex kinetics consistent with coverage-dependent phase behavior. Finally, by comparison of the measured desorption energies of the organosulfur compounds with those of the alkyl halides, for which the adsorption on GaAs(110) has been extensively investigated, an estimation of the chemical and physical contributions to the molecule-surface interaction is obtained. Our analysis suggests that both types of interactions play important roles in the adsorption energetics
16. Y. Luo, M. Han, D.A. Slater, and R. M. Osgood, Jr. Studies of heteroepitaxial growth of thin ii-vi semiconductor layers by sequential ultrahigh vacuum dosing. Journal of Vacuum Science & Technology A (Vacuum, Surfaces, and Films), 18(2):438-49, 2000.
An in situ molecular-level study of material growth using a binary reaction sequence of hydride and metalorganic precursors is presented. The study used a model material system of CdS/ZnSe(100) and focused on the material chemistry of heteroepitaxy growth. In the growth process, dimethylcadmium and H/sub 2/S precursors were sequentially dosed onto a c(2*2) ZnSe(100) substrate under high-vacuum conditions. At temperatures of 300 K, saturated chemisorption of a Cd and a S monolayer occurred during each cycle of the binary reaction sequence. Characterization of the growth surface was accomplished in the growth chamber using Auger electron spectroscopy, X-ray photoelectron spectroscopy and low-energy ion scattering spectroscopy for probing surface chemical composition and low-energy electron diffraction for determining surface order. These measurements showed layer-by-layer growth at a substrate temperature of 300 K, yielding an ordered stoichiometric CdS film. Strong variations in the composition of the grown surface layer were observed at different substrate temperatures; these variations were found to be related to the temperature dependence of the precursor reactions with the growth surfaces.
17. X.J. Shen, H. Kwak, A.M. Radojevic, and R. M. Osgood, Jr. Momentum-dependent ultrafast dynamics of image states on flat and stepped cu(111) surfaces. Technical Digest. Summaries of papers presented at the Quantum Electronics and Laser Science Conference, pages 230-1, 1999.
Summary form only given. Ultrafast time-resolved two-photon photoemission has recently been used to probe carrier dynamics at surfaces and interfaces. Here we describe a k-space state-resolved study of image-state lifetimes, which measures the time evolution of the photoexcited electron population at different points in the momentum space. Image electrons are weakly bound surface electrons with very narrow spectral features. Studies are done on both stepped and flat surfaces in order to understand image electron interactions with a step potential. The results suggest a new relaxation channel for image-state electrons on stepped surfaces
18. M. Han, Y. Luo, J.E. Moryl, and R. M. Osgood, Jr. An investigation of the surface reaction mechanisms of alternating-grown, ordered atomic layers: Cds on znse(100). Surface Science, 425(2-3):259-75, 1999.
Temperature-programmed desorption (TPD) is used for detailed investigation of the surface chemistry of a binary reaction sequence by sequential gas-phase dosing of a ZnSe(100) substrate with (CH/sub 3/)/sub 2/Cd and H/sub 2/S. Analysis of the TPD spectra shows that adsorbed DMCd irreversibly dissociates on a ZnSe(100)-c(2*2) surface at room temperature to form a fully methyl-terminated surface; this termination is responsible for the previously reported, self-limiting reaction. At 370 K, DMZn desorbs from this surface due to a methyl-exchange reaction. This desorption temperature is independent of coverage, indicating a first-order reaction. In addition, at high DMCd exposures, the adsorption/desorption process leads to replacement of surface Zn by Cd. The experiments also examine the reaction of H/sub 2/S with the methyl-terminated surface. This reaction, which is also self-limiting, forms a sulfur-hydride-terminated surface after the release of surface CH/sub 3/ groups in the form of CH/sub 4/. Studies of surfaces formed by more than one binary reaction sequence are also reported, which show that the alternating growth surfaces are terminated with either methyl groups or hydrogen. The methyl-passivated growth surface preferentially desorbs methyl radicals at 390 K instead of the metal-alkyl species. For the sulfur-hydride-terminated surface the recombinative reaction of the H/sub 2/S species causes desorption of H/sub 2/S at 480 K. In this case, the symmetric peak shape of the desorbed H,S signal and its shift to lower temperature with increasing the coverage suggest a second-order reaction mechanism. In more general terms, these results indicate that the relative strengths of bond for methyl-metal (II) and metal-VI element play an important role in the surface reactions
19. K.A. Khan, N. Camillone III, and R. M. Osgood, Jr. Photoinitiated electron transfer to selected physisorbed alkyl bromides: The effects of alkyl chain length on dissociation cross sections. Journal of Chemical Physics, 110(21):10526-38, 1999.
