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The remarkable antineoplastic peptide dolastatin 10, isolated from the opisthobranchia mollusc Dolabella auricularia, is currently in clinical development and further improvements in its total synthesis have been undertaken. Major effort has been directed at devising more stereoselective routes to the dolastatin 10 amino acid units dolaisoleuine 2 and dolaproine 3, each bearing three chiral centres. We report herein highly stereoselective routes to both natural (3R,4S,5S)-dolaisoleuine 2 and its 3S,4S,5S-isomer 14 (Z replaces H) using an asymmetric aldol methodology. Key reaction steps are condensation of chiral α-(methylsulfanyl)acetyloxazolidinone 4d with (S)-N-Z-N-Me-isoleucinal 6 using dibutylboron triflate followed by reductive desulfurization, O-methylation and cleavage of the oxazolidinone auxiliary to complete a simple route to N-benzyloxycarbonyldolaisoleuine 10. By substituting chiral oxazolidinone 5d for 4d the 3S-isomer of N-benzyloxycarbonyldolaisoleuine 14 was selectively obtained.

We compare growth of multiple quantum well modulators atop a p‐type or n‐type AlGaAs layer. We find the excitonic absorption edge of the quantum wells broadens 30% less under applied field when grown on a p‐down AlGaAs layer compared to the n‐down case. Auger spectroscopy shows Be surface concentration builds up to 0.3 ML on the p‐type AlGaAs layer whereas Si is smaller than 0.07 ML on an n‐type AlGaAs layer. X‐ray diffraction reveals quantum well crystallinity and interfacial roughness to be improved when grown on p‐down AlGaAs layer. We conclude Be is a surfactant that assists AlGaAs epitaxy.

The Micronesian marine sponge Phakellia sp. has been found to contain key members of the extraordinarily potent (antineoplastic) halichondrin/halistatin polyether macrolide family.

Conformational isomerism in a selection of N-Z-N-methylisoleucine derivatives was detected and investigated using variable temperature NMR spectroscopy. Thermodynamic values for the restricted rotation of conformational isomers were derived.

The specific contact resistances of annealed Au/Ti/W contacts on 0.2 and 2*10/sup 19/ cm/sup -3/ Be doped GaInAs are found to decrease by factors of approximately 1000 and approximately 3, respectively, when a thin Mn layer is inserted between Ti and W, reaching 2.7*10/sup -6/ Omega cm/sup 2/ in the latter case. This type of shallow and thermally stable p-type ohmic contact avoids the problems associated with very high Be, or Zn doping and should be well suited for applications in high performance devices on InP substrate. >

We examine growth of Ga1−xInxAs/GaAs1−yPy multiple quantum well structures by gas source molecular beam epitaxy for photonic switching applications at 1.06 μm. For appropriate x and y values these structures can be grown pseudomorphically at the GaAs lattice constant. Such structures show good morphology, room‐temperature photoluminescence and sharp excitonic absorption edges near 1.06 μm.

We evaluate Be and Si segregation in GaAs during growth by gas source molecular‐beam epitaxy and molecular‐beam epitaxy. We find that the dopant segregation lengths, when detected, are thermally activated with an energy of 0.6 eV. These kinetics are too small to be explained by classical models of dopant segregation, but instead lie close to motion energies corresponding to the elementary jumps executed by the dopant near the surface.

We present the results of a photoemission study of the annealed Ti/GaAs(110) interface, using soft x‐ray (SXPS), x‐ray (XPS), and ultraviolet (UPS) photoelectron spectroscopies to monitor the substrate and overlayer core levels and the valence band for Ti thicknesses between 6.7 and 20 monolayers and annealing temperatures between 200 and 425 °C. The SXPS data imply the formation of a stable Ti–As region near the Ti/GaAs interface that increases in width with temperature. Concomitant to this, the binding energy of the Ga changes from its room‐temperature position towards elemental Ga. The XPS data suggest the presence of elemental Ga at or near the overlayer surface. The UPS data suggest the possibility of cluster formation on the overlayer surface. In situ current–voltage (I–V) measurements were also made on thick‐film systems fabricated using similar methods to those used in the photoemission studies [i.e., cleaving, depositing metal (∼1000 A), and annealing in UHV]. The movement of Ti–As reaction produ...

