Optical spectroscopy of surfaces: Reflectance studies of chemisorption

A high precision technique has been used to study the small changes in near-normal incidence reflectance R caused by chemisorption of gases on an atomically clean metal surface maintained in ultrahigh vacuum. Examples are given for O₂, CO, and H₂ on Mo(100) in the photon energy range 1.9 <?ω < 4.8eV. The relative reflectance change $\text{ΔR}\text{R}$ is negative and reaches as much as 1%. Structure that appears in ΔR(e), where e=exposure (pressure × time), is attributed to different binding configurations of the adatoms as observed, e.g., in LEED experiments. The dependence of $\text{ΔR}\text{R}$ on ?ω suggests the presence of adsorbate-induced surface states within a few eV of the Fermi energy. Possible approaches to determining the dielectric function of the surface region from the data are discussed.     

Theoretical studies of chemisorbed oxygen on Ag(110)

The interaction of an oxygen atom with a 26-atom cluster model of the (110) face of Ag has been investigated with ab initio self-consistent-field and configuration-interaction theory. The SCF results for the bridge site (C_2v) predict $\text{r = 0.3}\text{A}\text{?}$ and ω_e = 327 cm^?1, in good agreement with the available experimental evidence. The calculated binding energy (D_e = 9 kcal/mole) is roughly an order of magnitude too small. The inclusion of electron correlation increases r_e and ω_e only slightly, but should have a dramatic effect on D_e. The ground state corresponds to a “surface oxide” state. The theoretical projected density-of-states curves exhibit “bonding” and “anti-bonding” O(2p) peaks, separated by ~ 6 eV, in good agreement with recent angle-resolved photoemission data.     

Surface self-diffusion on Ni(110): Temperature dependence and directional anisotropy

The surface self-diffusion coefficients, D_s, on a Ni(110) crystal are measured by a mass transfer technique in [1$\text{1}$0] and [001] directions in the temperature range 773–1573 K. The surface cleanliness was checked by Auger electron spectroscopy. LEED investigations showed that the sinusoidal surface profile consisted of (110) terraces and monatomic steps. The temperature dependence of D_s can be expressed by $\text{D}{}_{\mathrm{s}}\text{[1}\text{1}\text{0] = 0.009}\text{exp}\text{(}\text{?17.5}\text{kcal}\text{mole}\text{· RT)}$ and $\text{D}{}_{\mathrm{<mtext>s</mtext>}}\text{[001] = 470}\text{exp}\text{(}\text{?45}\text{kcal}\text{mole}\text{· RT)}$ at temperatures below 1150 K. Theoretical values for the activation energies of surface migration were calculated in the framework of the pairwise interaction model. Together with an estimate for the formation energy of adatoms of $\text{16.3}\text{kcal}\text{mole}$, one obtains for the activation energy of surface self-diffusion 17 and $\text{51 kcal}\text{mole}$ for [1$\text{1}$0] and [001] direction, respectively. At T > 1150 K the anisotropy in D_s begins to vanish. Surface diffusion in [1$\text{1}$0] direction at T < 1150 K is most likely taking place by a simple adatom hopping process. Circumstantial evidence indicates that diffusion in [001] direction does not occur by a simple hopping process but by a more complex mechanism involving higher energy surface diffusion states. This isotropic process is suggested to take place for both directions at T < 1150 K.     

Reducible scale invariance at short distances

It is proven that, using reducible scale invariance at short distances, conformal symmetry implies canonical (Bjorken) scaling, provided diagonal dimensions of dilatation multiplets occuring in the operator product expansion of two electromagnetic currents have the canonical value l_n = 2 + n. If the electromagnetic current itself belongs to such multiplets then the hadron production cross section in e⁺e^? annihilation falls off faster than $\text{1}\text{s}$ at asymptotic energy.     