We report the results of measurements of the cross section as a function of wavelength (351, 248, and 193 nm) for photoinitiated dissociative electron attachment to three normal alkyl bromides [CH/sub 3/(CH/sub 2/)/sub n-1/Br, n=1, 2, and 3] physisorbed on GaAs(110). Upon UV exposure, the molecules undergo C-Br bond cleavage due to a substrate-mediated electron-transfer process. The cross sections for all three molecules increase monotonically with decreasing wavelength. Our results suggest a 1 eV higher threshold for dissociation of ethyl and propyl bromide than for methyl bromide. A simple model of the electron-transfer process is employed to estimate the peak per-electron cross section for dissociative attachment in the monolayer. We find that the cross sections for the physisorbed molecules are approximately five times smaller than those for gas-phase molecules, due to a reduction in the lifetime of the molecular anion in the vicinity of the surface. In addition, we also find an increase in cross section with chain length very similar to that observed in the gas phase; the gas-phase behavior has been explained by an increase in the anion lifetime with chain length. Our results suggest that while quenching of the molecular anion at the surface is important, it does not eliminate the progression of anion lifetime with chain length
20. M. Han, Y. Luo, J.E. Moryl, R. M. Osgood, Jr. and J.G. Chen. A near-edge x-ray absorption fine structure study of atomic layer epitaxy: the chemistry of the growth of cds layers on znse(100). Surface Science, 415(3):251-63, 1998.
Atomic layer epitaxy (ALE) using a binary reaction sequence of the precursors dimethylcadmium (DMCd) and hydrogen sulfide (H/sub 2/S) is shown to produce epitaxial layers of CdS on a ZnSe(100) substrate. Near-edge X-ray adsorption fine structure (NEXAFS) spectra taken at the carbon K-edge and sulfur L-edge are consistent with a growth model based on self-limiting surface reactions and support a mechanism based on sequential surface ligand displacement. This evidence, combined with a TPD study, shows the presence of alternating methyl- and hydride-terminated surfaces, which are passivated to further uptake of Cd or S, respectively. Layer-by-layer growth of good quality CdS was confirmed by comparison of NEXAFS spectra of bulk CdS with our grown films
21. III Camillone, N., K.A. Khan, P.J. Lasky, L. Wu, J.E. Moryl, and R. M. Osgood, Jr. The wavelength dependence of photoinduced hot electron dissociative attachment to methyl bromide adsorbed on gallium arsenide (110).Journal of Chemical Physics, 109(18):8045-57, 1998.
The wavelength dependence of photoinduced dissociation of CH/sub 3/Br via dissociative electron attachment (DEA) of hot electrons for one monolayer CH/sub 3/Br adsorbed on GaAs(110) has been measured. The cross section for dissociation is found to decrease monotonically by two orders of magnitude as the incident wavelength is varied from 308 to 550 nm. There is an apparent threshold near 490 nm (2.5 eV), well below the gas phase photodissociation threshold near 250 nm (5.0 eV), but in good agreement with a simple estimate based on expected values for the decrease in the photoemission threshold and the lowering of the molecular affinity level upon adsorption of CH/sub 3/Br on a semiconductor surface. The observed threshold is found to move to higher energy as dissociation of the monolayer proceeds. Based on the work of Hasselbrink and co-workers [F. Weik, A. de Meijere, and E. Hasselbrink, J. Chem. Phys. 99, 682 (1993)], a simple theoretical model is developed which considers the tunneling of hot electrons through the interfacial barrier between the physisorbed CH/sub 3/Br and the GaAs. The results of our theoretical model in conjunction with those of earlier ab initio calculations [S. Black, R. Friesner, P.H. Lu, and R.M. Osgood, Jr., Surf. Sci. 382, 154 (1997)] suggest that the adsorbate affinity level is centered at 0.6 eV above the (adsorbate- modified) vacuum level of the substrate. This value corresponds to a 1.8 eV stabilization of the negative ion resonance upon adsorption
22. Y. Luo, D. Slater, M. Han, J. Moryl, R. M. Osgood, Jr., and J.G. Chen. In situ investigation of the surface chemistry of atomic-layer epitaxial growth of ii-vi semiconductor thin films. Langmuir, 14(6):1493-9, 1998.
