Study of the phase transitions in ammonium bromide by Raman spectroscopy

A complete Raman scattering study of an oriented NH₄Br single crystal has been carried out and new results are reported for temperature between 45 to 410°K. The study includes measurements and interpretations of the Raman spectral bandwidths, intensities and frequencies of the lattice and internal modes as a function of temperature. Because of the ability of generating a single domain crystal, unambiguous assignments of the Raman active modes are made. From the present data together with those presented previously significant information on the behavior of the ordering and crystal structure associated with the phase transitions in NH₄Br is obtained.     

UV Subshell photoionisation cross-section of atomic Zn,Cd, and Hg: Experiment and theory

Partial photoionisation cross-sections have been measured at photon wavelengths of 584, 304, 256 and 243 Å for the valence d and s shells of zinc, cadmium and mercury atoms. These cross-sections have also been calculated using the GIPM, a simple one-electron method which obtains the final-state potential by an inversion of the Schrödinger equation for the initial orbital of the photoelectron in the Hartree-Fock wave function. Comparison shows good qualitative agreement between theory and experiment. The quantitative agreement is typically within a factor of 2. Angular asymmetry parameters have also been calculated and the branching ratio for the splitting of the d-subshell cross-section into ²D_{$\text{5}\text{2}$} and ²D_{$\text{3}\text{2}$} contributions has been measured.     

Heavy-atom effect on radiative and radiationless transitions in charge-transfer complexes

The quantum yields of fluorescence and phosphorescence and decay times of fluorescence were measured for a series of charge-transfer complexes of hexamethylbenzene (an electron donor) with various electron acceptors containing heavy (halogen) atoms and the rate constants of different radiative and radiationless transitions were determined. It was found that radiative T₁ → S₀ transition, i.e. the phosphorescence, is strongly enhanced by the heavy-atom effect. The mechanism of the heavy-atom enhancement of charge-transfer phosphorescence is discussed and it is concluded that increasing spin- orbit mixing of radiative T₁ → S₀ charge-transfer transition with singlet-singlet, locally excited transition of acceptor molecule is responsible for the observed effects.     

Gas phase and solid state X-ray photoelectron spectra of the substituted acetylenes (SF5)n C2 (CF3)2−n (n = 0, 1, 2): A comparison of the pentafluorosulfur and trifluoromethyl groups

The core level X-ray photoelectron spectra (XPS) of CF₃CCCF₃, CF₃CCSF₅ and SF₅CCSF₅ have been measured in the solid state. Gas phase spectra of CF₃CCCF₃ and CF₃CCSF₅ have also been obtained. The XPS data, interpreted with the point charge potential model and semiempirical MNDO (minimum neglect of differential overlap) molecular orbital calculations, indicate that the electron withdrawing effect of the ?CF₃ group is greater than that of the ?SF₅ group. Results further suggest that sulfur 3d orbitals do not play a detectable role in the bonding or charge distribution in these molecules. Carbon 1s linewidths of ?CF₃ carbon atoms are found to be much narrower than those arising from the acetylenic carbon atoms. The narrower lines correlate with the much higher binding energy of the ?CF₃ carbon atoms. Large shifts (nearly 1 eV) in heteroatom core level binding energy differences (for example, F 1s — C 1s) between the gas phase and solid state data are observed. These shifts are attributed to solid state effects (Madelung potential, intermolecular bonding interactions, and/or extramolecular relaxation contributions). From these comparisons it is clear that solid state effects are not uniform in their influence on the photoionized sites in these molecules.     

A speckle-shearing interferometer: A tool for measuring derivatives of surface displacements

A speckle-shearing interferometer is developed using a sheared Michelson interferometer. The tool, like speckle interferometry, utilizes the speckle effect of coherent light. However, while speckle interferometry measures surface displacements, the speckle-shearing interferometer determines the derivatives of the surface displacements. Hence, it eliminates the necessity of differentiating the measured displacements to obtain strain.     

