Xps studies of donor and acceptor chemisorption of NO and CO on nickel oxide surfaces

Systematic variation on the defect (excess) oxygen concentration in the surface of nickel oxide preheated to 700, 1100 and 1450°C, has been revealed in O(1s) and $\text{Ni(2p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{)}$ X-ray photoelectron spectra. The magnitude of the surface charge, after evacuation at 500°C, is also directly related to the defect properties. The effect of chemisorption of NO and CO on the free hole concentration of the p-type semiconductor surface has been studied by monitoring the surface charge. NO adsorption, predominantly as negatively charged (electron acceptor) molecules removes much of the surface charge whereas CO adsorption, as CO^δ+ species (donor chemisorption), increases the surface charge on all samples.     

Deposition of nickel from Ni(CO)4 on palladium and iron surfaces studied by X-ray photoelectron spectroscopy

The interaction of nickel carbonyl, Ni(CO)₄, with evaporated palladium and iron surfaces has been studied at 90 and 290 K by X-ray photoelectron spectroscopy. The carbonyl is weakly adsorbed in molecular form at 90 K on the metals giving a $\text{Ni 2p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ peak at 854.6 eV, a C 1s at 287.2 eV and an O 1s at 533.8 eV. Some fraction of the carbonyl decomposes even at 90 K on iron to give deposited nickel atoms. In the interaction with palladium at 290 K, deposited nickel atoms $\text{(Ni 2p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{= 852.9 eV)}$ and chemisorbed CO are observed. A satellite feature of the $\text{Ni 2p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ peak varies depending on the quantity of the deposited nickel atoms; the main peak-satellite separation increases with increase in the quantity. The same variation is observed for evaporated nickel-palladium alloys. This can be ascribed to the difference in the electronic states of the nickel atoms. The difference is reflected in the reactivity of the atoms with O₂. With iron the deposited nickel atoms show an increase in binding energy of 0.4 eV in the $\text{Ni 2p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ Peak and no satellite when the number of nickel atoms is small. The oxidation of the surface is also studied.     

Elastic leed intensity-energy studies of clean (001), (110) and (111) nickel surfaces

Elastic low energy electron diffraction (LEED) intensity-energy (I-E) measurements for clean (001), (110), and (111) nickel surfaces were obtained at room temperature. Surface composition was monitored by Auger spectroscopy. I-E data from 15 to 220 eV were obtained at normal incidence for the non specular beams and for the specular beams at incidence angles from 4° to 20° on the 0° and 45° azimuths of (001), on the 0° and 90° azimuths of (110), and on the 0° azimuth of (111) nickel. Normalization of the data was performed electronically during data acquisition. Intensities were calibrated with the use of a shielded, biased Faraday collector. The effects of instrumental and experimental uncertainties were examined and minimized to obtain intensities accurate to ± 15 %, energy scales accurate to ± 0.35 eV, and incident and azimuthal angles accurate to ± 0.25° and ± 1.0° respectively.All nickel surfaces have I-E spectra which are characteristic of strong multiple scattering. Angular evolution features for (001) and (110) spectra may be correlated with intraplanar resonances associated with the onset of propagating beams. Only the (001) surfaces were found to have pronounced, sharp resonance features associated with surface barrier resonances and inelastic loss processes. Kinematic analysis of the Lorenzian-shaped I-E peaks on all surfaces in consistent with surface expansion using either an energy-dependent or a constant inner potential of 10.75 ± 0.5 eV. The widths of these same peaks on all surfaces were found to vary as $\text{E}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ above 40 eV and $\text{E}{}^{\mathrm{<mtext>1</mtext><mtext>3</mtext>}}$ below.     

X- ray photoemission study of the interaction of oxygen and air with clean cobalt surfaces

The interaction of oxygen with polycrystalline cobalt surfaces has been studied at 300 K (1 × 10^?6 to 1 × 10^?5 Torr) using high-resolution (monochromatized) X-ray photoemission. At high exposures (> 100 L nominal) CoO is identified as the product from the nature of the $\text{Co}\text{2}\text{p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{, 2}\text{p}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{, 3}\text{s}$, and valence band spectra. There is no evidence for measurable amounts of Co₃O₄ or Co₂O₃. Two O 1s features are observed at both high and low (10L) exposures. The dominant O 1s feature at 529.5 ± 0.2 eV corresponds to the oxide and a minor feature at 531.3 ± 0.2 eV is attributed to non-stoichiometric surface oxygen. Exposure to air produces quite different results, with a dominant O 1s feature at 531.5 ± 0.2 eV and dominant $\text{Co}\text{2}\text{p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ and $\text{2}\text{p}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ features centered at 781.3 ± 0.2 eV and 797.1 ± 0.2 eV. These three values are very close to those reported here for bulk Co(OH)₂. Ion etching of the air-exposed surface removes this dominant surface product rapidly revealing some oxide and finally metal.     

