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We present a new photo-fragment imaging spectrometer, which employs a movable repeller in a single field imaging geometry. This innovation offers two principal advantages. First, the optimal fields for velocity mapping can easily be achieved even using a large molecular beam diameter (5 mm); the velocity resolution (better than 1%) is sufficient to easily resolve photo-electron recoil in (2 + 1) resonant enhanced multiphoton ionization of N2 photoproducts from N2O or from molecular beam cooled N2. Second, rapid changes between spatial imaging, velocity mapping, and slice imaging are straightforward. We demonstrate this technique's utility in a re-investigation of the photodissociation of N2O. Using a hot nozzle, we observe slice images that strongly depend on nozzle temperature. Our data indicate that in our hot nozzle expansion, only pure bending vibrations - (0, v2,0) - are populated, as vibrational excitation in pure stretching or bend-stretch combination modes are quenched via collisional near-resonant V-V energy transfer to the nearly degenerate bending states. We derive vibrationally state resolved absolute absorption cross-sections for (0, v2 < 7, 0). These results agree well with previous work at lower values of v2, both experimental and theoretical. The dissociation energy of N2O with respect to the O(¹D) + N2¹Σg+ asymptote was determined to be 3.65 ± 0.02 eV. [ABSTRACT FROM AUTHOR]

Using visible and x-ray photoelectron spectroscopy, we measured the work function of a Au(111) surface at a well-defined submonolayer coverage of Cs. For a Cs coverage producing a photoemission maximum with a He–Ne laser, the work function is 1.61±0.08 eV, consistent with previous assumptions used to analyze vibrationally promoted electron emission. A discussion of possible Cs layer structures is also presented. [ABSTRACT FROM AUTHOR]

Using molecular beam techniques we find that incident D atoms can abstract CD3 from a Cu(111) surface to yield CD4 in a direct (Eley–Rideal) gas–surface reaction with a cross section of ∼10-16 cm2/D atom. Dynamical evidence for a direct reaction includes the observation of an extremely sharp angular distribution that is clearly displaced from the surface normal, and the determination of a very high translational energy of the product, Ef, which is ∼2 eV. For a 0.25 eV D-atom beam incident at 45° on a 95 K surface, this energy varies with the detection angle, θf, as Ef(θf)=(1.8+θf/45) eV, where θf<0° in the ‘‘backscattering’’ direction. For these conditions, the angular distribution approximately follows the function cos70(θf-5.5), being peaked 5.5° from the normal with a full width at half maximum of <17°. Lowering the beam energy to 0.07 eV gives a broader angular distribution peaked at about 1.5° from the normal, consistent with cos60(θf-1.5). The reaction with 0.25 eV H incident at 45° gives a similar distribution peaked at ∼3.5° from the normal. The shifts in the angular distributions are approximately consistent with parallel momentum conservation. The CD3/Cu(111) surface was prepared by thermal dissociation of CD3I on the surface or by adsorbing CD3 directly from a CD3 beam produced by the pyrolysis of azomethane. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We find that CO is displaced from a ∼90 K Cu(111) surface by an incident H atom beam with a cross section of ∼10-16 cm2/H atom. As for a previous study of the ejection of O2 from Pt(111), our results indicate that part of the heat of adsorption of the incident species is carried away by the ejected molecule in a ‘‘dynamic displacement’’ process. We have determined the internal-state distribution of the ejected CO using quantum-state-specific laser ionization detection. We have also determined its angular and velocity distribution using a rotatable quadrupole mass spectrometer. The rotational distribution of molecules displaced in the v=0 and v=1 vibrational states are close to Boltzmann distributions at 390 K and 940 K, respectively. While the v=1 population is approximately proportional to the CO coverage, that for v=0 has a more complex coverage dependence, approximately following the presence of the CO α state, which gives a distinct temperature-programmed desorption peak for coverages above 1/3 ML. The equivalent vibration temperature ranges from 1500 K at low coverage to 800 K for a saturated surface. The velocity distribution of the ejected molecules is close to a Boltzmann distribution at 1300 K, corresponding to a translational energy of ∼0.22 eV. The angular distribution is symmetric about the normal and is close to a cos5 θf at small angles, desorption angles, θf, approximately following a cosine distribution for θf>=40°. We discuss the results in terms of the dynamic displacement model, where desorption of CO(v=0) is driven by a sudden switch from the chemisorption to physisorption wells. In the case of CO(v=1), we suggest that desorption may follow the formation of a temporary HCO intermediate. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have determined the internal-state distribution for the HD product of the reaction of gas-phase D atoms with H atoms chemisorbed on Cu(111) and for the corresponding reaction of H atoms with chemisorbed D atoms. In the case of D-on-H, the populations of the vibrational states v=0, 1, 2, and 3 are comparable, while that for v=4 is considerably smaller, giving a mean vibrational energy of ∼0.7 eV. The vibrational state distribution for H-on-D is similar, but in this case there is a clear peak at v=1, even less population in v=4, and a somewhat smaller mean vibrational energy of ∼0.6 eV. The mean rotational energy falls with increasing v in both cases, ∼0.5 eV for v=0 to <0.2 eV for v=4, with an overall mean rotational energy of ∼0.4 eV. The rotational distributions are distinctly narrower for H-on-D than for D-on-H. The maximum internal energy observed is ∼2.3 eV, consistent with the total energy available to the product. Results are consistent with recent calculations. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We examine experimental data on the activated dissociative adsorption and associative recombination of hydrogen at copper surfaces with respect to the role played by molecular vibrational states. We use models describing the variation of the adsorption probability with the vibrational state, kinetic energy, and angle of the incident molecules, establishing the parameters of the models by a nonlinear least squares fit to adsorption data. Using the principle of detailed balance, we apply these models to the corresponding data on associative desorption thus comparing adsorption data with desorption angular, velocity, and internal state distributions. The most consistent picture resulting from this analysis is that the adsorption has significant contributions from both H2(v=0) and H2(v=1) and that these components have markedly different translational thresholds. Within the framework of this picture we are able to resolve the apparent contradiction between the strong angular dependence of the kinetic energy required for adsorption and the lack of angular dependence of the mean kinetic energy of desorption. We also partially resolve the apparent discrepancy in interpretation of the role of H2(v=1) in recent adsorption experiments. [ABSTRACT FROM AUTHOR]

