Penning ionization electron spectroscopy and photo-electron spectroscopy of molecular solids. II. Ammonia and water

The Penning ionization electron spectra (PIES) and ultraviolet photoelectron spectra (UPS) of ammonia and water molecules condensed on a cold metal substrate have been measured using thermal He*(2³S, 2¹S) metastable atoms and He(I) (58.4 nm) photons. The shifts of the observed positions of the PIES peaks relative to those of the UPS peaks in the condensed phase are roughly equal to the corresponding shifts in the gas phase. The relative intensities of the 3a₁ and 1e orbital peaks are reversed in the PIES and UPS for both gaseous and condensed ammonia; the origin of this reversal is interpreted as the difference between the interactions with metastable atoms and photons. On the other hand, the relative intensities of the 3a₁ orbital peak in the PIES and UPS for condensed water decrease as compared to the gas-phase spectra. This implies participation of the 3a₁ orbital of water in the hydrogen bonding.     

Aspects of crystal growth within carbon nanotubes

The comparative crystallisation and HRTEM imaging properties of simple binary halides formed by the alkali iodides MI (M = Li, K, Na, Rb and Cs) within single walled carbon nanotubes (SWNTs) are described. The most common structure type observed within SWNTs is the rocksalt archetype, although CsI was observed to form both bcc and rocksalt structure types. In SWNTs forming in the 1.2–1.6 nm diameter range, all of the incorporated halides showed preferred orientation, with the 〈100〉 growth direction predominating for rocksalt-type packing and 〈112〉 so far observed exclusively for bcc packing. Crystals with dimensions spanning 2–6 atomic layers thickness in projection invariably exhibited lattice expansions that were attributed predominantly to a net reduction in coordination at the crystal-carbon interface. The crystallisation behaviour of UCl₄–KCl and AgI–AgCl eutectic melts was compared in carbon nanotubes of different diameters and a pronounced ordering influence over the normally glassy melts was observed in narrower capillaries. HgI₂ crystallised within nanotubes with ultra-narrow (i.e., 0.8 nm) capillaries were observed to form helical 2 ×1 layer crystals. To cite this article: J. Sloan et al., C. R. Physique 4 (2003).     

Momentenanalyse derR 2-Nullphononen-Linie in NaF unter einaxialem Druck

The R₂ zero-phonon absorption line 4480 Å in NaF at 4.2 °K and 23 °K is studied under uniaxial stress along 〈100〉 and 〈100〉. According to the F₃ model of theR center the R₂ line is found to be due to a transition between a degenerate ground state (²E) and a nondegenerate excited state (²A) of this center. Instead of a splitting into single components under stress, changes in line shape are observed which are analyzed by means of the method of moments. The zeroth and first moment of the line are calculated as functions of the magnitude of stress and temperature. The theoretical moments are found to be in good agreement with those determined from the experimental data. From a comparison of the measured moments with the theoretical expressions the stress splitting parameters are obtained which describe the removal of both the orientational and the orbital degeneracy of theR center under uniaxial stress. The corresponding strain parameters of the R² line in NaF are compared with those in other alkali halides.     

Angle-resolved ultraviolet photoelectron spectroscopy of expitaxial films of vanadyl-phthalocyanine grown on alkali halides

The molecular orientation of Vanadyl-Phthalocyanine (VOPc) grown on alkali halides was studied by Angle-Resolved Ultraviolet Photoelectron Spectroscopy (ARUPS). To avoid charging up of the specimen during ARUPS measurment, thin epitaxial films of alkali halides grown on GaAs (0 0 1) were used as substrates. Photoelectrons emitted from the highest occupied molecular orbital band of VOPc showed strong angular distribution leading to the conclusion that the molecules stay almost parallel on the surface. This was in accordance with the expectation based on the reflection high-energy electron diffraction analysis of the film.     

Zur Theorie kraftfreier Magnetfelder

From the exciton spectrum of the alkali halides it can be shown that theP-function of the alkali atom seems to be necessary, if one wants to construct certain excitons by means of atomic orbital functions. A relation between the ionisation energy of the exciton and the optical dielectric constant has been obtained from the experiments. This is consistent with a hydrogenic model of the exciton, which we have used in an earlier paper.     

Measurement of the nuclear magnetic moment of110Ag(T 1/2=24.4 s)

Nuclear magnetic resonance of¹¹⁰Ag has been observed in silver halides using the polarized neutron capture,β decay anisotropy method. The magnetic moment was determined asμ _I(¹¹⁰Ag)=2.7084 (5) nm (uncorrected).     

The vapour-phase ultra-violet photoelectron spectroscopy of metal halides at elevated temperatures

Simple UV photoelectron spectrometers capable of operation with gaseous samples at temperatures up to 1000 K are described. He(I) spectra of halides of thallium, mercury, tin, lead and caesium are reported. Comparison of the spectra of the Group IVB MX₂ species with those of the TIX and HgX₂ species suggests a new assignment of the former spectra.     