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.     

SIMS,AES, work function and flash desorption measurements of the CO adsorption on NiCu alloy surfaces

The chemisorption of CO on Cu, Ni and CuNi alloy surfaces was examined by SIMS, work function measurements and desorption spectroscopy. Using a dynamic SIMS technique the M⁺, M⁺₂, MCO⁺ and M₂CO⁺ emission at different temperatures (100–400 K) was measured as a function of CO exposure. In agreement with the work function and desorption experiments an increase of M⁺ and MCO⁺ emission due to the CO adsorption on Cu was found only at low temperatures (100–190 K). On the Ni surface an increase of Ni⁺, NiCO⁺ and Ni₂CO⁺ was measured up to 400 K. The adsorption of CO on CuNi alloy surfaces — as derived from the work function measurements — can be described by the assumption of two different states of adsorbed carbon monoxide. They can be characterized by different binding energies and from sign and magnitude different work function changes. These states were interpreted as adsorption at Ni or Cu sites of the alloy surfaces, respectively. To a certain extent the SIMS results from the alloy surfaces are incompatible with the work function measurements and desorption spectroscopy and the SIMS studies on the pure metals. A Cu⁺ emission with comparable intensity to the Ni⁺ emission was found for alloys with bulk concentrations of 60 and 40 at% Cu at 300 K. The ratio $\text{Ni}{}^{\mathrm{+}}\text{Cu}{}^{\mathrm{+}}$ was nearly independent of CO pressure and temperature. The measured ratios of $\text{Cu}{}^{\mathrm{+}}{}_2\text{(}\text{Cu}{}^{\mathrm{+}}\text{+}\text{Ni}{}^{\mathrm{+}}\text{)}$, $\text{Ni}{}^{\mathrm{+}}{}_2\text{(}\text{Cu}{}^{\mathrm{+}}\text{+}\text{Ni}{}^{\mathrm{+}}\text{)}$ and $\text{CuNi}{}^{\mathrm{+}}\text{(}\text{Cu}{}^{\mathrm{+}}\text{+}\text{Ni}{}^{\mathrm{+}}\text{)}$ with values about 10^?2 can be explained the basis of a statistical arrangement of Cu and Ni atoms in the alloy surface. The intensities of the MCO⁺ emissions are 10² times smaller than the corresponding values of the pure metals. No emission of M₂CO⁺ was found on CuNi during CO adsorption.     

Photoemission of Xe and Kr adsorbed on the W(110) plane

The UPS and XPS spectra of Xe adsorbed on clean, O, CO, and Xe covered W(110) surfaces and the UPS spectrum for Kr on clean and O covered W(110) surfaces have been investigated. On clean W, Xe and Kr show a splitting of the $\text{5p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ and $\text{4p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ hole states respectively. For Xe the coverage dependence of this splitting was investigated in detail; neither the positions nor the intensity ratio of the substates are coverage dependent for θ ? 0.04, suggesting that splitting is due to differences in the image interaction of the $\text{m}{}_{\mathrm{j}}\text{= ±}\text{3}\text{2}$ and $\text{m}{}_{\mathrm{j}}\text{= ±}\text{1}\text{2}$ components. For Xe equal shifts, relative to vacuum, of ~1.0 eV were observed for 5p, 4d, and 3d levels, suggesting that initial state effects are small. Image interaction for Xe and Kr on clean W could best be fitted by assuming an increase, rather than a decrease in the effective hole-image separation from the nominal value, suggesting that the image plane is moved back into the metal by a screening length. For Xe adsorbed on $\text{Xe}\text{W}$(110), or on virgin-$\text{CO}\text{W}\text{(110)}$ polarization of the intermediate layers was found to contribute significantly to relaxation. Coadsorbed oxygen broadened Xe 5p and Kr 4p peaks. There was an almost linear relation between O 2p UPS intensity at the energies of the various peaks and the amounts of broadening, suggesting that the latter results from resonance neutralization by electrons from the O 2p states.     

