Surface temperature modulation in molecular beam relaxation spectrometry applied to catalytic decomposition of CH3OH on NiO

A new modification of molecular beam relaxation spectrometry (MBRS) of surface processes is described making use of partial modulation in order to study nonlinear processes: a constant particle beam is directed towards the catalyst surface, the surface temperature is modulated due to absorption of a modulated beam of UV light, reaction products are analyzed by use of phase sensitive mass spectrometric detection. The application of the method is shown by a study of catalytic decomposition of methanol on polycrystalline NiO. Formation of CO was found to be a monomolecular, formation of H₂ and H₂O bimolecular processes. The resulting mechanism may be described as follows:

Rate constants in dependence from surface temperature T₀ are η = 1.8 × 10³$\text{exp(?}\text{46}\text{RT}{}_{\mathrm{o}}\text{kJ}\text{mol}\text{)}$; k_d1 = 1.8 × 10¹⁰$\text{exp(?}\text{92}\text{RTl}{}_0\text{kJ}\text{mol}\text{)}$ s^?1; k_d2 = 1.2 × 10^?2$\text{exp (?}\text{88}\text{RT}{}_0\text{kJ}\text{mol}\text{)}$ cm² particles^?1 s^?1; k_d3 = 3.5 × 10^?4$\text{exp(?}\text{88}\text{RT}{}_0\text{kJ}\text{mol}\text{)}$ cm² particles^?1 s^?1. Average surface residence times of the intermediates are: $\text{27 ?}{}^{\mathrm{\tau }}\text{HCO}\text{}\text{?}\text{1 ms}$ at 550 ? T₀ ? 650 K; $\text{42 ?}{}^{\mathrm{\tau }}\text{H ? 7 ms}$ at 540 ?T₀ ? 610 K; $\text{177 ?}{}^{\mathrm{\tau }}\text{OH ? 19 ms}$ at 550 ? T₀ ? 645 K.     

The microwave spectrum of the SiCl radical

The microwave spectrum of ²⁸Si³⁵Cl, the most abundant isotopic species of the silicon monochloride radical, was observed in both the ${}^2\text{Π}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ and ${}^2\text{Π}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ spin states of the ground vibronic state. The SiCl radical was produced in a flow cell by a dc discharge in SiCl₄. The observed transitions were $\text{J =}\text{7}\text{2}\text{←}\text{5}\text{2}\text{up to}\text{21}\text{2}\text{←}\text{19}\text{2}$ for both the spin states, and the observed frequencies were subjected to the least-squares analysis to yield accurate molecular constants as follows: B₀ = 7652.3048(23), D₀ = 0.007017(14), A_J = ?0.8392(16), p₀ + 2q₀ = 138.660(98), q₀ = 0.20(17), $\text{a + (b + c)}\text{2}\text{= 37.50(28)}$, $\text{a ? (b + c)}\text{2}\text{= 49.84(73)}$, b = 9(12), d = 46.40(94), and eQq₁ = ?23.13(96), all in MHz with 3σ in parentheses. The positive sign of the Λ-doubling constant, p₀, indicates that the contributions of ²Σ^? states dominate over those of ²Σ⁺ states. The spin and orbital averages of the unpaired-electron distribution were calculated from the observed hyperfine coupling constants and were discussed in terms of the electronic structure of the molecule.     

Doppler-limited dye laser excitation spectroscopy of HCF

The cw dye laser excitation spectrum of the $\text{A}\text{?}{}^1\text{A″(000) ←}\text{X}\text{?}{}^1\text{A′(000)}$ vibronic band of HCF was observed between 17 188 and 17 391 cm^?1 with the Doppler-limited resolution, 0.04 cm^?1. The HCF molecule was produced by the reaction of discharged CF₄ with CH₃F, and 853 lines were observed, of which 516 transitions were assigned to K′_a ← K″_a = 3 ← 4, 2 ← 3, 1 ← 2, 0 ← 1, 1 ← 0, 2 ← 1, 0 ← 0, 1 ← 1, 2 ← 2, 3 ← 3, 2 ← 0, and 0 ← 2 subbands. A rotational analysis yielded the rotational constants and quartic and sextic centrifugal distortion constants for both the $\text{A}\text{?}$ and $\text{X}\text{?}$ states and the band origin, with good precision. The molecular constants determined reproduce the observed transition frequencies with an average deviation of 0.0038 cm^?1. Small rotational perturbations in the excited state were found at J = 5, 6 and J = 10, 11 of J_1,J and at J = 15, 16 of J_2,J?1 levels.     