Atomic-layer epitaxy (ALE) can provide atomic-scale control of the single-crystal growth of thin semiconducting layers. Despite the widespread investigation of the epitaxial properties of this technique, few detailed studies of the fundamental surface chemistry of the process have been performed. This paper describes an overview of such an in situ study of heterogrowth using a binary reaction sequence with the precursors H/sub 2/S and dimethylcadmium to grow CdS on ZnSe(100). The surface chemistry was investigated using thermal desorption spectroscopy and near-edge X-ray absorption fine structure (NEXAFS). Epitaxial growth was also characterized in the growth chamber using electron and ion surface probes
23. Yi Luo, D. Slater, Ming Han, J. Moryl, and R. M. osgood, Jr. Low-temperature, chemically driven atomic-layer epitaxy: in situ monitored growth of cds/znse(100). Applied Physics Letters, 71(26):3799-801, 1997.
A chemical reaction scheme for room-temperature atomic-layer epitaxial growth of II-VI semiconductors is described and demonstrated. Growth is accomplished by dosing in an ultra-high-vacuum chamber, and the surface structure and composition is probed in situ by a variety of electron and ion probes
24. X.Y. Wang, X.J. Shen, and R. M. Osgood, Jr. Surface electron motion near monatomic steps: two-photon photoemission studies on stepped cu(111). Physical Review B (Condensed Matter), 56(12):7665-74, 1997.
The electronic structure of three surface or near-surface states on stepped Cu(775) has been investigated using angle resolved, resonant, two photon photoemission. Since the electron wave function in each of these electronic states has a different average distance from the crystal plane, the measurement allowed the step potential at each distance to be sampled. The energy dispersion of the n=1 image-potential state was found to be oriented by the (111) terrace and the n=2 state was oriented by the (775) surface plane. The band structure of the embedded sp-like surface state (n=0) was determined by the projection of the bulk band gap onto Cu(775)
25. Y. Luo, D.A. Slater, M. Levy, and R. M. Osgood, Jr. Chemical preparation of cdte(100) and (110) surfaces using atomic hydrogen. Applied Surface Science, 104-105:49-56, 1996.
We present the results of an AES, XPS, LEED and photoluminescence study of the reaction of oxide and contaminant overlayers on CdTe(100) and (110) surfaces with atomic hydrogen. For both surfaces exposure of an oxide overlayer to a flux of atomic hydrogen produced by `cracking' ambient molecular hydrogen on a hot tungsten filament results, in contrast to the case of GaAs substrates, in a rapid, quantitative removal of the oxide overlayer at substrate temperatures as low as 300 K. This process results in surfaces free of contaminants which have sufficient surface order to produce clear LEED patterns characteristic of well known reconstructions of these surfaces. Additionally in situ photoluminescence spectroscopy allows direct correlation of surface cleaning with reduced nonradiative carrier recombination
26. P.J. Lasky, P.H. Lu, Y. Luo, D.A. Slater, and R. M. Osgood, Jr. The adsorption and thermal reaction of dimethylcadmium, dimethylzinc and trimethylgallium on gaas(110). Surface Science, 364(3):312-24, 1996.
The adsorption of dimethylcadmium (DMCd), dimethylzinc (DMZn) and trimethylgallium (TMGa) on GaAs(110) has been studied using TPD and XPS spectroscopies. In the first monolayer, the dimethyl species adsorb molecularly at 80 K, but, upon heating, undergo dissociation. Thermal elimination of the hydrocarbon fraction of these precursors is found to proceed mainly via the formation and desorption of TMGa despite the fact that the only available gallium is strongly bound in the GaAs lattice. For the cases of 1 ML DMCd and DMZn adsorption, TMGa desorption is seen as two distinct peaks at 435 and 380 K, while Cd or Zn deposited in this manner subsequently desorbs in a single broad feature between 400 and 700 K. The desorption of TMGa, obtained by TMGa molecular adsorption at 80 K, is found to be strongly coverage dependent with the sequential population of two states giving rise to peak desorption temperatures of 435 and 340 K as a function of increasing coverage. For all these adsorbates, it is suggested that this effect is due to reaction-limited TMGa desorption, resulting from distinct kinetic pathways to the formation of TMGa at low and high coverages. For all species, the results provide a useful comparison with similar earlier studies on GaAs(100)
27. X.Y. Wang, X.J. Shen, R. M. Osgood, Jr. R. Haight, and F.J. Himpsel. Observation of lateral superlattice effects on stepped cu(001). Physical Review B (Condensed Matter), 53(23):15738-42, 1996.
Lateral superlattice effects are observed for image-state electrons on a stepped Cu(001) surface via angle-resolved two-photon photoemission. Adsorption of Na atoms ( 0.01 ML) on the stepped surface enhances the step regularity, yielding clear zone folding with a reduced Brillouin zone given by the reciprocal step lattice
28. D.A. Slater, Y. Luo, and R. M. Osgood, Jr. Chemical preparation of ordered cdte(110) and (100) surfaces using atomic hydrogen. Journal of Crystal Growth, 159(1-4):754-60, 1996.