The vibrational energy dependence of the dissociative chemisorption probability of CH4 on W(110) is investigated with the use of a seeded supersonic molecular beam. By variation of the beam source temperature and seed gas mixture the degree of vibrational excitation of the incident CH4 molecules can be varied while the incident kinetic energy is held constant. The results are consistent with a model in which all vibrational modes are equally effective and vibrational energy and translational energy are approximately equivalent in promoting this highly activated process. Previous unsuccessful attempts to promote CH4 chemisorption on rhodium via vibrational excitation are consistent with our findings; we are also able to account for previous observations of enhancement of dissociative chemisorption in heated effusive beam experiments. [ABSTRACT FROM AUTHOR]

Measurements of the velocity and angular distributions for trapping-desorption scattering of argon from a clean, well characterized Pt(111) single crystal are reported. For certain experimental conditions, both the characteristic velocity and angular distributions deviate markedly from that predicted using equipartition arguments (i.e., a Maxwellian flux distribution in velocity and a cosine distribution in angle). The average kinetic energy for the flux exiting normal to the surface at 100 K is only 80% of that expected for a Maxwellian at TS. This kinetic energy deficit decreases and approaches zero as the detector is rotated away from the surface normal. The angular flux distribution is found to be broader than cosine. These results are discussed in terms of microscopic reversibility which permits estimates of the velocity dependent condensation coefficient to be obtained. [ABSTRACT FROM AUTHOR]