Ninety college juniors were randomly assigned to one of three treatments differentiated by access to feedback. In each treatment, subjects completed a computer-based tutorial that presented the five coordinate concepts defined as intellectual skills (Gagne, 1977). In the Early Adaptive Control treatment subjects were required to select from four types of available feedback, varying in amount of information, after responding to each of the first five practice items. The type of feedback to be displayed continued to be under the control of the learner for the remaining items if the learner’s performance score was at or above 70%. If learner performance dropped below criterion the most elaborative feedback was displayed until criterion was reached again. The Late Adaptive Control treatment used a similar strategy except that it began “adapting” feedback control after the tenth item. Subjects in the third treatment, Program Control, automatically received the most elaborated feedback after each respon...

The purpose of this paper is to describe a new approach for determining the relationship between aggregate pollutant emissions and ambient concentrations in an air basin. The approach is to use a detailed meteorological model to generate a large set of pseudodata on specific source-receptor relationships. This data is then statistically reduced to a simple relationship between aggregate emissions and pollution concentration averages or maxima. In the paper, we give a justification for the approach and an empirical application to the Petrified Forest region of Arizona.

Synchrotron radiation photoelectron spectroscopy was used to study the interface between the rare earth metal ytterbium, and the GaAs(110) cleaved surface. From trends in line shape and intensity of the Ga 3d, As 3d, and Yb 4 f core level signals, we have deduced a picture of the formation and the final structure of the interface at room temperature. For metal coverages up to 1 ML, there is a weak chemical interaction between deposited metal and substrate. The coverage range between 1 and 2 ML is marked by strong interdiffusion and reaction, and the formation of an Yb arsenidelike phase at the surface that is stable with further metal deposition. This reacted layer appears to restrict further interdiffusion; the As is trapped at the interface, and additional metal deposition results in the growth of an Yb metal rich overlayer. From the continuous shift of the Ga 3d signal with coverage, and the behavior of the Yb 4 f line shape, the overlayer appears to be an Yb/Ga alloy that becomes more dilute in Ga as ...

Alpha ketoamides may be deprotonated twice with strong base. The dianions so formed react with alkyl halides to yield α-amido tertiary alcohols.

One hundred fourteen sixth graders participated in a study designed to test the effects of a learner-control strategy involving choice of elaborative material in a computer-based tutorial. An Options treatment in which children were allowed to choose a variety of types of elaborative material was contrasted with two program-control treatments, including one in which all the elaborative material from the Options treatment was mandatory (Full treatment) and the other in which students saw only a core presentation but no elaborative material (Lean treatment). A second variable in the study was level of task persistence. Four levels of task persistence were created based on initial task behavior. Students within each of these four task persistence groups were randomly assigned to one of the three treatment groups. A multivariate repeated measures analysis of covariance (using reading ability as the covariate) was conducted on immediate and retention posttests. Results showed that under both program-control treatments, persistence was curvilinearly related to performance; that is, extremely high and low persistence groups performed more poorly than did medium persistence groups. Under the learner-control treatment, however, the highest task persistence groups performed best. Additionally, when amount of material seen was controlled for, the learner-control treatment group outperformed the program-control groups.

In this paper, we give a short status report on the electronic properties of metal/III–V semiconductor interfaces. A large numer of metals, ranging from strongly reactive Ni, Co, Cr, Mn, Pd and Al to less reactive or nonreactive Cu, Ag, Au, Ga, Sn and In, have been studied for GaAs(110) and InP(110) systematically with the intent to correlate the interfacial chemistry with the Schottky barrier formation. Soft X-ray photoemission spectroscopy based on synchrotron radiation has been used to characterized the chemical reactions at the interface on an atomic scale, and the barrier heights for these structures have been compared to those for thick metal films prepared under identical conditions and measured with standard electrical methods. This review will illustrate, with a few examples for metal overlayers (Ni, Al, In) on InP(110), the current status of this field.

Thirty-seven sixth graders were assigned to one of two computer-based treatments to learn about the propaganda coordinate concepts of bandwagon, transfer, testimonial, and repetition. In both treatments, students were allowed to choose to see additional instruction in the form of paraphrased definitions, additional expository and practice instances, and analytical feedback. In one treatment, general statements of encouragement to select options were included. In the second treatment, no such encouragement was included. Scores on the Otis-Lennon Mental Ability Test and the Internal Achievement Rating Survey were collected for all students. Students who were shown encouragement statements did elect to see more options than did those without encouragement statements. No differences were found on the posttest scores for the two groups. Consistent with findings from earlier studies, students who chose to see one type of option also chose to see other types.