Vibrational study of H3OUO2PO4·3 H2O (HUP) and related compounds. Phase transitions and conductivity mechanism: Part II,H3OUO2PO4·3 H2O

Infrared and Raman spectra of polycrystalline H₃OUO₂PO₄·3 H₂O (HUP) and its D and P¹⁸O₄ derivatives, in the form of dense transparent disks and wet powder, have been investigated at various temperatures in the 100–300 K region. The bands due to framework vibrations are similar to those of KUP, whereas those for the protonic species are different. OH stretching and bending bands of the oxonium ion have been identified at 2920, 1740 and 1160 cm^?1 in the low-temperature spectrum of HUP. Differential scanning calorimetry (DSC) and infrared (IR) intensity investigations show a phase transition between 274 and 260 K. The mechanism of the phase transition consists, as in the case of KUP, of ordering of the protonic species, which induces ordering of PO₄ tetrahedra. The ordering can be influenced by excess water content, stacking faults and stress (ferroelastic behaviour is evidenced). The conductivity mechanism in HUP is discussed.     

A mirror-free cavity configuration for a CW ring dye laser

A new cavity configuration without mirrors is described for a CW ring dye laser. The cavity design is based on a new type of focusing prism which uses the Brewster angle of incidence and total internal reflection. Special roof prisms allow wavelength tuning. All of these elements are made from uncoated quartz thus allowing dye laser operation over a large range of wavelengths. CW operation as well as synchronously pumped mode locked operation with intracavity doubling is reported using the Rhodamine 6G dye.     

A new electron spectrometer design:II

A novel charged particle energy analyzer of simple geometry is described. Expressions for the potential distribution and electric field components defined by the geometry are given. A raytrace program using these components is discussed and results of its application are presented in graphical and tabular form. Also presented are experimental results for two versions of the analyzer in the form of representative spectra of well known species (Ar⁺, H₂⁺, O₂⁺). Finally, a comparison of the new instrument with the well known cylindrical mirror analyzer is outlined which suggests the superiority of the new instrument under the specific conditions given.     

A modified plane wave model for calculating UV photo-ionization cross-sections

Photoionization cross-sections are calculated for a number of molecules, using a plane wave method. Agreement with experimental data is considerably improved with respect to common plane wave results if the energy of the photoelectron is assumed to equal the incident photon energy.     

Transferability of dipole moment derivatives: A general method for the calculation of absolute rotational contributions

A general method for the calculation of absolute rotational corrections for the dipole moment derivatives is given based on the definition of pure geometrical distortions. The method is generally valid and allows the rotational contributions to be calculated for all modes of any molecule. The relation between absolute and relative corrections is discussed and the method is demonstrated by calculating the rotational corrections for a few asymmetrical molecules.     

A pulsed NMR study of hydrogen diffusion in hydrides of group IVa transition metals

Activation energies E_A for hydrogen diffusion in hydrides of Group IVa transition metals have been determined by ¹H nuclear magnetic resonance measurements of spin lattice relaxation in both the laboratory (T₁) and rotating (T_1p) frames. For the HfHx system both activation energies obtained from T₁ data, and the value of T₁ at the minimum appear to be insensitive to hydrogen content x in the range 1.58 ? x ? 1.98. For hydrides of titanium and zirconium of approximately stoichiometric composition MH₂ (where M = metal), there is excellent agreement between activation energies obtained from T₁ and T_1p data. Mean activation energies obtained were 0.51 eV for TiH_1.98 and 0.83 eV for ZrH_1.96, consistent with a single diffusion mechanism in each case over the temperature range 260–600K and 400–800K respectively. In the case of HfH_1.98 the agreement was less good, values of 0.64 and 0.55 eV being obtained from T₁ and T_1p respectively.     