Soft X-Ray appearance potential spectroscopy of C,N and O adsorbed on Cr,Mo and W

The 1s level soft x-ray appearance potential spectra of C, N and O chemisorbed on polycrystalline Cr, Mo and W surfaces have been observed. The adsorbate ls spectra show the following characteristic features in the chemisorption regime: (i) Cr: one single peak; (ii) Mo and W: a doublet structure. We tentatively assume that the doublet structure is due to a splitting in the antibonding levels of the surface complex. Oxidation of Mo and W at high temperature causes changes in the metal $\text{(Mo 2p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ and $\text{W 3d}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}\text{)}$ and in the oxygen 1s spectra as compared with the spectra obtained after room temperature adsorption of oxygen.     

Electrical conductivity of the system Y2O3CeO2

The electrical conductivity of the system Y₂O₃CeO₂ was measured in the temperature range 500–1100°C and P_o2 range 10^–7?10^?1 atm. Possible defect models were suggested on the basis of conductivity data, which were investigated as a function of temperature and of P_o2. The observed activation energies were 0.40 eV and 1.79 eV in the low- and high-temperature regions, respectively. The observed conductivity dependences on P_o2 were in the temperature range 500–750°C and at temperatures from 750–1100°C. It is suggested that the system Y₂O₃CeO₂ shows a mixed ionic plus hole conduction due to an O^″_i defect and an electronic hole conduction due to a V^'''_Y defect in the low- and high-temperature regions, respectively.     

Interaction of inert gases with a nickel (100) surface: I. Adsorption of xenon

The adsorption of Xe on a Ni(100) surface has been studied in UHV between 30 and 100 K using LEED, thermal desorption spectroscopy (TDS), work function (Δφ) measurements, and UV photoemission (UPS). At and below 80 K, Xe adsorbs readily with high initial sticking probability and via precursor state adsorption kinetics to form a partially ordered phase. This phase has a binding energy of ~5.2 kcal/mole as determined by isosteric heat measurements. The heat of adsorption is fairly constant up to medium coverages and then drops continuously as the coverage increases, indicating repulsive mutual interactions. The thermal desorption is first order with a preexponential factor of about 10¹² s^?1, indicative of completely mobile adsorption. Adsorbed Xe lowers the work function of the Ni surface by 376 mV at monolayer coverage. (This coverage is determined from LEED to be 5.65 × 10¹⁴ Xe molecules/cm^-2.) For not too high coverages, θ, Δφ(θ) can be described by the Topping model, with the initial dipole moment μ₀ = 0.29 D and the polarizability α being 3.5 × 10^?24 cm³. In photoemission, the $\text{Xe 5p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ and $\text{5p}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ orbitals show up as intense peaks at 5.56 and 6.83 eV below E_f which do not shift their position as the coverage varies. Multilayer adsorption (i.e. the filling of the second and third layers) can be seen by TDS. The binding energies of these α states can be estimated to range between 4.5 and 3.5 kcal/mole. The results are compared and contrasted with previous findings of Xe adsorption on other transition metal surfaces and are discussed with respect to the nature of the inert-gas-metal adsorptive bond.     

The kinetics and mechanism of the autocatalytic decomposition of HCOOH on clean Ni(110)

The flash decomposition of DCOOH was studied on a clean nickel (110) surface following adsorption at 37°C. The reaction proceeded by a two-dimensional autocatalytic mechanism to form D₂, CO₂ and CO products. The results indicated DCOOH adsorbed dissociatively at 37°C by splitting off H₂O and forming an adsorbed molecule composed of DCO and DCOO. Above ten percent of saturation coverage these molecules formed a condensed phase or island structure. The decomposition of the molecules was rate determining for the formation of CO₂ and D₂ products. Theoretical calculations for branched chain mechanisms and coadsorption experiments with CO and H₂ separately with DCOOH indicated the intermediate involved in the explosion was not associated with the observed product molecules. The intermediate in the explosive decomposition was shown by interrupted flashes to be stable at 37°C. The autocatalytic flash decomposition curves were explained by reaction occurring at bare metal sites within the islands, and as product molecules desorbed the number of sites increased, causing the rate to accelerate. The rate of decomposition was well described by the $\text{equation Rate = ?k(}\text{c}\text{c}{}_{\mathrm{I}}\text{)(c}{}_{\mathrm{I}}\text{?c + fc}{}_{\mathrm{I}}\text{)}$, where c is the surface concentration, c_I is the initial surface concentration, and f is the density of initiation sites. The activation energy of 26.6kcal/mol was determined from heating rate variation. The narrow flash curves were fit with a first order pre-exponential factor of 1.6 × 10¹⁵ sec^?1 with a density of initiation sites of 0.004.     