The visible and photographic infrared spectrum of osmium oxide

The emission spectrum of OsO has been photographed in the region 405–875 nm where many new bands have been observed. In favorable cases the ${}^{190}\text{OsO}{}^{192}\text{OsO}$ isotopic splittings have been resolved and aid in vibrational assignments. Three visible bands in the region 433–470 nm have been assigned as (1,0), (0,0), and (0,1) of a $\text{ΔΩ}\text{= 0}$ electronic transition. The (0,0) and (0,1) bands have been rotationally analyzed, yielding principal constants (cm^?1) for the visible system of ν₀ = 22 273.3, B′₀ = 0.3657, D′₀ = 2.8 × 10^?7, B″_e = 0.4023, D″_e = 3.2 × 10^?7, $\text{Δ}\text{G″(}\text{1}\text{2}\text{) = 780.7}$, and $\text{Δ}\text{G″(}\text{1}\text{2}\text{) = 884.9}$. A band at 825.4 nm has been found to be a $\text{ΔΩ}\text{= +1}$ (0,0) band with the same lower state as in the analyzed visible bands. Constants for the upper state of the ir system are ν₀ = 12 109.7, B′₀ = 0.3845, and D′₀ = 3.1 × 10^?7 cm^?1.     

Chemisorption of indium on (111) silicon substrates: I. Adsorption and nucleation by mass-spectrometric technique

The adsorption and nucleation of indium on clean (111) silicon surfaces are studied by a UHV molecular beam mass-spectrometric technique. The thermal accommodation of the adatoms on the surface is complete. At very low surface coverages θ, an adsorption energy of $\text{57}\text{kcal}\text{mole}$ and a preexponential term τ₀ of the Frenkel relation equal to 8 × 10^?13 s are found from transient response measurements. The isosteric heat of adsorption E_a varies very slowly with θ, E_a is equal to $\text{59}\text{kcal}\text{mole}$ for θ ～ 10^?3 and $\text{57}\text{kcal}\text{mole}$ for θ = 0.9. The nucleation occurs without supersaturation in an adsorbed layer near a monolayer.     

Microwave optical double resonance and fluorescence spectrum of NO2 excited by the 4579 Å line of the argon-ion laser

A microwave-optical double resonance (MODR) involving the $\text{J =}\text{21}\text{2}\text{←}\text{19}\text{2}$ and $\text{J =}\text{21}\text{2}\text{←}\text{17}\text{2}$ components of the 10_{0, 10} ← 9_{1, 9} rotational transition of the ground vibronic state of ¹⁴N¹⁶O₂ has been observed using the 4579 Å Ar⁺ laser line for excitation. By means of high resolution study of the laser excited fluorescence, the optical transition has been assigned to the $\text{J =}\text{19}\text{2}\text{←}\text{21}\text{2}$ spin component of the 9_{0, 9} ← 10_{0, 10} transition from the ground vibronic state to an unknown vibrational state of A₁ symmetry of the ²B₂ electronic state.     

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.     

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.     

The microwave spectrum of 14NH3 in the v = 000011 state

The frequencies and assignments of 50 lines in the pure inversion spectrum of ¹⁴NH₃ in the 0001¹ vibrational state are reported in the microwave frequency region 18–53 GHz and in selected regions up to 58 GHz.The J = 0 inversion frequency, K-type doubling constant K, l = 2, ?1 and molecular dipole moment in this state are 32 904.7 ± 2.0 MHz, 1.958 ± 0.040 MHz and 1.459 ± 0.002 D, respectively, where model inadequacies are included in the uncertainties of the first two parameters. The dipole moment measurements for this and the ground state are in excellent agreement with Stark laser measurements. An expression containing the effective l-type doubling constant is obtained from the combination of frequencies $\text{[ν(1, 1, 1) ? ν(1, 1, ?1) ? ν(2, 1, 1) + ν(2, 1, ?1)]}\text{8}\text{= 10 361.894 ± 0.004}\text{MHz}$. A preliminary value for the l-type doubling constant is 10 655 ± 20 MHz.     

Attenuation and dispersion of first sound in a 19.4% 3He-4He mixture near the λ-transition

Attenuation α and dispersion D=u(ω) ? u(0) of first sound have been measured in a 19.4% ³He-⁴He mixture for frequencies $\text{2.25}\text{kHz}\text{?}\text{ω}\text{2π}\text{?594}\text{kKz}$, and at temperatures 1 μK?|T ? T_λ|?10 mK. They are interpreted as arising from a relaxation and a fluctuation process.     

Non-perturbative QCD effects explain the ΔI = 12 rule in the chiral limit

It is shown that in the chiral limit, the enhancement of the $\text{ΔI=}\text{1}\text{2}$ transitions for mesons is explained by the large size of non-perturbative QCD matrix elements. For $\text{〈}\text{s}\text{s〉 = 〈}\text{u}\text{u〉}$ we obtain ∣M(K_S⁰ → 2π⁰) ∣ =(5.2±0.6) × 10^?7m_K in excellent agreement with experiment.     