Kondo effect and spin filtering in triangular artificial atoms

We studied, strongly correlated states in triangular artificial atoms. Symmetry-driven orbital degeneracy of the single particle states can give rise to an SU(4) Kondo state with entangled orbital and spin degrees of freedom, and a characteristic phase shift δ=π/4. Upon application of a Zeeman field, a purely orbital Kondo state is formed with somewhat smaller Kondo temperature and a fully polarized current through the device. The Kondo temperatures are in the measurable range. The triangular atom also provides a tool to systematically study the singlet-triplet transitions observed in recent experiments [Phys. Rev. Lett., 88 (2002) 126803, cond-mat/0208268 (2002)].     

Spin-orbit interaction in molecular photoelectron spectra An intermediate coupling approach

A new and general semiempirical method for calculating ionization energies of molecules containing heavy atoms is presented. The extended Hückel hamiltonian is amended with a phenomenological one-electron spin-orbit interaction operator, and ionization energies are equated to orbital energies according to Koopmans' theorem. Calculations are presented for molecules with Br and I as heavy atoms. The systems considered are the hydrogen halides, diatomic halogens and interhalogens, haloacetylenes, halomethanes, and boron trihalides. Good agreement with the observed spin-orbit splitting is obtained. New assignments are proposed for the trihalides considered.     

Photoelectron-spectroscopic determination of lattice self-potentials in lanthanide trifluorides

Since the photoelectron spectroscopically measured binding energies probe the potentials at the cation and anion sites separately in solids, they should be a direct determination of the lattice self-potentials at these sites through comparisons with the respective ionization potentials of the gaseous ions. In a study of the photoionization of the 1s orbital electrons on the fluoride ion and of the outer 5p or 4f orbital electrons on the cations of the lanthanide trifluorides, it is found that the so determined lattice self-potentials at the anion sites deviate from those calculated from the electrostatic interactions in the point charge model by an amount which can be explained in terms of ionicity. For the cations, however, the deviations for all except La³⁺ and perhaps Gd³⁺ exceed those expected for ionicity in the point charge model by as much as 2.3 eV in the case of the maximum at Pr³⁺. These deviations are discussed in terms of crystal field interactions, covalency, polarization, and complication caused by final state relaxation during photoionization.     

Rydberg transitions of aniline and N,N-diethylaniline

The absorption spectra of aniline and N,N-diethylaniline in the gaseous and condensed phases were measured in the 30 000–60 000 cm^?1 region. The magnetic circular dichroism (MCD) spectra of these compounds in the gas phase and in solution were also measured in the region 30 000–49 000 cm^?1. The MCD signals due to the Rydberg transitions were newly observed for N,N-diethylaniline. On the basis of these data, we demonstrated that in the region below 50 000 cm^?1 the Rydberg transition bands appear at 47 180 cm^?1 for aniline and at 36 520, 38 880, and 42 000 cm^?1 for N,N-diethylaniline. Besides these, in the region higher than 50 000 cm^?1, a number of Rydberg transition bands were observed for both compounds in the gaseous state. The observed Rydberg transition bands were analyzed with the aid of the Rydberg formula: they were assigned for aniline to the np- and nd-type series converging to the first and second ionization potentials, and for N,N-diethylaniline to the us- and two np-type series converging to the first ionization potential.     

NMR chemical shifts in cuprous salts: The magnetic moment of Cu

An extensive study has been made of chemical shifts of the ⁶³Cu resonance in a number of cuprous salts. The observed trend in chemical shifts for the cuprous halides is in disagreement with theoretical ionicity estimates of either Pauling or Phillips. This trend has been explained by taking into account the relative importance of π bonding for the different halides. On the basis of the present data, CuBr is the most suitable Knight shift reference compound. The measurements indicate μ = 2.2206 nm for ⁶³Cu and μ = 2.3791 nm for ⁶³Cu for reference moment, without diamagnetic correction.     

Normal coordinate analyses for trimethylantimony(V) dihalides: A comparison of three force fields

Approximate normal coordinate analyses have been carried out for the series (CH₃)₃SbX₂ (X = F, Cl, Br, and I) assuming a molecular symmetry of D_3h and that the methyl groups behave as single atoms. Sets of seven force constants have been evaluated for each molecule using each of the following force fields: general valence, modified Urey-Bradley, and orbital valence. The fundamental vibrational frequencies have been evaluated using these force constants and have been compared with the experimentally observed values published previously. The orbital valence force constants generally led to better agreement between calculated and observed frequencies. Trends in the force constant values are discussed briefly.     