A thermo-chemical study of the defect structure of barium oxide

A study of BaO has been made by use of thermogravimetric analysis, oxygen concentration analysis, and X-ray lattice parameter measurements in the temperature range 850°C ? T ? 1420°C and oxygen pressure range 7 × 10^-6 atm ? p_O2 ? 0·820 atm. Both the weight gain by the BaO samples and subsequently determined excess oxygen concentration were found to be directly proportional to . The enthalpy of incorporation oxygen in the lattice

$\text{1}\text{2}\text{O}{}_2\text{(g)=O(excess)}$

was determined to be ?0·395 ± 0·034 eV. Creation of vacancies on cation sites or of oxygen interstitials are consistent with the experimental results. As an alternative, the formation of O₂^2? ions, (as in BaO₂) as a result of incorporation of excess oxygen in the lattice, has been suggested.     

Rotation-vibration analysis of the Coriolis-coupled ν5g, ν6g, ν8g and ν9 bands of H2CCO

Medium resolution infrared grating spectra of gaseous ketene, H₂CCO were recorded between 1000 and 400 cm^?1, both at instrument temperature (40°C) and with cooling (?40°C). Interferometric Fourier spectra were also measured at ?70°C with resolution 0.22 cm^?1 between 450 and 330 cm^?1. The K structure of the fundamentals ν₅, ν₆, ν₈, and ν₉ was assigned. These fundamentals are coupled by a-axis Coriolis interactions. These couplings were analysed on the symmetric top basis for setting up the perturbation matrix and by utilizing the K-dependent Coriolis shifts of levels. A preliminary analysis of the Coriolis intensity anomalies was also undertaken.Band center values from combination differences are $\text{ν}{}_5{}^0\text{= 587.30 (27)}$ and $\text{ν}{}_6{}^0\text{= 528.36 (39)}\text{cm}{}^{\mathrm{?1}}$. Synthetic spectra indicate the band origins of ν₈ and ν₉ to be close to 977.8 and 439.0 cm^?1, respectively. Estimates of Coriolis coupling constants obtained from synthetic spectra are $\text{ζ}{}_{58}{}^{\mathrm{a}}\text{= + 0.33 (5)}$, $\text{ζ}{}_{68}{}^{\mathrm{a}}\text{= + 0.714 (20)}$, $\text{ζ}{}_{59}{}^{\mathrm{a}}\text{= ? 0.774 (20)}$, and $\text{ζ}{}_{69}{}^{\mathrm{a}}\text{= ? 0.30 (2)}$. Approximate ratios of unperturbed vibrational transition moments obtained from spectral simulations are $\text{M}{}_8{}^0\text{:±iM}{}_5{}^0\text{:±iM}{}_6{}^0\text{:M}{}_9{}^0\text{≈ +2:?9:+10:+0.5}$.     

Bulk-to-surface precipitation and surface diffusion of carbon on polycrystalline nickel

The time evolution of the KLL Auger spectrum of carbon as a function of temperature is used to derive the kinetics of the surface diffusion and bulk-to-surface precipitation of carbon on polycrystalline nickel. The results show that the activation energy for the surface diffusion of carbon atoms on polycrystalline nickel is 6.9 ± 0.6 $\text{kcal}\text{mole}$, and the activation energy for bulk-to-surface precipitation is 9.4 ± 0.6 $\text{kcal}\text{mole}$. The dependence on the surface diffusion coefficient D_s (cm²s^?1), on the absolute temperature T can be represented, over the experimental temperature range, 350–425° C, by: $\text{ln}\text{D}{}_{\mathrm{s}}\text{= 10.27 ?}\text{3568}\text{T}$.     