The room-temperature interaction of Co overlayers with the cleaved InP(110) surface has been studied using soft-x-ray core-level photoemission spectroscopy. We show that Co strongly reacts with the surface to form phosphide. For small metal coverages the reacted overlayer grows in a laminar fashion. The pinning position for the 1.5-A\r{} Co coverage is approximately 0.30 eV below the conduction-band minimum. For higher coverages the pinning is difficult to follow due to the uncertainty in accounting for the chemical reactions in the core-level spectra.

We report soft-x-ray valence-band and core-level photoemission studies of the growth and chemical reaction of column-III (Al, Ga, In, and Tl) overlayers on cleaved InP(110). We find that the deposition of submonolayer coverages of these metals leaves the valence-band spectra devoid of any structure other than that characteristic of the clean InP(110) surface. Strong bonding between the Al, Ga, or In adatoms relative to the bond between the adatoms and the substrate leads to cluster formation on the surface. Growth of a uniform overlayer cannot, however, be excluded for Tl. Core-level spectra of substrate atoms and overlayer atoms for Al and Ga interfaces clearly indicate a room-temperature exchange reaction accompanying the islanding of nonreacted metal. No indications of such a reaction are found for In and Tl. The trends of the reaction are in agreement with predictions of bulk thermodynamics. The overall trend in the growth mode agrees with the theoretical predictions of Zunger.

Soft X-ray photoemission spectroscopy measurements have been carried out on cleaved n-type GaAs (1 1 0) surfaces covered with Ni overlayers ranging in thickness from 0.05 to 53 A. The results of these room temperature measurements show that we have band bending effects occurring in conjunction with strong interfacial chemical reactions. Deconvolution of the Ga 3d core line into substrate and metallic components shows dissolution of the substrate at the interface with Ga diffusing into the surface of the metal overlayer for the intermediate coverages (1–15 A). Observation of the As 3d core level shows out-diffusion of As to the surface over the entire Ni coverage range. Using this deconvolution scheme we are able to follow the band bending of the Schottky barrier formed here up to the 8 A coverage. The Schottky barrier height is 1.0 ± 0.1 eV for this overlayer thickness.

A novel form of laser polarization spectroscopy is described which allows vibrational spectra of submonolayers of some adsorbates on metal surfaces to be observed. As a demonstration of its potential, spectra for CO adsorbed on a Pt(111) single crystal were obtained using a continuously tunable pulsed IR laser. The temperature dependence of the lineshape for the CO stretch in an on‐top adsorption site was measured and interpreted in terms of exchange‐coupling dephasing. Since sensitivity achieved with this spectroscopy is not yet as high as that obtainable with conventional IRRAS, the issue of noise sources and sensitivity in the laser spectroscopy is examined in some detail, and possibilities for future enhancements are discussed.

Using valence‐band and core‐level photoemission spectroscopy (PES) and electrical device measurements, the effects of annealing Ag, Al, and Au on n‐type (110)GaAs Schottky diodes fabricated in ultrahigh vacuum have been studied. PES was used to monitor the annealing‐induced changes in the interface Fermi level position and the chemical nature of the metal–semiconductor interface for submonolayer and several monolayer coverages. Barrier height determinations were also performed using current–voltage (I–V) and capacitance–voltage (C–V) device measurements on annealed thick metal film metal–semiconductor junctions. The results of this study show that the annealing‐induced microscopic changes in the electronic and chemical structure of the metal–semiconductor interface can be strongly correlated with the macroscopic changes in the electrical properties of thick film metal–semiconductor Schottky diodes.

Many photoemission measurements of the valence‐band discontinuity are based on the relative positions of the centroids of poorly resolved photoemission peaks. While such measurements are in principle accurate for abrupt, nonreactive interfaces, real interfaces are rarely so simple. We present high‐resolution soft x‐ray photoemission core level spectra which monitor the band bending and chemistry at two representative heterojunctions. The Si/InP(110) interface is highly reactive, has well‐resolved chemically shifted In and P components, and has preferential outdiffusion of In while the Si/GaAs(110) heterojunction has very subtle chemical reactions which are confined to the interfacial region. The effects of even such subtle chemical shifts can obscure the determinations of band bending and lead to errors as large as 0.3 eV for the Si/GaAs(110) heterojunction discontinuity. Such results emphasize the importance of obtaining high‐resolution photoemission spectra and applying careful analysis for accurate det...

