M. Narayana

Inelastic processes in ion/surface collisions: Scattered ion fractions and VUV photon emission for Ne+ and Ar+ collisions with Mg and Y surfaces

Time‐of‐flight (TOF) scattering spectra and vacuum ultraviolet (VUV) photon emission spectra resulting from 1–10 keV Ne+ and Ar+ ions impinging on magnesium and yttrium surfaces and the corresponding oxidized and hydroxylated surfaces have been measured. Measurements of the scattered neutrals plus ions and neutrals only are used to calculate scattered ion fractions Y+ for the single scattering collisions. The Y+ values rise steeply at low ion energies E0 (∼1–2 keV), reaching values of 70% and 38% at 10 keV for Ne+/Mg and Ar+/Y, respectively, and are very sensitive to adsorbate coverage. The dominant photon emission observed from the clean metals in the VUV range 30–200 nm is 1,3P → 1S resonance radiation from the excited neutral projectile atoms; emission was also observed from excited H and O for adsorbate covered surfaces. A model is developed for electronic transitions in keV ion/surface collisions which considers Auger and resonant transitions along the ion trajectory and electron promotions in the qu...

Analysis of nuclear quadrupole interaction effects in electron spin‐echo modulation spectra by second‐order perturbation methods

The incorporation of nuclear quadrupole interaction into electron spin‐echo modulation spectra by second‐order perturbation methods has been developed for both two‐pulse and three‐pulse echo sequences. The results have been compared with exact numerical diagonalization of the Hamiltonian and with a first‐order perturbation treatment. Model calculations have been carried out for deuterium (I=1) and aluminum (I=5/2) nuclei. It is shown that the second‐order expressions can be used to obtain relatively accurate values for the number and distance of interacting nuclei at electron–nuclear distances greater than 0.26 nm. The procedure is more limited when the quadrupolar interaction exceeds the dipolar interaction when neither can be neglected.

X-ray photoelectron and electron spin resonance spectroscopic studies of CuNaY zeolites

Abstract X-Ray photoelectron spectroscopic (XPS) studies of hydrated, dehydrated and, reduced Cu NaY zeolites were carried out. With sufficient analyzer resolution, both the Cu 2 p 1 2 and 2 p 3 2 transitions were resolved into two components in hydrated CuNaY. The higher-binding-energy component and the shake-up satellites decreased in intensity with dehydration. By correlation with electron spin resonance (ESR) and X-ray diffraction results the higher-binding-energy component is assigned to octahedrally coordinated Cu 2+ in the large α-cages of the zeolite and the lower-binding-energy component to tetrahedrally coordinated Cu 2+ in the small β-cages of the zeolite. On hydrogen reduction of dehydrated CuNaY the XPS transitions shift by 1 eV to lower binding energies which correlated with an order of magnitude decrease in the Cu 2+ ESR intensity. These shifted transitions are assigned to Cu + or possibly to Cu 0 .

Proton environment of paramagnetic centres in natural and solvent refined bituminous coal

Electron spin resonance measurements at 35 GHz have revealed two spectral components in natural bituminous coal. Comparison with results at 9 GHz together with lineshape analyses at both frequencies identifies one e.s.r. component due to exchange narrowing. Electron spin echo measurements of natural and solvent refined coal show proton modulation with the modulation being much deeper in the solvent refined coal. For the solvent refined coal 16 protons at 0.5 nm are indicated. These results clearly indicate more hydrogen within 0.6 nm of the paramagnetic centres in solvent refined versus natural coal.

Determination of the coordination geometry of supported vanadium catalysts by electron spin echo modulation analysis

Abstract Electron spin echo modulation analysis was used to characterize the coordination geometry of the paramagnetic vanadium species formed in the 0.25% V 2 O 5 in SiO 2 with D 2 O adsorbate. The deuterium modulation indicates one deuteron at 0.23 nm and four deuterons at 0.26 nm. The four farther deuterons are interpreted as due to two-coordinated water molecules.