Pressure dependence of chemical bonds in copper(I) halides

The pressure dependence of the chemical bonds in copper(I) halides is studied using the DV–Xα cluster method. The values of the bond overlap population (BOP) are discussed with varying the bond length. The different tendencies of the BOP are shown with increasing the pressure in copper(I) halides. These tendencies are discussed in connection with the p–d hybridization.     

Dispersion of the stress optic coefficient of the alkali halides

The dispersion of the stress optic coefficient C=n³/2 (q₁₁?q₁₂) of the alkali halides, NaCl, KCl, KBr and KI have been measured from the visible to the ultraviolet region. In general the value of “C” decreases with wavelength for all crystals. While the dispersion is only a few per cent in the visible region of wavelengths, it is enormous in the ultraviolet. NaCl shows a dispersion of about 100% from 5800 to 2400 Å; KCl about 200% from 5000 to 2400 Å; KBr about 300% from 5000 to 2400 Å; and KI about 400% from 5000 to 2800 Å. Also the potassium halides exhibit a change in sign of their “C” values in the ultraviolet. In KCl the sign reversal occurs at about 2550 Å; in KBr at 2760 Å and in KI at 3380 Å. Below these wavelengths, the potassium halides belong to the same class inMueller's classification as sodium chloride. The theory ofRamaseshan andSivaramakrishnan based on the assumption that a stress causes a change in the frequencies and oscillator strengths of atoms is unable to explain the observed behaviour of the alkali halides. On the other hand, the mere variation of the ionic refractivities with wavelength is also unable to explain the observed dispersion onMueller's theory. One is forced to assume that the strain polarisability constantK inMueller's theory varies with wavelength. When “K” is calculated from the experimentally observed values of “C”, it is found to increase with decreasing wavelength for all alkali halides. The variation with wavelength of “K” for all the alkali halides can be fitted up well by a formula of the type given byRamaseshan andSivaramakrishnan. Hence it appears that the total dispersion ofC can be explained only when we take into account the variation with wavelength of 1. theLorentz andCoulomb contributions fromMueller's theory and 2. the strain polarisability constant fromRamaseshan andSivaramakrishnan's theory.     

Theoretical study of the superhyperfine parameters for Cu2 + in K2PdX4 (X = Cl, Br)

The superhyperfine parameters T _j (j?=?x, y, z) for Cu^2?+? in the square-planar K₂PdX₄ (X = Cl, Br) are theoretically studied from the perturbation formulas of these parameters for an octahedral 3d⁹ cluster, by considering both the contributions from the crystal-field and charge-transfer mechanisms. The related molecular orbital coefficients are determined from the cluster approach in a uniform way. Based on one adjustable proportional factor ρ for the orbital admixture coefficients, the calculated results of present work show reasonable agreement with the observed values.     

Rydberg states with a 2Π core: The b3Πi and C1Π states of DCl

The rotational structure in the lowest Rydberg complex of hydrogen chloride, [X²Π]4sσ, was reinvestigated. The study is limited to the spectrum of D³⁵Cl, the HCl bands being too diffuse for a detailed analysis of second-order effects. The Λ-type doubling in both component states, b³Π_i and C¹Π, is small since it arises from the uncoupling of the core rather than Rydberg orbital angular momentum. It can be interpreted in terms of pure precession relations that are known to exist between the ground and first excited states of DCl⁺. By contrast, the spin-orbit interactions, also originating in the core, are strong. In addition to distorting the triplet splitting in b³Π, they lead to an avoided crossing between the nearly coinciding levels b³Π₀ (v = 1) and C¹Π₁ (v = 0). They are also responsible for anomalies in the b³Π₀ ← X¹Σ⁺R-branch intensities of DCl as well as of HBr, DBr, and HI. From the J values at the observed R-branch minima we have estimated the ratio $\text{μ}{}_{\mathrm{\parallel }}\text{μ}{}_{\mathrm{\perp }}$ of the transition moments associated with the excitation of a 3pσ or 3pπ core electron to the 4sσ Rydberg orbital of DCl and, correspondingly, of the other hydrogen halides.     

Unrestricted hartree-fock cluster analysis of F and F A centres in some alkali halides

The optical absorption energies and nearest-neighbour ground state hyperfine interaction constants are evaluated for F centres in lithium and potassium halides, and for FA(Li) centres in KC1 and KBr. For these defects, the electron-ion interaction includes self-consistent unrestricted Hartree-Fock treatment of nearest-neighbour ions, in terms of Gaussian-localized atomic orbital basis functions. The results, with the exception of FA-centre hyperfine constants, are qualitatively correct.     

An electron-paramagnetic-resonance study of the dynamic Jahn-Teller effect for Sc2+ in ZnS

Electron-paramagnetic-resonance spectra for ZnS:Sc²⁺ were observed from 1.3 K to 4.2 K. The spectra are characteristic of the dynamic Jahn-Teller effect for orbital E states.