Glenn D. Kubiak

Recombinative desorption dynamics: Molecular hydrogen from Cu(110) and Cu(111)

The rotational and vibrational distributions of H2 and D2 recombinatively desorbing from clean Cu(110) and Cu(111) surfaces following atomic permeation are studied using multiphoton ionization combined with time‐of‐flight mass spectrometry. Rotational distributions are found to be non‐Boltzmann and to possess mean rotational energies which are 80%–90% of the surface temperature, Ts. These distributions are identical to within the experimental accuracy for H2 and D2 and also for desorption from the (110) and (111) faces. Moreover, the ortho and para nuclear spin modifications of both isotopes are statistically populated. In contrast, the vibrational population ratio, Pv‘=1/Pv‘=0, is found to be as much as 100 times greater than the ratio corresponding to a Boltzmann vibrational population at Ts. Specifically, the Pv‘=1/Pv‘=0 ratio for H2 (D2) is 0.052±0.014 (0.24±0.20) desorbing from Cu(110), and 0.084±0.030 (0.35±0.20) desorbing from Cu(111). For comparison the Boltzmann‐at‐Ts ratios would be 0.0009 for H...

Direct inelastic scattering of nitric oxide from clean Ag(111): Rotational and fine structure distributions

The internal state distribution of scattered NO is determined by laser fluorescence excitation spectroscopy when a pulsed, supersonically cooled beam of NO is incident upon the (111) face of a clean Ag single crystal. It is found that the mean rotational energy 〈Er〉 depends linearly on the surface temperature Es(=Ts) and the incident kinetic energy normal to the surface En according to 〈Er〉=a(En+〈Ew′〉)+bEs. The three parameters a, b, and 〈E′w〉 are constants independent of En and Es. Arguments are presented showing that 〈E′w〉 is some measure of the average NO/Ag(111) well depth. For the Ω=1/2 fine structure component we estimate that 〈E′w〉=2850±450 K (245±40 meV), a=0.88±0.009, and b=0.18±0.04 while for Ω=3/2, 〈E′w〉=2080±150 K (180±13 meV), a=0.132±0.005, and b=0.11±0.02. The results are compared to the predictions of one‐dimensional impulsive models of gas‐surface scattering. These models are able to describe qualitatively the dependence of 〈Er〉 on En and Es but only when trapping fractions that are incom...

Recombinative desorption of H2 and D2 from Cu(110) and Cu(111): Determination of nonequilibrium rovibrational distributions

Rotational and vibrational state distributions have been determined for H2 and D2 recombinatively desorbing from clean Cu(110) and Cu(111) surfaces in ultra‐high vacuum. A 2+1 resonance‐enhanced multiphoton ionization technique is employed. Rotational distributions for both isotopes are found to be mildly non‐Boltzmann and having mean rotational energies less than the surface temperature Ts and equal for both surfaces. The (v″=1)/(v″=0) vibrational population ratio for both H2 and D2, however, is ∼50 times greater than the value expected for an equilibrium ensemble at Ts for desorption from Cu(110) and ∼100 times greater from Cu(111).

Resonance-enhanced multiphoton ionization of molecular hydrogen via the E,F1Σg+ state: Photoelectron energy and angular distributions

Abstract Photoelectron energy and angular distributions are measured for the 2+1 multiphoton ionization process H 2 X 1 Σ g + (ν = 0, J ) + 2hv → E,F 1 Σ g + (ν E , J E = J ) + h ν → H 2 + X 2 Σ g + (ν + ) + e − , for ν E = 0, 1, or 2 and for J E = 0 or 1 of the inner well of the double-minimum E,F state. Although a strong preference is found for ν + = ν E , the detailed H 2 + vibrational distribution does not exhibit Franck-Condon behavior, and the photoelectron angular distributions vary markedly with both the J E value of the intermediate state and the ν + value of the ion.

Statistical averaging in rotationally inelastic gas-surface scattering: The role of surface atom motion

Abstract Within the framework of the one-dimensional cube model, the predicted appearance of rotational rainbows and trapping cutoffs in the final rotational state distribution in the limit of zero surface temperature is shown to depend strongly on statistical processes. In particular, inclusion of a distribution of initial surface cube velocities acts to broaden these features.