This paper reports detailed molecular beam measurements of the sticking coefficient at zero coverage for O2 on a Pt(111) surface as a function of initial energy (Ei), angle of incidence (θi), and surface temperature (Ts). Under most conditions the sticking coefficient measures the probability for dissociative chemisorption. These results demonstrate that both precursor mediated and quasi‐direct dissociation can be observed, depending upon the initial conditions. The quasi‐direct process is revealed by a step increase in the sticking with Ei. This feature scales intermediately between Ei and the normal component En, and is weakly dependent on Ts. The precursor mediated sticking is well described by standard precursor kinetic models. At low Ei and Ts, sticking measures trapping into a molecularly adsorbed state. This trapping decreases more rapidly with Ei than anticipated from simple models and scales intermediately between Ei and En. The sticking results are discussed in terms of likely dynamic processes ...

Notetaking is a universal activity in college lecture courses, but little research has been done to examine students' perceptions of this study strategy as it relates to their overall study routine. In the current study, students in a large lecture course in introductory macroeconomics were asked to complete the Notetaking Perceptions Survey (NPS), an instrument that assesses students' perceptions of the worth or value of notetaking, their perceived level of notetaking activity, and their degree of confidence about their own notetaking skill. Additionally, students' learning style, as assessed by the Learning Style Inventory (Smith and Kolb, 1986), gender, high school rank, and year of high school graduation were included as predictor variables within a multiple regression analysis to predict scores on the there notetaking perception factors. Notetaking perceptions were predicted by one of the learning style dimensions and by gender. The relationships of final course grades with the three notetaking factors from the NPS and the other learner characteristics were also determined. Grades were predicted by one of the notetaking perception factors and by high school rank.

The effect of the room temperature interaction of an increasing coverage of the rare earth metal, Yb, on the Fermi level pinning behavior of n-GaAs (110) has been studied in detail. Soft X-ray photoemission spectroscopy was used to obtain information on the variation of the Fermi level position in the band gap via photoelectron emission from the As 3d and Ga 3d core level states. Both As and Ga interact with the overlayer to such a degree that we have used two independent methodologies to obtain a reliable estimate of the Schottky barrier height. The established limits for the barrier height are 0.61–0.72 eV. The results of this study show that the pinning position of the Fermi level is established well before the interface constituents are fully reacted and is stable for higher overlayer coverages. The Schottky barrier height is consistent with the low electronegativity of the rare earth metals.

The results of an investigation of the photochemical and photophysical properties of a series of complexes of the type (RuL{sub 2}L{prime})Cl{sub 2} are reported, where L and L{prime} are bipyridine, phenanthroline, and methyl and phenyl derivatives of those ligands. All measurements were conducted in CH{sub 2}Cl{sub 2}. Luminescence lifetime and quantum yield data indicate that the ligand which accepts the electron in the localized MLCT excited state plays a significant role in the excited-state decay. HPLC analysis of the organic photoproducts from mixed-ligand complexes shows that flexible bipyridyl ligands are preferentially photolabilized, consistent with a dissociative mechanism involving an open-ended bipyridine in the transition state. 55 refs., 2 tabs.

Soft x‐ray photoemission spectroscopy studies were performed in ultrahigh vacuum on cleaved n‐InP(110) with sequential room‐temperature Cr and Mn depositions of up to 30 A. The In 4d, P 2p, and Cr 3p or Mn 3p core levels were monitored as a function of coverage. Both systems exhibited strong metallic In‐phase separation and surface segregation, formation of a metallic phosphide, and no evidence of island formation. Fermi level‐pinning positions of 0.6 and 0.3 V below the CBM were estimated for the Cr and Mn systems, respectively, by deconvoluting the substrate from the interfacial components of the In core lines. These results are discussed in the context of different models of Schottky barrier formation.