Monopoles,duality, triality

The existence of magnetic charges could be a raison d'être not only for the quantization of electricity in units $\text{1}\text{3}\text{e}$ but also for the confinement of the quarks at the end of “observable”, “electric” Dirac strings (quarks have no magnetic charge in this scheme). We first review the Dirac quantization condition, using a “sum over histories” approach, and get a more general result: A string attached to the dyon (e₁, g₂) is observable by a dyon (e₂, g₂) unless (1?x) e₂g₁ ? xe₁g₂ = nh, where x is an arbitrary parameter which reflects an ambiguity in the action principle. The Dirac and Schwinger-Zwanziger quantization rules are special cases, with $\text{x = 0}\text{and}\text{1}\text{2}$, respectively. Then, we look for the values of x and of the magnetic charges to “explain” that (i) the electric charge is quantized in units $\text{1}\text{3}\text{e}$, (ii) the string attached to an electron is unobservable, (iii) the string attached to a quark is observable. We find a denumerable set of solutions. In most cases, the magnetic charges also are connected with observable strings.     

A monte-carlo/molecular dynamics study of the diffusional recombination kinetics of C(a) + O(a) → CO(g) on Pt(111)

The diffusion constants for C and O adsorbates on Pt(111) surfaces have been calculated with Monte-Carlo/Molecular Dynamics techniques. The diffusion constants are determined to be D_C(T)=(3.4 × 10^?3e^{$\text{?13156}\text{T}$})cm²s^?1 for carbon and D_O(T) = (1.5×10^?3 e^{$\text{?9089}\text{T}$}) cm² s^?1 for oxygen. Using a recently developed diffusion model for surface recombination kinetics an approximate upper bound to the recombination rate constant of C and O on Pt(111) to produce CO_(g) is found to be (9.4×10^?3 e^{$\text{?9089}\text{T}$}) cm² s^?1.     

Pressure effects on the quadrupole coupling constant of 7Li in first stage lithium graphite

The pressure dependence of the experimental ⁷Li NMR spectra is reported for first stage lithium graphite (LiC₆) intercalation compound at temperatures T = 232 and 293 K. This experiment together with the presented point charge model calculation of the ⁷Li quadrupole coupling constant $\text{(}\text{e}{}^2\text{qQ}\text{h}\text{)}$ allows an unambiguous determination of the sign of e²qQ/h which is negative: $\text{e}{}^2\text{qQ}\text{h=-52 kHz}$ at p = 1 bar and T = 232 K. The averaged location of the electrons transferred from the Li intercalant to the graphite layers, as estimated in this study, is in excellent agreement with earlier theoretical energy-band calculations. The compressibility of LiC₆ in the c-direction is predicted to be k_c = 1.7 × 10^-12cm²dyn^-1, it agrees with estimates derived from the available phonon dispersion relations.     

Scaling of the “superstable” fraction of the 2D period-doubling interval

The scaling properties of a “superstable” parameter interval, $\text{C}$, where the eigenvalues about a period-2n orbit are complex, are derived for 2D period-doubling maps. The ratio of $\text{C}$ to the whole parameter interval, between the nth and the (n+1)st bifurcation, is shown to be a universal function of the effective jacobian, B_e, only (B_e≡B²ⁿ, B is thejacobian of th e map). Unlike the whole period-2ⁿ interval, $\text{C}$ has a convergence rate that behaves as $\text{4.6692016×B}{}^{\mathrm{<mtext>-1</mtext><mtext>4</mtext>}}{}_{\mathrm{e}}$ as B_e↓), wh ile its complement has a convergence rate 8.7210972/4 as B_e↑1.     

New evaluation of muon (g − 2) hadronic anomaly

The hadronic part a_H of the muon g-factor anomaly $\text{a ≡}\text{(g ? 2)}\text{2}$ is evaluated from latest data on σ(e⁺e^? → hadrons). For a p-wave ππ scattering length of a₁ = 0.04±0.005 we calculate a_H = (66±10) × 10^?9, compared to a(experiment) ? a(QED) = (60±29) × 10^?9. Half of the uncertainty on a_H is associated with the energy interval $\text{0.92 <}\text{s}\text{< 2}\text{GeV}$.     