Microwave and photoelectron study of cis- and trans-isocyanato ethene,CH2CHNCO (vynyl isocyanate)

The microwave and photoelectron spectra of isocyanato ethene CH₂CHNCO have been studied. The microwave results indicate that the species is planar and possesses both a cis and a trans form. The appearance of dense and complicated vibrational satellite lines indicates that the molecule is quite flexible, a general property of molecules containing the isocyanate group. The rotational constants are:

$\text{cis: A}{}_0\text{= 20 146.8, B}{}_0\text{= 3107.267, C}{}_0\text{= 2689.513}\text{MHz}\text{; trans: A}{}_0\text{= 62 584.051, B}{}_0\text{= 2437.730, C}{}_0\text{= 2346.507}\text{MHz}$

These constants are shown to be consistent with structures in which $\text{r(}\text{CN}\text{) = 1.382 ± 0.005}\text{A}\text{?}$, ∠(CCN) = 122 ± 1° (for both conformers), and ∠(CNC) = 142.4 ± 0.5° (cis) and 138.4 ± 1.5° (trans). The dipole moments are μ(cis) = 2.120 ± 0.015 and μ(trans) = 2.207 ± 0.007 D. Several distinct peaks are observed in the photoelectron spectrum; however, the structure is not resolved into features belonging to the different isomers. The first ionization potential lies at 9.80 ± 0.1 eV. The spectrum has been assigned with the aid of theoretical calculations.     

Trapping probabilities and desorption energies of alkali atoms on a clean and an oxygen covered tungsten (110) surface

Alkali atoms were scattered with hyperthermal energies from a clean and an oxygen covered (θ ≈ 0.5 ML) W(110) surface. The trapping probability of K and Na atoms on oxygen covered W(110) has been measured as a function of incoming energy (0–30 eV) and incident angle. A considerable enhancement of trapping on the oxygen covered surface compared to a clean surface was observed. At energies above 25 eV there are still K and Na atoms being trapped by the oxygen covered surface. From the temperature dependence of the mean residence time τ of the initially trapped atoms the pre-exponential factor τ₀ and the desorption energy Q were derived using the relation: $\text{τ = τ}{}_0\text{exp}\text{(}\text{Q}\text{kT}{}_{\mathrm{<mtext>s</mtext>}}\text{)}$. On clean W(110) we obtained for Li: $\text{τ}{}_0\text{= (8 ±}\text{8}\text{4}\text{) × 10}{}^{\mathrm{?14}}\text{sec}$, Q = (2.78 ± 0.09) eV; for Na: τ₀ = (9 ± 3) × 10^?14 sec, Q = (2.55 ± 0.04) eV; and for K: τ₀ = (4 ± 1) × 10^?13 sec, Q = (2.05 ± 0.02) eV. Oxygen covered W(110) gave for Na: τ₀ = (7 ±3) × 10^?15 sec, Q = (2.88 ± 0.05) eV; and for K: $\text{τ}{}_0\text{= (1.3 ±}\text{0.9}\text{0.6}\text{) × 10}{}^{\mathrm{?14}}\text{sec}$, Q = (2.48 ±0.05) eV. The adsorption on clean W(110) has the features of a supermobile two-dimentional gas; on the oxygen covered W(110) adsorbed atoms have the partition function of a one-dimen-sional gas. The binding of the adatoms to the surface has a highly ionic character in the systems of the present experiment. An estimate is given for the screening length of the non-perfect conductor W(110):k_s^?1≈ 0.5 Å.     

Nuclear magnetic resonance and relaxation in liquid zinc

Measurements of the Knight shift K and the nuclear spin-lattice relaxation rate $\text{(}\text{1}\text{T}{}_{\mathrm{l}}\text{)}$ of Zn⁶⁷ are reported for liquid Zn. Measurements of K extend from 409°C (supercooled) to 700°C; measurements of $\text{(}\text{1}\text{T}{}_{\mathrm{l}}\text{)}$ cover the range 425 – 620°C. The Knight shift shows a weak positive temperature dependence attributed to thermal expansion. The reciprocal enhancement factor of the Korringa relation K(α) is found to increase with temperature from .77 ± .04 to .82 ± .04 in the range 425 – 620°C. The quadrupolar contribution to the relaxation rate was concluded to be very small with an estimated upper limit given by $\text{(}\text{1}\text{T}{}_{\mathrm{l}}\text{)}{}_{\mathrm{Q}}\text{≤ 15 sec.}{}^{\mathrm{-l}}$     