We have determined the alignment of D2(v,J) desorbed from Cu(111). The measurements reveal a small preference for ‘‘helicoptering’’ motion that increases with increasing J. At low J, the alignments are much smaller than predicted by recent calculations. We believe that the anisotropic potential may scramble the alignment as the molecules leave the surface. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have determined the dependence of the dissociative adsorption probability in the zero coverage limit, S0, for H2 on Cu(111) as a function of translational energy, Ei, and incidence angle, θi, vibrational state, v, and rotational state, J. We have also obtained information on the effect of surface temperature, Ts, on this probability. These results have been obtained by combining the findings of two separate experiments. We have obtained the form of the dependence of S0 on Ei at Ts=925 K for a range of quantum states from desorption experiments via the principle of detailed balance. We have obtained absolute S0 values from direct molecular beam adsorption experiments, which reveal that S0 scales with the so-called ‘‘normal energy,’’ En=Ei cos2 θi. The desorption experiments provide detailed information for J=0 to 10 of H2(v=0) and for J=0 to 7 of H2(v=1). The beam experiments additionally provide information on the adsorption of H2(v=2), averaged over J. All measurements are consistent with adsorption functions with an s-shaped form, which can be described by S0=A(1+erf(x))/2, where x=(En-E0)/W. Values of W are ∼0.16 and 0.13 eV for v=0 and v=1, respectively, at Ts=925 K, falling by about 0.05 eV for Ts=120 K, and with only a slight dependence on J. Values of A are insensitive to v and J, with a value of ∼0.25. S(En,v,J) curves are thus similar for different v and J, but shifted in En.In contrast, we find that the values of E0, which determine the mid-point of the curves, have a strong dependence on v and J. Specifically, E0 for H2(v=0) molecules is about 0.6 eV, falling to 0.3 and 0.1 eV for H2(v=1) and H2(v=2), respectively. Translational energy is thus about twice as effective as vibrational energy in promoting dissociation. E0 rises with increasing J at low J, before falling at high J, indicating that rotational motion hinders adsorption for low rotational states (J<4), and enhances adsorption for high... [ABSTRACT FROM AUTHOR]

We have investigated the dependence on the rotational and vibrational states of the translational energy of D2(v,J) formed in recombinative desorption from Cu(111). These results provide information about the effect of rotational energy relative to that of vibrational and translational energy on the dissociative chemisorption of D2 on Cu(111). The range of rovibrational states measured includes rotational states J=0–14 for vibrational state v=0, J=0–12 for v=1, and J=0–8 for v=2. D2 molecules were detected in a quantum-state-specific manner using three-photon resonance-enhanced multiphoton ionization (2+1 REMPI). Kinetic energies of desorbed molecules were obtained by measuring the flight time of D2+ ions in a field-free region. The mean kinetic energies determined from these measurements depend strongly on the rotational and vibrational states. Analyzing these results using the principle of detailed balance confirms previous observations that vibrational energy is effective, though not as effective as translational energy, in promoting adsorption. Rotational motion is found to hinder adsorption for low rotational states (J≤5) and enhance adsorption for high rotational states (J≥5). Even for high J states, however, rotational energy is less effective than either vibrational energy, which is 30%–70% more effective than rotational energy, or translational energy, which is 2.5–3 times more effective than rotational energy in promoting adsorption. The measured internal state distributions for the rovibrational states listed above are consistent with the observed dependence of the kinetic energy of the de- sorbed molecules with the rotational state. In addition, the analysis performed yields the dependence of the adsorption probability on kinetic energy separately for each rovibrational state. These functions have very similar sigmoidal shapes for all states examined. Changing the quantum state is primarily associated... [ABSTRACT FROM AUTHOR]

We have determined the velocity distributions of individual quantum states of NO scattering from Ag(111) at specific scattering angles θf using molecular beam techniques to control the incidence energy Ei and angle θi. We find that the mean energies of scattered species Ef depend weakly on θf at low collision energies, but become increasingly independent of this parameter as Ei approaches 1.0 eV. This is true for all final rotation states J. The previously reported insensitivity of the final kinetic energy to J is found to apply at all scattering angles, so that Ef vs θf curves for high J fall only slightly below those for low J. This system is highly translationally inelastic at high incidence energies, with up to 55% of Ei being lost to phonons at Ei=1.0 eV. Angular distributions are relatively insensitive to J at low Ei , but for high Ei the peak flux is found to shift away from the surface normal as Ei increases. The effect of the surface temperature only becomes apparent at low incidence energies. A search for supernumerary rotational rainbows reveals no discernible oscillations even for the lowest surface temperatures. We believe that these supernumerary oscillations may be damped by ‘‘surface corrugation’’ effects for this system. Discussion focuses on the observed anticorrelation between kinetic energy transfer to phonons and to rotation, the extent to which parallel momentum is conserved in this system, and energy-angle scaling laws for energy transfer. In this latter case we show that energy transfer in this system scales approximately with the quantity Ei cos θi, over the full range of conditions covered. [ABSTRACT FROM AUTHOR]