Production of direct single electrons in proton-nucleus collisions at 13 GeV/c

Direct production of single electrons in proton-nucleus collisions at 13 GeV/c has been measured. A rising $\text{e}\text{π}$ ratio with decreasing p_T above the 10^?4 level is observed for low p_T single electrons. The source of the rising $\text{e}\text{π}$ ratio at low $\text{e}\text{π}$ ratio at low p_T is discussed in connection with the low mass dielectron continuum below the ? mass region.     

Effect of pressure on the Curie temperature of PdFe,PdCo, PtFe and PtCo alloys

The Curie temperatures of $\text{Pd}$Fe, $\text{Pd}$Co, $\text{Pt}$Fe and $\text{Pt}$Co alloys with Fe or Co concentrations between 1 and 10 at. % have been measured by thermal scanning of the ⁵⁷Fe Mössbauer resonance under pressures up to 170 kbar. Systematically, the largest (positive) values of the logarithmic derivative, dlnT_e/dlnV, are found in the most dilute alloys. The data are compared with models, which relate the variation of T_c under pressure to changes in the impurity spin-conduction electron coupling parameter, J_i, and to changes of the pure host metal susceptibility, χ_h. The discrepancies between the experimental data and these models are discussed.     

The 404.5-nm system of the ReO molecule

Observations on the emission spectrum of ReO in the region 375–870 nm are reported. Five bands of a $\text{ΔΩ}\text{= 0}$ system with (0, 0) band at 404.5 nm have been rotationally analyzed and the principal results for ¹⁸⁷ReO are (in cm^?1) ν₀ = 24 709.90, B′_e = 0.3819, B″_e = 0.4252, ω′_e = 874.8₂, and $\text{Δ}\text{G″(}\text{1}\text{2}\text{) = 979.1}{}_2$. Data on the minor isotopic species ¹⁸⁵ReO are also reported. It is suggested that broad rotational profiles found in bands near 842 nm may be due to nuclear hyperfine structure.     

Site-selective adsorption of xenon on a stepped Ru(0001) surface

The adsorption of xenon has been studied with UV photoemission (UPS), flash desorption (TDS) and work function measurements on differently conditioned Ru(0001) surfaces at 100 K and at pressures up to 3 × 10^?5 Torr. Low energy electron diffraction (LEED) and Auger electron spectroscopy (AES) served to ascertain the surface perfectness. On a perfect Ru(0001) surface only one Xe adsorption state is observed, which is characterized by$\text{Xe}\text{5}\text{p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$,$\text{1}\text{2}$ electron binding energies of 5.40 and 6.65 eV, an adsorption energy of E_ad≈ 5 kcal/mole and dipole moment of μ'_T ≈ 0.25 D. On a stepped-kinked Ru(0001) surface, the terrace-width, the step-height and step-orientation of which are well characterized with LEED, however, two coexisting xenon adsorption states are distinguishable by an unprecedented separation in$\text{Xe}\text{5}\text{p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$,$\text{1}\text{2}$ electron binding energies of 800 meV, by their different UPS intensities and line shapes, by their difference in adsorption energy ofΔE_ad ≈ 3 kcal/mole and finally by their strongly deviating dipole moments of μ_S = 1.0 D and μ_T = 0.34 D. The two xenon states (which are also observed on a slightly sputtered surface) are identified as corresponding to xenon atoms being adsorbed at step and terrace sites, respectively. Their relative concentrations as deduced from the UPS intensities quantitatively correlate with the abundance of step and terrace sites of the ideal TLK surface structure model as derived from LEED. Furthermore, ledge-sites and kink-sites are distinguishable via E_ad. The E_ad heterogeneity on the stepped-kinked Ru(0001) surface is interpreted in terms of different coordination and/or different charge-transfer-bonding at the various surface sites. The enormous increase in Xe 5p electron binding energy of 0.8 eV for Xe atoms at step sites is interpreted as a pure surface dipole potential shift. —The observed effects suggest selective xenon adsorption as a tool for local surface structure determination.     