Magnetic field dependence of the nuclear spin—lattice relaxation of 93mMo and 94,95Tc in Fe

The nuclear spin—lattice relaxations of ^93mMo$\text{Fe}$ and ^{94, 95}Tc$\text{Fe}$ at T ? 10 mK have been investigated with frequency- modulation-pulsedNMR on oriented nuclei as a function of an external magnetic field B₀. In the range 0?B₀?6kG the relaxation constant varies by a factor >10 and ?6 for ^93mMo$\text{Fe}$ and ^94,95Tc$\text{Fe}$, respectively. The high-field Korringa constants for ^93mMo, ⁹⁴Tc and ⁹⁵Tc in Fe were determined to be 2.5(4), 1.3(4) and 0.55(10) sK.     

Are monojets due to unusual Z0 decays?

We examine and compare implications of several recent suggestions that events of the form $\text{p}\text{p}\text{→}\text{jet}\text{+(}\text{missing transverse momentum}\text{)}$ could be due to unusual Z⁰ decays. Two possibilities are noted: (a) $\text{Z}{}^0\text{→}\text{N}\text{N}$, where N (or $\text{N}$) is a neutral heavy lepton decaying to three neutrinos with a 10% branching ratio, and (b) Z⁰→h₁h₂, where h₁ and h₂ are two distinct spinless particles, and only the decay of h₁ is seen in the detector. Early tests of both possibilities are noted.     

A study of SO2 adsorption on CaO (100) surfaces using X-ray photoelectron spectroscopy

The adsorption of SO₂ on CaO (100) at 300 K has been studied using X-ray photoelectron spectroscopy. Under ultrahigh-vacuum conditions, the surface was exposed to 0–500 Langmuirs of SO₂. The resulting adsorption yields a single SO surface species with an S 2p peak at 168.2 eV and an O 1s_{$\text{sol1}\text{2}$} peak at 531.7 eV. Subsequent heating of the exposed surface to 673 K indicated no desorption or changes in the binding energies of the S 2p and O 1s_{$\text{1}\text{2}$} peaks. On the basis of these data and binding-energy data for standard compounds, the adsorbed species is identified as SO₄^2?. The surface coverage due to the SO₄^2? species was also measured as a function of SO₂ exposure. From these data, the initial adsorption is found to be first-order in surface coverage, and the initial sticking probability is found to have a value of 0.4.     

Excitation intensity dependence of shallow donor bound exciton luminescence in n-GaAs

Luminescence measurements were performed on high purity epitaxial n-GaAs $\text{(}\text{1 × 10}{}^{14}\text{cm}{}^3\text{< n <}\text{3 × 10}{}^{15}\text{cm}{}^3\text{)}$ for various excitation intensities I₀ in the range $\text{8 mW}\text{cm}{}^2\text{< I}{}_0\text{<}\text{4 W}\text{cm}{}^2$. The luminescence line corresponding to the radiative decay of the shallow donor bound exciton, (D⁰, X), broadens with increasing I₀ and appears as a doublet for $\text{I}{}_0\text{? 1}\text{W}\text{cm}{}^2$, while the two-electron replica of the (D⁰, X) remains a single narrow line. The doublet structure of the (D⁰, X) at elevated excitation levels is due to missing luminescence intensity in the center of the line as a consequence of low (D⁰, X) concentration in a layer extending 1–2 μm from the sample surface into the bulk. The low concentration of (D⁰, X) is attributed to capture of (D⁰, X) quanta into surface states, extending to lower energies from the Fermi level fixed by the shallow donors. Comparison of the present results with luminescence spectra obtained by various authors reveals, that unexplained spectral features in the (D⁰, X) region of n-GaAs reported in the literature are a consequence of high excitation intensity and correspond to the effect reported here. In partly compensated p-GaAs with donor concentrations as given above, the (D⁰, X) did not transform into a doublet structure even at $\text{W}\text{cm}{}^2$ excitation intensity.     