The Los Alamos Scientific Laboratory (LASL) visibility model is capable of producing simulated “before and after” pictures that illustrate visual effects of smoke plumes. Although the model has been under development for a few years, until recently there had been very little testing of the model against field experience or testing of sensitivity of the model results to numerical approximations used in the model. Further validation and sensitivity testing of the LASL model began in late 1979. The work focused on three areas: (1) comparison of the LASL model results with plumes encountered in the field, (2) comparison of LASL background-atmosphere model results with measured sky intensities and (3) examination of the variation of model results with changes in the numerical approximations. The field study took place during August of 1979 in the vicinity of coal-fired power plants in northwestern New Mexico and northern Arizona. Telephotometer, NOx plume measurements and aerosol size distribution measurements of aerosols were made in the plumes of three different coal-fired power plants. Photographs were taken of the plumes and simulated photographs were prepared by the model. Light intensities calculated by the background radiative transfer model were compared to measured light intensities in a very clean atmosphere and in a moderately hazy atmosphere. The measured intensities were derived from photographic densities, or from telephotometer measurements. In addition to the field measurements, differences resulting from increased numbers of wavelengths in the color representation were examined. We also examined other changes in the numerical approximations. This paper describes the results of these studies.

A study was made to determine the effect of environmental parameters on the germination, growth, and development of Suaeda depressa (Pursh) Wats. Germination tests showed that seeds germinated in solutions containing up to 4 % NaCl with no toxic effects indicated after treatment with distilled water. The rate of germination and the percentage germination decreased with increased salinity. The effect of environmental parameters on growth was measured by shoot height, side shoot development, leaf length, and dry weight. Growth was greatest in 1 % NaCl solutions with adequate available nitrogen. With increased salinity and low available nitrogen levels plant growth decreased. A 10-hr photoperiod stimulated immediate floral induction. Although flowering and completion of the life cycle occurred in solutions containing up to 4 % NaCl, increased salinity decreased the rate of floral induction and the dry weight of flowers and fruit produced. This study indicates that environmental parameters such as salinity, available nitrogen, and photoperiod can create a variety of growth forms, causing taxonomic confusion. SUAEDA DEPRESSA, an annual leafy succulent, is distributed west of the Mississippi River from southern Canada to Mexico. In this area, S. depressa is narrowly restricted to wet saline soils with poor drainage (Ungar and Capilupo, 1969). The restriction of certain species to saline environments was reported by Miller and Egler (1950) and Adams (1963). Coupland (1950) and Ungar (1964, 1965) concluded that distribution and zonation in saline soils was controlled by competition among species and individual tolerances to salinity. Laboratory studies by Binet (1963) indicated that S. vulgaris Moq. could grow under nonsaline conditions but made its best growth in 1.0 % NaCl nutrient solutions. The distribution of halophytes such as S. depressa is apparently controlled by soil salinity, with other edaphic and climatic factors playing a secondary role (Ungar and Capilupo, 1969). Gates, Stoddart, and Cook (1956) indicated that changes in the chemical factors of soils in saline areas were inconsistent with the distribution and zonation of the vegetation. Studies by Ungar (1968) and Ungar, Hogan, and McClelland (1969) showed that even though S. depressa most commonly occurred in heavy clay soils, the soil texture varied greatly and apparently had little influence on its distribution. Suaeda depressa has ' Received for publication 17 January 1972. 2 Present address: Department of Biological Sciences, University of Arizona, Tucson 85721. We wish to thank Dr. J. W. O'Leary and Dr. L. A. Larson for critically reading this manuscript. This research was partly supported by National Science Foundation research grant GB-6009. been found growing in soils ranging from 0.374.44 % in salinity, in which NaCl made up 93 % of the total salts (Ungar, 1968). Macroclimatic factors vary greatly along S. depressa's northsouth distributional axis and appear to have little significance in determining its distribution (Gleason and Cronquist, 1963). The purpose of this study was to observe the effects of different environmental conditions on the germination and growth of S. depressa. Specific attention was placed on height, branching, and leaf length and how they correlated with saline environments. METHODS AND MATERIALS-The seeds of Suaeda depressa were collected on October 3, 1964 at Cloud County, Kansas, and stored at 5 C until the initiation of the study in April 1970. There was no loss in viability over the storage period. Germination stludies-The seeds were germinated on two sheets of Whatman #2 filter paper in 100 mm x 15 mm petri dishes. All germination studies were initiated in the dark in 6 ml of distilled water with NaCl as noted. The results are based on 10 replicates of 25 seeds each for each test solution. Emergence and radicle growth to 0.5 cm in length were used as germination criteria. A temperature regime of 15 + 1 C for 14 hr and 5 + 1 C for 10 hr was used. This temperature regime was used to approximate the 11 C mean under natural conditions for April when seeds germinate in nature. All germination

Massive defects of the upper aerodigestive tract present a reconstructive challenge. We report a case in which a large defect of the naso-oropharyngeal and oesophagus was reconstructed with a combination of a gastric pull-up and a pectoralis major muscle flap. Postoperative function was good and survival was in excess of 16 months. The history of such reconstructions and possible alternative techniques are also discussed.