We have studied reaction of oxide and contaminant layers on CdTe(100) and (110) surfaces with atomic hydrogen using a variety of analytical techniques. For these surfaces we observe a rapid, quantitative removal of oxide overlayers at room temperature resulting in surfaces free of contaminants with sufficient surface order to produce clear LEED patterns. In situ photoluminescence spectroscopy allows direct correlation of the enhancement of radiative carrier recombination with the production of an ordered (100)c(2*2) surface by such means
29. P.H. Lu, P.J. Lasky, Q.Y. Yang, and R. M. Osgood, Jr. Dynamics of hot-electron transfer in oriented methyl halides on gaas(110). Chemical Physics, 205(1-2):143-58, 1996.
The ordered molecular orientation of CH/sub 3/X(X=Cl, Br and I) on GaAs(110) has been studied by angle-resolved time-of-flight (TOF) spectra of methyl fragments ejected by dissociation due to photoinduced electron transfer processes. A hot electron, produced in the substrate by UV irradiation, induces scission of the C-X bond in a manner similar to gas phase dissociative electron attachment (DEA). The methyl fragments ejected from a 1 ML layer have a unique angular distribution peaked about -45 degrees +or-5 degrees in the  direction defined by the GaAs(110) surface. The dynamics of electron transfer reactions in these systems, including the wavelength (193, 248 and 351 nm) dependence of the DEA reaction and the dependence of the dissociation cross section on the different methyl-halide molecules are discussed. The molecular orientation has been probed by near-edge X-ray adsorption fine structure (NEXAFS) experiments which indicate that the tilted orientation of adsorbed molecules accounts for the angular distribution of methyl fragments ejected from hot-electron transfer reactions in these systems
30. Wang, R. Paiella, and r. M. Osgood, Jr. Two-photon photoemission study of two-dimensional scattering processes on na-covered cu surfaces. QELS `95. Summaries of papers presented at the Quantum Electronics and Laser Science Conference. Volume 16. 1995 Technical Digest Series Conference Edition, page 150, 1995.
Summary form only given. We present an angle-resolved two-photon photoemission (2PPE) investigation of the scattering of the n=1 image-state electrons by ionized species, introduced through Na adsorption at a Cu(111) surface. The use of angle-resolved photoemission allows us to selectively study the electron dynamics as a function of a given, unique lateral velocity
31. Wang, R. Paiella, and r. M. Osgood, Jr. Two-dimensional electron-scattering processes on na-dosed cu(111): a two-photon photoemission study. Physical Review B (Condensed Matter), 51(23):17035-9, 1995.
Angle-resolved two-photon photoemission was used to investigate scattering of image-state electrons on Cu(111) at very low coverages of adsorbed Na atoms. The observed adsorption-induced broadening and quenching of the image-state peak are attributed to weakened conservation of the parallel momentum as well as interband, inelastic electron scattering. A phenomenological adsorbate scattering cross section, which is proportional to the parallel momentum, is introduced to characterize the nonlifetime linewidth broadening
32. Lasky, P.H. Lu, M.X. Yang, R. M. Osgood, Jr. B.E. Bent, and P.A. Stevens. Nexafs measurements indicating a tilted molecular orientation for methyl halides on gaas(110). Surface Science, 336(1-2):140-8, 1995.
The orientation of methyl halides (CH/sub 3/X, X=Cl, Br, I) molecularly adsorbed at 80 K on GaAs(110) has been investigated by near-edge X-ray absorption fine structure (NEXAFS) spectroscopy at the carbon K edge. Analysis of the angular dependence of the C-X sigma * resonance indicates that the molecules are oriented with the C-X axis tilted with respect to the surface normal. In the first monolayer, the molecules are inclined in the (01) direction, the C-X axis approximately 45 degrees from the surface normal. In the second monolayer the molecules are inclined in the opposite (01) direction from the surface normal. Correlations between the orientation results obtained from these studies and other observations on these same surface adsorbate systems are discussed
33. Y. Luo, D.A. Slater, and R. M. Osgood, Jr. Low-damage processing of cdte(110) surfaces using atomic hydrogen. Applied Physics Letters, 67(1):55-7, 1995.
We present the results of an AES, XPS, and LEED study of the reaction of oxide and contaminant overlayers on the CdTe(110) surface with atomic hydrogen. Exposure of an oxidized surface to a flux of atomic hydrogen produced by crackingambient molecular hydrogen on a hot tungsten filament results in a rapid, quantitative removal of the oxide overlayer with substrate temperatures as low as 300 K to produce a stoichiometric surface free of contaminants which has sufficient surface order to produce a sharp (1*1) LEED pattern