Two-photon photoelectron spectroscopy of Pd(111)

Angle‐resolved two‐photon photoelectron spectroscopy has been used to study the normally unoccupied band structure of Pd(111). Photoemission features are studied as functions of excitation photon energy hν, polarization state, and emission angle. Below hν=4.9 eV, photoemission is dominated by excitation of bulk states near the Fermi level of Λ3 symmetry through Λ1 excited states between the Fermi and vacuum level. The kinetic energy of the corresponding spectral peak is found to increase according to 2 hν−Φ, where Φ is the Pd(111) work function. At hν=4.9 eV, a surface state with 0.65±0.1 eV binding energy is found. The dispersion of this state with k∥ is well‐described by an effective mass m* equal to the free electron mass me. This state is assigned as the n=1 member of the image potential series of states. The relationship between the dispersion of this state and its position in the (111) projected bulk band gap is discussed.

Dynamics of recombinative desorption of H2 and D2 from Cu(110), Cu(111), and sulfur‐covered Cu(111)

Rotational and vibrational state distributions have been determined for H2 and D2 recombinatively desorbing from clean Cu(110) and Cu(111) surfaces in ultrahigh vacuum, and also from sulfur‐covered Cu(111). A 2+1 resonanced‐enhanced multiphoton ionization technique is employed. For clean copper the (v‘=1)/(v‘=0) vibrational population ratio for both H2 and D2 is ∼50 times greater than the value expected for an equilibrium ensemble at the surface temperature Ts for desorption from Cu(110) and ∼100 times greater from Cu(111). In contrast, for sulfur‐covered Cu(111), the H2 (v‘=1)/(v‘=0) ratio is 10 to 100 times less than for clean Cu(111) at the start of permeation but recovers sharply about 45 min later to a value close to that measured for the clean surface. We find that the total near‐surface sulfur concentration appears to remain constant over the same time period, but that the overlayer structure is radically altered.Rotational and vibrational state distributions have been determined for H2 and D2 recombinatively desorbing from clean Cu(110) and Cu(111) surfaces in ultrahigh vacuum, and also from sulfur‐covered Cu(111). A 2+1 resonanced‐enhanced multiphoton ionization technique is employed. For clean copper the (v‘=1)/(v‘=0) vibrational population ratio for both H2 and D2 is ∼50 times greater than the value expected for an equilibrium ensemble at the surface temperature Ts for desorption from Cu(110) and ∼100 times greater from Cu(111). In contrast, for sulfur‐covered Cu(111), the H2 (v‘=1)/(v‘=0) ratio is 10 to 100 times less than for clean Cu(111) at the start of permeation but recovers sharply about 45 min later to a value close to that measured for the clean surface. We find that the total near‐surface sulfur concentration appears to remain constant over the same time period, but that the overlayer structure is radically altered.

Measurements of weakly bound hydrogen on Pd(111) between 5 and 80 K

The adsorption of hydrogen and deuterium onto a low‐temperature surface of Pd(111) has been studied experimentally using electron‐stimulated desorption (ESD) and temperature programmed desorption spectrometry (TPD). At a surface temperature TS=7.5±1 K, exposure to D2 leads to multilayer formation which is reflected in the desorption of deuterium ion clusters D+n by ESD (Ee−=100 eV). Cluster sizes up to n=21 have been observed which, except for n=2, consist exclusively of odd numbers of atoms. The temperature dependence of the ESD signal following exposure at TS=7.5±1 K suggests that by 9.5±1 K the multilayer desorbs leaving a diatomic molecular species which, in the high‐coverage limit, is stable up to ∼20 K. Exposure to hydrogen at these low temperatures gives rise to two TPD peaks: the β (chemisorption) peak and a new TPD peak denoted γ (near 60 K at low coverage) which is assigned to desorption of a surface molecular hydrogen species. Mixed isotope experiments show that there is no mixing in the γ stat...

The exposure dependence and emission spectrum of chemiluminescence produced during the oxidaton of Si(111) by O2

Abstract Chemiluminescene produced during the low pressure oxidation of a Si(111) surface by O 2 has been investigated. Carefully cleaned and annealed samples yield chemiluminescence at pressures as low as 6×10 −7 torr, corresponding to a peak chemiluminescence yield of 2.2×10 −7 photons per incident molecule and comparing favorably with the value of 1×10 −7 measured by Bruce and Comas (ref.1). The evolution of the emission shows a hyperbolic decrease in time at pressures above 1×10 −4 torr but becomes more complex at the lowest exposures studied. The spectrally broad chemiluminescence was dispersed at medium resolution and recorded with an Optical Multichannel Analyzer in an attempt to identify and characterize the energy content of the emitting species.