A study of the excited electronic states of 9-fluorenone

Some spectroscopic properties of the low-energy electronic states of 9-fluorenone have been examined. The spectra in paraffin matrices at 4.2°K show detailed vibrational spectra. Two fluorescence spectra are observed; a diffuse emission arises from 9-fluorenone crystals in the paraffin matrix, and a sharp emission is characteristic of the molecule. The sharp fluorescence is analyzed in terms of known a₁ vibrational fundamentals. The sharp absorption is a near mirror-image to the fluorescence, so Herzberg-Teller vibrations are not prominent. The polarization in the crystal spectrum allows this low-energy transition near 23 000 cm^?1 to be assigned ${}^1\text{B}{}_2\text{←}{}^1\text{A}{}_1$. Because there is no vibronic perturbation in fluorescence, and certainly no out-of-plane modes, a $\text{π}{}^1\text{← n}$ transition seen at about 26 000 cm^?1 is tentatively assigned ${}^1\text{B}{}_1\text{←}{}^1\text{A}{}_1$. Another sharp absorption system is seen at 31 000 cm^?1 in the paraffin matrices at 4.2°K (linewidth 6 cm^?1) but no fluorescence was detected. The polarized crystal spectrum indicated the assignment of this system and another very strong system at 40 000 cm^?1 to be ${}^1\text{B}{}_2\text{←}{}^1\text{A}{}_1$, while other systems at about 34 000 cm^?1 and 44 000 cm^?1 are ${}^1\text{A}{}_1\text{←}{}^1\text{A}{}_1$.The phosphorescence spectrum of pyrene-d₁₀ held in a single crystal of 9-fluorenone at 4.2°K has been recorded. No delayed fluorescence from the host crystal is observed at 4.2°K but is intense at 77°K. The energy difference between host and guest triplet levels is estimated to be about 900 cm^?1 allowing the lowest triplet state of 9-fluorenone to be placed at 17 800 cm^?1.     

Surface segregation of PdAg alloys induced by ion bombardment

A Pd-rich phase has been observed to segregate on the surface of PdAg alloys under the influence of 500 eV Ar⁺ and N₂⁺ bombardment. By examining the X-ray photoemission spectra of these surfaces, we report changes in the structure of the Pd 3d peaks which are specifically linked to this phase. Studies of the spectral changes with Ar⁺ dose show that one $\text{Pd 3d}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}$ peak arises from electrons originating from the altered overlayer and a second $\text{Pd 3d}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}$ peak arises from the underlying alloy with a binding energy dependent on the underlying concentration. The altered layer is found to vary in thickness between one and five layers, depending on the bulk alloy concentration. The thickness of this layer is also dependent on the nature of the primary ion since a different altered layer thickness is found using nitrogen ion bombardment.     

Coverage and adsorption-site dependence of core-level binding energies for tin and lead on Al(111) and Ni(111)

Thin layers (0.2–10 monolayers) of Pb and Sn were prepared on Al(111) and Ni(111) surfaces and characterized by means of LEED, AES, UPS and work-function measurements. The binding energy of the shallow Sn 4d and Pb 5d core-levels was investigated with respect to coverage and adsorption-site-dependent changes. On Al(111) the Sn and Pb monolayers exhibit ordered, two-domain, aligned but not in registry structures. For these layers core-level binding energies were found identical to those of the bulk metals. On Ni(111), Pb gives rise to a 3 × 3 structure, followed then by a 4 × 4 structure at higher coverages. The Pb 5d core-level binding energies shift continuously to higher values. A final shift of 0.42 eV is reached after about 2 monolayers. Sn on Ni(111) exhibits two welll separated peaks lying at 23.70 and 23.97 eV for the $\text{4d}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}$ line. These two lines can be correlated with two different adsorption sites which have to be assumed for the $\text{(}\text{3}\text{×}\text{3}\text{)R30°}$ and the (2 × 2) structure found at different coverages. The binding energy shifts are discussed in a model based on a Born-Haber cycle.