Showing papers by "Emil Roduner published in 2000"

Mass and temperature effects on the hyperfine coupling of atomic hydrogen isotopes in cages

Abstract: The hyperfine coupling constant A of a hydrogen isotope confined in a cage differs from its vacuum value, demonstrating that the atom acts as a probe of its environment. Room-temperature values of A for muonium in different cube-shaped Si8O12 units (T8 units or octasilsesquioxanes), in H2O, and in D2O were determined with an accuracy of about 1 MHz. The results are compared with those obtained from high-resolution ESR on H in H2O, and D in D2O, and in different silsesquioxanes in the temperature range 40–300 K. Both the strong isotope effect and the temperature dependence are well described by a single-oscillator model of Roduner et al. (J. Chem. Phys. 102 (1995) 5989). Striking differences seen between the different silsesquioxanes and between the porous Optipur and bulk Suprasil reflect varying spatial constraints and electronic interactions.

The addition reaction of X to O2 (X = Mu, H, D): isotope effects in intra- and intermolecular energy transfer

Abstract: Rate constants are calculated for the addition reaction of the light hydrogen isotope muonium (Mu) to oxygen and compared with the analogous additions of H and D and with experimental results. The origin and magnitude of kinetic isotope effects is discussed. Both the pressure dependence and the temperature dependence in different pressure regimes are analysed with the goal to check the applicability of statistical reaction theories to these systems. Two different theories are applied. One is based on approximations as introduced by Troe (J.Chem.Phys., 1977, 66, 4758; J.Phys.Chem., 1979, 83, 114; and J.Phys.Chem., 1981, 75, 226), while in the second one, the master equation is solved numerically. The microcanonical rate constants going into the master equation are calculated using RRKM theory, and for the activation/deactivation as a consequence of collisions with moderator (N2), an exponential energy transfer mechanism is assumed. Comparison with low pressure experiments leads to the conclusion that collisions of the moderator with highly excited MuO2* molecules are much less efficient than those with HO2*, as far as activation/deactivation is concerned. This result may be explained by the larger vibrational frequencies of MuO2*. The linear dependence with the moderator concentration of the Mu rate constants up to 300 bar, as observed in the experiment, could partly be simulated, but some discrepancy remains. In this context, the possibility of non-RRKM behaviour is discussed. Finally, the major influence of tunneling, especially for the light Mu atom, is analysed. It is suggested that the effect of tunneling near the low pressure limit may exceed significantly its high pressure limiting value.

Reorientational dynamics of aza-cyclohexadienyl radicals in pyridinium tetrafluoroborate

Abstract: Pyridinium tetra#uoroborate is a ferroelectric with a paraelectric}ferroelectric phase transition of second order, which is exceptional for multidirectional ferroelectrics. Avoided-level-crossing muon-spin resonance has been used to show that the Mu substituted aza-cyclohexadienyl radical derived from the pyridinium ions undergoes fast uniaxial rotation around the axis perpendicular to the molecular plane, con"rming earlier results by Czarnecki et al. The changes in reorientational dynamics between 239 and 244 K also verify the existence of a phase transition in this range, as found by Czarnecki. Furthermore, the results give strong evidence that superimposed on the rotation there is a wobbling motion of the rotational axis which freezes in at the "rst transition temperature, „ 1 "238.7 K. ( 2000 Elsevier Science B.V. All rights reserved.

Hfcs of the C6H6Mu radical in NaY zeolites

Abstract: The hyperfine interactions of the MuC6H6 radical in NaY zeolites at low to moderate benzene loadings have been measured by both the FT-μSR and ALC-μSR techniques over a wide temperature range. From a preliminary interpretation of the data, the hyperfine coupling constants for the muon and proton of the – CHMu methylene group in two different orientations for the radical bound to the SII cation site have been determined. At 322 K these values are: A μ (1)=606±2 MHz and A p (1)=108±2 MHz , for the muon on the opposite side of the ring from the cation; and A μ (2)=430±2 MHz and A p (2)=70±5 MHz , for the muon on the same side. These results as well as their trends with temperature demonstrate that the MuC6H6 radical adopts a non-planar equilibrium geometry due to the strong interaction of the π electron density with the Na cation.

A novel set-up for fast muon-spin-rotation experiments

Abstract: A novel set-up for fast muon-spin–rotation experiments with an effective time resolution of (150±10) ps (full-width at half-maximum) of the ‘prompt peak’ is described. A new method of extending the histogram length of the hitherto existing data-acquisition system allows the use of the full time range of the clock with a histogram length of approximately 5 μs and pile-up and second-muon gates with the same length. The whole electronics consists of only four NIM modules and an EG & G Ortec ‘Picosecond Time Analyzer’ mod. 9308 connected to a personal computer. The performance of the novel spectrometer is demonstrated in measurements of the hyperfine precession frequency of muonium in fused quartz, liquid water, and different silsesquioxanes at room temperature.

Production of pulsed ultra slow muons and first μSR experiments on thin metallic and magnetic films

Abstract: At ISIS, RAL (UK) we have produced a pulsed ultra-slow muon beam (E≲20 eV ) and performed the first μSR experiments. Thanks to the pulsed feature, the implantation time is automatically determined and, by adjusting the final muon energy between ∼8 keV and 20 eV, depth slicing experiments are possible down to monolayers distances. We report slicing experiments across a 20 nm copper film on quartz substrate with evidence for a 2 nm copper oxide surface layer. A preliminary experiment on a hexagonal cobalt film suggests the existence of muon precession in the local magnetic field.

Replacing muons by protons: polarized beam NMR

Abstract: Numerous applications of μSR use the muon as a light substitute for the proton. One key reason why μSR is used in many cases rather than NMR on ‘real’ protons is the huge difference in sensitivity. Conventional NMR uses the tiny nuclear polarization which is due to the Boltzmann distribution and typically of the order of 10−5. About five orders of magnitude in sensitivity can thus be gained if protons (or other nuclei) from highly polarized beams are used. Such beams have already been used for decades in nuclear physics research. The high polarization achieved renders the free induction decay signal of a few μAs of protons detectable. We demonstrated this with polarized protons implanted into D2O, liquid Br2, and crystalline Si.

Production of pulsed ultraslow muons and first μSR experiments on thin metallic and magnetic films

Abstract: Standard muon spin rotation (μSR) spectroscopy implants 4 MeV spin-polarized positive muons to investigate the bulk properties of matter. Success in producing epithermal muons opens interesting possibilities for studying ultrathin films, interfaces, and even surfaces. At the ISIS Facility, Rutherford Appleton Laboratory (Chilton, UK), we have produced a pulsed ultraslow muon beam (E< 20 eV) and have performed the first μSR experiments. Due to the pulsed feature, the implantation time is automatically determined and, by adjusting the final muon energy between about 8 and 20 eV, depth slicing experiments are possible down to monolayers distances. We describe slicing experiments across a 20 nm copper film on quartz substrate with evidence for a 2 nm copper oxide surface layer. A preliminary experiment on a hexagonal cobalt film suggests the existence of muon precession in the local magnetic field. The results are discussed in relation to the morphological features of the film.