Surface relaxation and thermal expansion for the (001) face of α-Fe and Cu

The surface relaxation and the near-surface enhancement of thermal expansion have been calculated for the (001) face of a bcc crystal, α-Fe, and an fcc crystal, Cu. The calculations make use of the anharmonic perturbation formalism of Dobrzynski and Maradudin; the results for certain equal-time vibrational correlation functions which arise in this formalism are also presented. The crystal potential is described in terms of several kinds of short-range empirical interatomic potentials, such as have been used in studies of defects in bulk; in the near-surface region, the effects of surface redistribution of the electron distribution are modelled by the addition of a simple surface Madelung (SSM) force. The effect of the SSM force is to limit severely the usual outward relaxation driven by short-range interatomic potentials. For Fe(001), the five and one-half percent outward static relaxation driven by the short-range potentials acting alone is changed to a one percent inward static relaxation when the SSM force is incorporated; for Cu(001), the comparable change is from a one percent outward relaxation to a one-half percent outward relaxation. On the other hand, the SSM force makes only a small effect on the surface-enhanced thermal expansion coefficients (STEC) for interplanar spacings. The STEC for the outermost spacing is between 2.5. and 3.0 times of that for the bulk at the Debye temperature for both Fe(001) and Cu(001); for the second interplanar spacing, the STEC is smaller than 1.5 times of that of the bulk at the Debye temperature. The ratios of the near-surface mean-square amplitudes (MSA) to those of the bulk at high temperatures are, for Fe(001), about 1.75 for z-components (normal to surface) and 1.55 for x-components (parallel to surface) in the surface layer; for Cu(001), about 1.95 for z-components and 1.30 for x-components. The interplanar correlation functions, while smaller than the MSA on an absolute scale, do show considerable surface-enhancement, particularly for the zz-compoments. For example, the zz-correlation between an atom in the outermost layer and its nearest neighbor in the next layer is nearly twice the comparable bulk correlation above the Debye temperature for both Fe(001) and Cu(001).     

Vibrational study of H3OUO2PO4 · 3 H2O (HUP) and related compounds. phase transitions and conductivity mechanisms: part I,KUO2PO4 · 3 H2O (KUP)

Infrared and Raman spectra of polycrystalline KUO₂PO₄ · 3 H₂O (KUP) and its isotopic derivatives KUO₂P¹⁸O₄ · 3 H₂O and KUO₂PO₄ · 3 D₂O have been investigated in the 4000-10-cm^?1 range at different temperatures. An assignment of the bands in terms of UO₂, PO₄ and H₂O vibrations has been proposed. Combined differential scanning calorimetry and spectroscopic data show two diffuse phase transitions near 130 and 230 K. Comparison of the vibrational spectra of phase I at 300 K and phase IV at 100 K indicates that ordering of the water molecules with subsequent ordering of PO₄ tetrahedra on a site with lower symmetry appears to be the main mechanism responsible for the phase transformation. All the six O-H distances of water molecules in phase IV are found to be crystallographically nonequivalent. Conducting ion frequencies and the corresponding force constants have been determined for the analogous compounds MUP with M = K⁺, Na⁺, Ag⁺, NH⁺₄, Tl⁺ and H₃O⁺ and compared with other properties of these ionic conductors. Conductivity mechanisms in these materials are discussed.     

Wetting transitions at triple points; A study based upon the analysis of stepwise adsorption isotherms

This adsorption isotherm study finds that ethylene adsorption on PbI₂ is limited to only a few layers at T<T_3t (the triple point temperature of bulk ethylene) but that almost unlimited adsorption can take place as T_3t, is approached. This behaviour is qualitatively identical to the behaviour of the ethylene/graphite system previously reported. In both cases wetting by ethylene is incomplete below T_3t since the adlayers formed have an orientational order which is in compatible with that which is found in the bulk solid ethylene. However, C₂H₄ shows a strong tendency to wetting somewhat below its bulk triple point temperature because it can form a liquid or at least an orientationally disordered solid film of limited thickness. The similarities and differences between these two systems are discussed. In addition, we discuss in general systems for which wetting is temperature dependent. The first part of this paper is devoted to outlining in detail how stepwise adsorption isotherms on powdered samples can be used to give information about wetting of the exposed basal face. The merits and limitations of this method are discussed in the context of the ethylene/PbI₂ example.     