Spectroscopic information on ground-state Ar2, Kr2, and Xe2 from interatomic potentials

Calculations of vibrational and rotational level spacings of homonuclear inert gas diatomic molecules by numerical integration of the radial Schrödinger equation are presented. The potentials which were used for the ground states of Ar₂, Kr₂, and Xe₂ were obtained from accurate fits to the molecular beam scattering data. From the calculated $\text{Δ}\text{G}{}_{\mathrm{<mtext>v+</mtext><mtext>1</mtext><mtext>2</mtext>}}\text{'}\text{s}$ and B_v's, the following spectroscopic constants (in cm^?1) were fitted: for Ar₂ω_e = 31.92, ω_ex_e = 3.31, ω_ey_e = 0.11, B_e = 0.060, α_e = 0.004; for $\text{Kr}{}_2\text{ω}{}_{\mathrm{e}}\text{? 23.99}$, $\text{ω}{}_{\mathrm{e}}\text{x}{}_{\mathrm{e}}\text{? 1.30}$, $\text{ω}{}_{\mathrm{e}}\text{y}{}_{\mathrm{e}}\text{? 0.021}$, $\text{B}{}_{\mathrm{e}}\text{? 0.024}$, $\text{α}{}_{\mathrm{e}}\text{? 0.001}$; for $\text{Xe}{}_2\text{ω}{}_{\mathrm{e}}\text{? 21.26}$, $\text{ω}{}_{\mathrm{e}}\text{x}{}_{\mathrm{e}}\text{? 0.75}$, $\text{ω}{}_{\mathrm{e}}\text{y}{}_{\mathrm{e}}\text{? 0.008}$, $\text{B}{}_{\mathrm{e}}\text{? 0.013}$, $\text{α}{}_{\mathrm{e}}\text{? 0.0004}$.     

An investigation of the processes e+e−→μ+μ−γ and e+e−→e+e−γ

The reactions e⁺e^?→μ⁺μ^?γ and e⁺e^?→e⁺e^?γ have been studied with the CELLO detector at the PETRA storage ring. Data have been collected from $\text{s}\text{=14}\text{GeV up to}\text{46.8}\text{GeV}$. In a Dalitz plot analysis of the data, good agreement is found with QED, except for the region Mx_μγe>0.8 where the probability that QED describes the high energy data is at the percent level.     

Rotationally resolved spectra of two van der Waals states of I + Cl

Three-step optical resonance is used to execute state-selected transitions from the ground state of ICl to two van der Waals states, $\text{b(}\text{Ω}\text{= 1)}$ and $\text{b′(}\text{Ω}\text{= 2)}$, both of which correlate with the second dissociation limit, $\text{I}\text{(}{}^2\text{P}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{) +}\text{Cl}\text{(}{}^2\text{P}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{)}$, of ICl. Since the B(0⁺) state also belongs to this limit, three out of five states converging to I + Cl¹ are now accounted for. Principal constants of these states are: b′(2): T_e = 18275.84, ω_e = 31.093, ω_ex_e = 1.672, ω_ey_e = 0.0070, B_e = 0.034834, α_e = .001587, and D_e = 164.09 cm^?1; b(1): T_e = 18273.30, ω_e = 26.75, ω_ex_e = 0.882, B_e = 0.03579, q = 0.00084, and D_e = 166.63 cm^?1. In both states the equilibrium distance is near 4.2 Å, slightly greater than the sum of van der Waals contact radii, $\text{r}{}_{\mathrm{<mtext>I</mtext>}}\text{+ r}{}_{\mathrm{<mtext>Cl</mtext>}}\text{= 3.95}\text{A}\text{?}$. The large value of q in the b(1) state indicates that, in the basis set $\text{|j}{}_{\mathrm{a}}\text{j}{}_{\mathrm{b}}\text{j}\text{Ω}\text{〉}$ (a = I, b = Cl, j = j_a + j_b) the b(1) and b′(2) states belong to j = 1 and j = 2 “complexes,” respectively.     