It is shown that at the Al/n‐GaAs(110) interface grown in ultrahigh vacuum at −80 °C the Fermi level remains unpinned at least up to a 3 monolayer coverage. In contrast, at room temperature the pinning near midgap is established after a deposition of approximately 1 monolayer of Al. The low‐temperature behavior is correlated with the growth of a more uniform overlayer which inhibits cluster and defect formation. This result provides a critical test of models of Schottky barrier formation.

The initial sticking coefficient (probability for dissociative chemisorption) has been measured for O2 on a Pt(111) surface as a function of initial energy, incident angle, and surface temperature using molecular beam techniques. The results reveal two distinct mechanisms for sticking; precursor mediated and direct dissociation, either of which can dominate depending upon the initial adsorption conditions.

The room‐temperature growth of the Pd/n‐GaAs (110) intimate interface has been studied by soft x‐ray core level photoemission spectroscopy. It is shown that Pd reacts with the GaAs surface forming an arsenide compound in which phase segregated Ga metal is included. In addition some As is segregated on the surface of this composite. Using deconvolution of the different core level contributions, the Schottky barrier height of this system is experimentally determined to be 0.9±0.05 eV. The pinning is attributed to the deeper level of the unified defect model.

Previously a donor defect level at about −0.1 eV below the conduction band minimum has been associated with InP, in part to explain the nonmonotonic movement of the Fermi level within the band gap during the oxidation of n‐InP. We present here new soft x‐ray photoemission spectroscopy results which associate this behavior with a level in the oxide on the surface of the InP rather than with a native defect such as a P vacancy. Specifically, we have observed that the reversal in Fermi level movement occurs at exactly the same oxygen exposure as the appearance of structure in the In 4d core line indicative of the formation of In oxide (s). Phosphorus oxide is also indicated, as expected, along with elemental P, but the In oxide is particularly relevant because In2O3 has a sufficient density of shallow donor states to make it an n‐type semiconductor. These oxide donor states, rather than native defect states, can compensate native acceptor defect states and cause the observed Fermi level movement. The Fermi l...

The chemical reaction at the Ni/InP (110) and Ni/GaAs (110) interfaces produced by sequential deposition of thin Ni overlayers onto cleaved semiconductor surfaces has been investigated with valence band (VB) and core level photoemission and Auger spectroscopies using synchrotron radiation as the excitation source. By monitoring changes in the VB, P 2p, In 4d, Ga 3d, As 3d, and Ni 3p photoemission spectra and the lineshape of the P LVV Auger transition during the initial stage of Schottky barrier formation, we found that for both interfaces the first few Å of Ni react strongly with the surface resulting in the formation of a nickel phosphide or nickel arsenide. At the same time, segregation of metallic In or Ga is observed.

With recent changes provided by the Clean Air Act Amendment of 1977, the effect of industrial smoke plumes on scenic landscapes assumes heightened importance. The impact of large coal-fired power plants is most easily understood through the use of before-and-after photographs. A technique has been developed to modify a clean "before" scene as dictated by solutions to the radiation transfer problem in a polluted atmosphere. This allows one to produce simulated "after" scenes, which can illustrate the visual effects of pollutants emitted under a variety of circumstances. Application of this technique to very large coal-fired power plants suggests that such facilities may impair scenic vistas under some circumstances, unless stricter pollution controls and standards are enforced.© (1978) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

This paper reports detailed molecular beam measurements of the sticking coefficient at zero coverage for O2 on a Pt(111) surface as a function of initial energy (Ei), angle of incidence (θi), and surface temperature (Ts). Under most conditions the sticking coefficient measures the probability for dissociative chemisorption. These results demonstrate that both precursor mediated and quasi‐direct dissociation can be observed, depending upon the initial conditions. The quasi‐direct process is revealed by a step increase in the sticking with Ei. This feature scales intermediately between Ei and the normal component En, and is weakly dependent on Ts. The precursor mediated sticking is well described by standard precursor kinetic models. At low Ei and Ts, sticking measures trapping into a molecularly adsorbed state. This trapping decreases more rapidly with Ei than anticipated from simple models and scales intermediately between Ei and En. The sticking results are discussed in terms of likely dynamic processes ...