Atom-surface scattering with velocity-selected H and D atomic beams from LiF and NaF (001)

Hydrogen and deuterium atoms with an energy between 20 and 140 meV have been scattered from LiF and NaF (001) cleavage planes in UHV. The atomic beam was nearly monoenergetic by use of a slotted disk type velocity selector with a velocity resolution of $\text{Δv}\text{v}\text{=12\%}$ (FWHM). The scattering showed diffraction with a pronounced specular beam. The energies of several bound surface states have been determined by analysis of selective adsorption minima. These energies together with relative intensities of diffracted beams and specular intensity versus incident energy measurements may be represented by an interaction potential of Morse-type with additional terms which are periodic parallel to the surface. The parameters of the Morse-potential have been found to be: depth D=17.8 meV and reciprocal range $\text{κ=1.04}\text{A}\text{?}{}^{\mathrm{?1}}$ for H₁/LiF (001) and D = 17.9 meV, k = 1.15 Å^?1 for H₁/NaF (001); the additional periodic terms have relative strength β₁₀=0.04 and $\text{β}{}_{11}\text{≈}\text{β}{}_{10}\text{2}$ in the case of H ₁/LiF, but β₁₁≈β₁₀=0.03 in the case of H₁/NaF. From the observed influence of surface temperature on the position of selective adsorption minima the coefficient of thermal expansion of the topmost surface layer turns out to be approximately twice the bulk value.     

The cosmic microwave background and the hadron era of the early universe: A possible connection

A cosmological model is constructed which is a Friedman model, but with a finite ultimate temperature (T_F). A plausible argument is presented which suggests that the existence of T_F and the cosmic microwave background restricts the form of the hadronic level density:

, where A, B = constants. In our model, $\text{B =}\text{7}\text{2}\text{?}\text{3}\text{s}\text{,}\text{where}\text{s =}\text{positive}$ integer. The case $\text{s = 3 (B =}\text{5}\text{2}\text{)}$ is the well-known Hagedorn model; the Frautschi model corresponds to s = 6 (B = 3); the cases $\text{s = 1 (B =}\text{1}\text{2}\text{)}$ and s = 2 (B = 2) have not been considered before now. For each value of $\text{B <}\text{7}\text{2}$, the model gives the temperature (T_γ0) of the microwave background as a function of A, T_F and ?₀ (the present energy density of the universe). Two fascinating results then emerge: first, all estimates of T_γ0 favor a low density Friedman universe (?0 = 10^?31g/cm³), which rules out a universe with positive curvature; second, for ?₀ = 10^?31gr/cm³ the best estimate of T_γ0 (?3K) occurs for the Hagedorn model.     

Intersections of quasi-energy levels in the relativistic theory of multiply charged ions

The relativistic generalization of the quasi-energy method is given with the help of which the influence of an alternating electric field on the energy levels (2 $\text{1}\text{2}$ 0), (2 $\text{1}\text{2}$ 1) of multiply charged H-like ions is investigated. Intersections are found of quasi-energy levels in external fields with frequencies ω₀ ? ΔE_L and definite values of the field amplitude.     

Rotational and hyperfine structure in the yellow system of CuF

Previous spectroscopic studies of copper monofluoride have suggested that the excited electronic state involved in the yellow system is either ³Σ^? or a (¹Π, ¹Σ) “p-complex”. A Doppler-limited (HWHM ? 0.4 GHz), laser-induced fluorescence study of the (0, 0) band of the ⁶³Cu¹⁹F isotopic species was performed in order to make a more definite assignment of the upper state. As a result of increased resolution, many of the rotational lines appear as partially resolved quartets resulting from the ⁶³Cu ($\text{I =}\text{3}\text{2}$) nuclear hyperfine splitting. It was not possible on the evidence available to decide between the alternative assignments for the upper state. Assuming a ³Σ^? state, 200 features were assigned, measured, and fitted by a least-squares procedure to determine values for the parameters (cm^?1) of CuF in the excited state: T₀ = 17 574.9663(12), λ₀ = ? 27.7149(11), λ_D0 = 0.001161(1), B₀ = 0.375035(2), 10⁶D₀ = 0.510(1), γ₀ = 0.01074(6), $\text{b}{}_{\mathrm{F}}\text{(}{}^{63}\text{Cu}\text{) = 0.032(2)}$, $\text{c(}{}^{63}\text{Cu}\text{) = ?0.039(7)}$. The ⁶³Cu hyperfine parameters are interpreted in terms of possible electronic models of CuF in the A state.     

Thermal changes in the optical properties of SnSxSe2−x crystals

Optical measurements on crystals in the series SnS_xSe_2?x for 0 ? x ? 2, have yielded information on the changes in the ordinary refractive index $\text{Δn}\text{ΔT}$ and the energy gap $\text{ΔEg}\text{ΔT}$ in the temperature range 125–425 K. The coefficient $\text{Δn}\text{ΔT}$ has values +40 to +160 × 10^?6K^?1 and this confirms that covalent bonding predominantly exists in these materials. The coefficient $\text{ΔEg}\text{ΔT}$ remains fairly consistent for all values of x with an average value of -8.0×10^-4eV K^-1.