Rotational coupling of the β 1 and E 0+ ion-pair states of lodine bromide

The $\text{E(}\text{Ω}\text{= 0}{}^{\mathrm{+}}\text{)}$ and $\text{β(}\text{Ω}\text{= 1)}$ ion-pair states of IBr are coupled by an electronic Coriolis interaction. A nonlinear, least-squares fit of term values for both states yields a set of potential and rotational constants purged of the effects of this interaction. The magnitude of the coupling constant, |W_1,0| = 2.41 cm^?1, is 98% of the value predicted for pure precession. Principal constants of the previously unreported β state of I⁷⁹Br are T_e = 39507.76, ω_e = 122.09, ω_eχ_e = 0.2546, B_e = 0.02937, and α_e = 8.2 × 10^?4cm^?1.     

Isospin bounds for energy partition in ee and NN annihilation

In reactions such as $\text{e}$e → anything, $\text{N}$N → anything, the final state (neglecting secondary isospin violating decays) is customarily supposed to have isospin zero or one. We show that for such states the average fraction (X) of the energy carried away by neutral pions is bounded by $\text{X<}\text{(9+√41)}\text{20}\text{≈0.77}$, when I₃ = 0 (e.g., for ee, pp, nn) and by $\text{X>}\text{(11?√41)}\text{40}\text{≈0.115}$, when I₃=±1 (e.g., for pn or np).     

Analysis of the 2770-Å emission system in I2

A weak emission spectrum of I₂ near 2770 Å is reanalyzed and found to to minate on the A(1u³Π) state. The assigned bands span v″ levels 5–19 and v′ levels 0–8. The new assignment is corroborated by isotope shifts, band profile simulations, and Franck-Condon calculations. The excited state is an ion-pair state, probably the 1g state which tends toward $\text{I}{}^{\mathrm{?}}\text{(}{}^1\text{S) +}\text{I}{}^{\mathrm{+}}\text{(}{}^3\text{P}{}_1\text{)}$. In combination with other results for the A state, the analysis yields the following spectroscopic constants: T″_e = 10 907 cm^?1, $\text{D}$″_e = 1640 cm^?1, ω″_e = 95 cm^?1, $\text{R″}{}_{\mathrm{e}}\text{= 3.06}\text{A}\text{?}$; T′_e = 47 559.1 cm^?1, ω′_e = 106.60 cm^?1, $\text{R′}{}_{\mathrm{e}}\text{= 3.53}\text{A}\text{?}$.     

A spectrum of rhenium oxide

The arc emission spectrum of the ReO molecule has been photographed in the region 590–860 nm and three bands of a single electronic transition have been rotationally analyzed. The separation of lines of the isotopic molecules ¹⁸⁵ReO and ¹⁸⁷ReO leads to the conclusion that the vibrational assignments for these bands are 1-0, 0-0, and 0–1. It is conceivable that an electronic isotope shift of ～0.08 cm^?1 exists. The following vibrational and rotational data (cm^?1) have been determined: ν₀(0-0) = 14 038.42, $\text{Δ}\text{G′(}\text{1}\text{2}\text{) = 867.85}$, $\text{Δ}\text{G″(}\text{1}\text{2}\text{) = 979.14}$; B′_e = 0.3889, α′_e = 0.0019, B″_e = 0.4257, α″_e = 0.0043. It is concluded that Λ′ ? Λ″ = +1 with Λ″ ≥ 2.