Microwave spectral study of the axial and equatorial conformers of cyanocyclopentane

The microwave spectrum of cyanocyclopentane has been observed and analyzed in the 26.5 to 40.0-GHz frequency region. Rotational transitions were assigned for both an axial and an equatorial conformer. These data and those for the molecule deuterated in the 1-position (1-d-cyanocyclopentane) show that both conformers adopt the C_s envelope structure. For the normal isotopic species the rotational constants have the following values, in megahertz: axial, A = 4297.196, B = 2210.245, C = 2057.205; equatorial, A = 6324.905, B = 1790.937, C = 1497.792. Relative intensity measurements indicate that the ground-state energy difference of the two conformers is 0 ± 200 cal/mole. The low-frequency (pseudorotational) potential surface has been discussed and compared with that of chlorocyclopentane.     

Collisional redistribution of radiatively-excited levels of Tl and Ga atoms in an O2-acetylene-Ar flame

Several useful expressions for the population ratios of collisionally excited levels have been derived and different transitions of Tl and Ga atoms, seeded in a premixed atmospheric flame, have been excited in order to verify that the levels higher than the laser-excited level are populated approximately according to a Boltzmann equilibrium. This result confirms the possibility of using thermally assisted flourescence lines to measure local temperatures in flames. The Einstein spontaneous emission probability for the transition 6S_1/2→4P_3/2 at 2719.65Å of Ga has also been calculated (A_2719.65=0.41×10⁸s^-1) to fit the experimental point.     

Surface relaxation and reconstruction in diamond-like crystals

The MNDO semiempirical method has been used to study the structure of the (111) surface of diamond-like crystals employing two model clusters. Numerical results are in agreement with previous computations for the small X₄H₉ clusters and become very similar to experimental data when employing the larger X₉H₁₅ clusters. These latter clusters allow also the study of surface reconstruction. Our results suggest that the Haneman model of 2 × 1 reconstruction could be equally valid for silicon and diamond, but in the latter case the difference in vertical shifts is too small to be experimentally observed.     

Photoelectron spectra of some aromatic mono- and di-ketones

The photoelectron spectra of benzophenone, 4-OH benzophenone, benzil, 4-OH benzil, dibenzoylmethane, 2,2-dibenzoylpropane, 1,2-dibenzoylethane and 2-(p-methoxy-benzoyl)-2-benzoylpropane have been determined and ionization potentials lying below 15 eV have been assigned. These assignments lead to the result that for compounds containing substituents on the phenyl ring, an aromatic π orbital is the highest occupied. For the enolizable ketone, di-benzoylmethane, an enol π is the HOMO, whereas for all other compounds investigated an n orbital is the highest occupied.     

The electronic structure,x-ray photoelectron and emission spectra of YOF

The results of electronic structure calculations for YOF are presented, making use of Xα-DVM together with experimental X-ray photoelectron and emission spectra for this compound. Three versions of calculations have been performed, depending on the relation between oxygen and fluorine charges in the [YO₄F₄] cluster. The interpretation of experimental spectra is carried out on the basis of these calculations.     

The UPS of some compounds containing the heteroatoms phosphorus,nitrogen, and oxygen

The “lone-pair” regions of the He(I) UPS of 25 compounds containing phosphorus, nitrogen, and oxygen as heteroatoms are presented and discussed. The semiempirical MNDO SCF MO method has been employed to compute the orbital energies of many of the molecules. Although this theoretical treatment seems to predict correct geometries and conformations for the molecules, it does not always reproduce the most logical orbital-ordering sequence. In particular, the phosphorus “lone-pair” orbitals tend to be computed too stable. The controversial interpretation of the UPS of tris-dimethylaminophosphine is reviewed.