Detection of a new trigonal bipyramidal copper species in Cu–CaX zeolite by electron spin resonance and electron spin echo modulation analysis

Electron spin resonance and electron spin echo modulation analysis are used to identify the location and coordination of Cu2+ in hydrated and dehydrated CaX zeolite. A new, trigonal bipyramidally coordinated Cu2+ is found to be the dominant copper center in this zeolite. It is located in a six ring window (site SII) and coordinated to three six ring oxygens and two hydroxyls, one in the α cage and the other in the β cage of the zeolite. This Cu(Oz)3(OH)2 species is found to be stable for dehydration up to 180 °C. At higher temperatures the Cu2+ species seems to be reduced irreversibly. However, a Cu2+ species can be regenerated by oxidation and hydration but most of it is only trigonally coordinated.

Analysis of nuclear quadrupole interaction in three pulse electron spin echo modulation spectra

The introduction of nuclear quadrupole interaction in electron spin echo modulation by perturbation methods as proposed by Shubin and Dikanov [J. Magn. Reson. 52, 1 (1983)] is extended to the analysis of three pulse electron spin echo envelope modulation spectra. It is shown that an additional frequency difference occurs apart from the amplitude differences discussed by Shubin and Dikanov, because of an oversimplification of the expressions for the ENDOR levels. The effects of such inclusion of quadrupole interaction on the individual components contributing to the final modulation but with different time dependences are critically examined along with the effect of possible nonalignment of the quadrupole and dipolar tensors. The validity of the expressions obtained by the perturbation approach seems to be limited to relatively long electron‐nuclear distances (r>0.35 nm for nuclei with I=1 and greater for nuclei with I>1). An independent determination of the magnitude of the quadrupole interaction may be n...

Nuclear quadrupole effects on two and three pulse electron spin echo patterns: A first order perturbation approach for nuclear quadrupole interaction only

The nuclear modulation of electron spin echo signals is evaluated by an approach which handles only the nuclear quadrupole interaction as a perturbation of the eigenvectors and eigenvalues of the nuclear Hamiltonian including nuclear Zeeman and hyperfine terms. The theory is shown in detail for a first order perturbation treatment for both two and three pulse methods. Final formulas are obtained which are valid for any value of the electron–nuclear distance r, thus removing the limit of previous approaches. The modulation envelopes for I=1(2D) and I=5/2 (27A1) nuclear spin systems are critically analyzed at different electron–nuclear distances including possible misalignment between quadrupole and dipolar hyperfine tensors. Comparison with a first order perturbation approach for both hyperfine and nuclear quadrupole interactions as perturbations of the nuclear Zeeman Hamiltonian is made and shows that the exact treatment of the hyperfine interaction considerably reduces the apparent nuclear quadrupole eff...

Location and movement on dehydration of Cu2+ in Cs7Na5A zeolite from electron spin echo modulation analysis

It is shown that paramagnetic ions like Cu2+ can be located and that their motion upon dehydration in zeolites can be monitored by electron spin echo modulation analysis. In Cs7Na5A zeolite, Cu2+ is located inside the β cage about 0.1 nm below a hexagonal window between the β and α cages. The Cu2+ is coordinated to three lattice oxygens and one water molecule. On dehydration Cu2+ moves out of the β cage along the threefold axis of the hexagonal window to about 0.1 nm above the hexagonal window into the α cage.

Selective study of paramagnetic centres in coals by e.s.r. and e.s.e. spectrometry

Electron spin resonance (e.s.r.) and electron spin echo (e.s.e.) studies of two natural and heat-treated bituminous coals are presented. In all samples, two main paramagnetic centres have been detected and studied selectively by obseving e.s.e. decays at different parts of the e.s.r. signal. These centres interact differently with neighbouring protons and the interaction is reduced in both centres by heat-treatment. The e.s.r. measurements reflect the heterogeneity of the region around the radical and are sensitive to the presence of proton hyperfine and dipolar broadening interactions. E.s.e. decays are sensitive to the presence of signals with longer spin memory time TM. The TM values obtained for all samples can be rationalized by assuming an exchange frequency in the range 1 × 108 − 1 × 109s−1.