Molecular beam techniques have been used to study the dissociative chemisorption of nitrogen on W(110). Chemisorption probabilities have been measured as a function of incidence angle θi and kinetic energy Ei surface coverage and temperature. In addition, angular scattering distributions have been measured for a range of conditions and LEED has been used to examine surface structure. The initial (zero coverage limit) sticking probability is found to depend strongly on the incidence energy, scaling approximately with Ei, rather than with the velocity component normal to the surface. This probability is ≤3×10-3 for Ei≤30 kJ mol-1, and rises by more than a factor of 100 by ∼100 kJ mol-1, where it levels off at ∼0.35. It is argued that this behavior arises due to a strong chemical interaction prior to the barrier to dissociation. Angular scattering distributions revealed predominately quasispecular scattering with evidence as well for a diffuse component at low energies. The sticking probability falls steadily with increasing surface coverage and a saturation coverage of ∼0.25 atomic ML is observed for Ei∼10 kJ mol-1. At higher incidence kinetic energies, this saturation coverage increases to ∼0.5 ML at 200 kJ mol-1. LEED structures are also reported, corresponding to coverages of 0.25, 0.3, 0.5, and 0.52 ML. The 0.25 and 0.5 ML structures are identified as p(2×2) and c(4×2), respectively, for which structure models are proposed. [ABSTRACT FROM AUTHOR]

The dissociative chemisorption of oxygen on W(110) has been studied using molecular beam techniques. Chemisorption probabilities have been measured as a function of incidence angle, θi, and kinetic energy, Ei, and of surface coverage and temperature. In addition, angular scattering distributions have been measured for a range of conditions and LEED has been used to examine surface structure. The initial (zero coverage limit) sticking probability is found to depend strongly on the incidence energy, scaling with En=Ei cos2 θi. This probability is ∼10% at En =0.1 eV, rising to essentially unity above En =0.4 eV. At half a monolayer coverage of atomic oxygen, the sticking probability is close to zero up to a threshold of ∼0.25 eV, above which it rises to over 50% by 1.3 eV. In most cases, the sticking probability is found to fall roughly linearly with increasing surface coverage. However, a less-than-linear fall-off is observed for En ≥1 eV and for En ≤0.03 eV, the sticking probability actually rises with increasing coverage reaching a maximum at ∼0.2 ML. These results indicate that while dissociation may proceed via a classical precursor at the lowest energies, such a state can play little rolefor En≥0.1 eV. For En≤0.3 eV, the chemisorption probability falls to less than 5% for a coverage of about 0.5 ML; however, this apparent saturation coverage rises to 0.75 ML at ∼0.25 eV and to about 1.0 ML at about 0.85 eV. These ‘‘favored’’ coverages of 0.5, 0.75, and 1.0 ML are found to be associated with p(2×1), p(2×2), and p(1×1) LEED patterns, respectively. Angular scattering distributions recorded with a differentially pumped rotatable mass spectrometer, revealed predominantly quasispecular peaks, and velocity distributions are also characteristic of direct-inelastic scattering. The variation of the sticking probability with En is analyzed to obtain barrier... [ABSTRACT FROM AUTHOR]

We have determined the angular and velocity distributions of the CO2 product of the interaction of gas-phase O atoms with CO adsorbed on Pt(111) at 90 K. The desorption intensity is found to be strongly peaked along the surface normal, falling approximately as cos12 θf with increasing final angle, θf. The velocity distributions are found to be highly non-Boltzmann with mean energies of ∼0.6 eV, decreasing slightly with increasing θf. [ABSTRACT FROM AUTHOR]

We have determined the angular distributions of H2, HD, and D2 desorbing from Cu(111) for surface temperatures in the range 370–800 K. These are found to be strongly peaked and symmetric about the surface normal in every case. Results for all three isotopes are found to be indistinguishable, being close to a cos 12θf distribution at 600 K, slightly narrower at 370 K, and slightly broader at 800 K. Results are discussed in terms of other previous desorption measurements and related to adsorption data via detailed balance. [ABSTRACT FROM AUTHOR]