Experimental Linewidth Research Articles

Linewidth of the Radiation Emitted by a Josephson Junction

A quasiparticle-current-fluctuation contribution to the radiation emitted by Josephson tunnel junctions is calculated. The result is combined with Stephen's result for the pair-current-fluctuation contribution and compared with experimental linewidth data for Sn-I-Sn and Pb-I-Pb junctions.

Determination of the mean life of some excited nuclear states by Moessbauer experiments

Mean lives of several excited nuclear states have been determined using the Moessbauer effect. The mean life is deduced by interpolating the experimental line width to zero effective thickness of the resonance absorber. An improved expression is used for the interpolation. The results are: τ(140 keV level of99Tc)=277(14)ps τ(136 keV level of181Ta)=55 (3)ps τ(129 keV level of191Ir)=129 (2)ps τ(139 keV level of193Ir)=115 (3)ps τ(77 keV level of197Au)=2.73 (2)ns.

Errors in the Measurement of Electron-Transfer Rates by Means of Electron Spin Resonance

Rates of electron-transfer reactions can be conveniently determined from linewidths of electron spin resonance spectra in the limit of fast exchange by using an equation derived by Piette and Anderson (PA). Here the range of validity of this equation is investigated by a comparison of its predictions with exact linewidths obtained by numerical calculations. A correction term, depending on the second and fourth moments of the uncollapsed spectrum, is also derived which extends the range of validity of the PA fast-exchange equation. The ratio of the experimental linewidth to the square root of the second moment is a good indicator of the fast-exchange limit. This ratio must be less than 0.2 for the uncorrected fast-exchange equation to hold within 5%.

An improved source for 161Dy Mössbauer experiments

A simple and convenient source, Gd 0.5Dy 0.5F 3, for 161Dy Mössbauer effect measurement, is described. Experimental linewidths of 0.4 cm/sec (fwhm) have been obtained, far better than those previously reported.

Electron Magnetic Resonance Spectrum and Magnetic Susceptibility of K4MnCl6 from 1.4° to 300°K

The electron magnetic resonance spectrum and magnetic susceptibility of polycrystalline K4MnCl6 have been investigated over the temperature range from 1.4° to 300°K. The susceptibility follows a Curie–Weiss behavior above 95°K with a Weiss constant θ of −7.8°K. Between 13° and 80°K, a Curie–Weiss behavior is still observed but with a θ value of −6.0°K, different than that observed in the higher temperature range. Coinciding with the susceptibility change is a resonance linewidth change from 23 G above 90°K to approximately 175 G below 70°K. The origin of these changes is not known. No evidence for a transition to a magnetically ordered state has been observed, even down to 1.4°K. Comparison of experimental and theoretical resonance linewidths suggest that the dominant superexchange interaction involves next-nearest-neighbor Mn2+ ions. This interaction is conveyed by two intervening Cl− ions. The exchange integral for the next-nearest-neighbor interaction is estimated to be 0.22°K. Results suggest that there is approximately a 0.8% probability for transfer of an electron from a Cl− ion to a Mn2+ ion.

Microwave Spectral Linewidth of OCS Molecule Due to Molecular Collisions

A double-modulation microwave spectrograph has been used to determine the width of j 1→2 line of OCS molecule at different pressures due to collisions with the following perturbers: carbonyl sulfide, argon, oxygen, air, methyl bromide, and water vapor. Experimental linewidth parameters for OCS–OCS, OCS–Ar, OCS–O2, and OCS–N2 molecular collisions are interpreted using Anderson—Tsao—Curnutte theory for the mixed-interactions case. The molecular quadrupole moments of OCS, O2, and N2 are found to be 2.0±2.0, 2.0±0.2, and 5.0±0.5 D·Å, respectively.

Wide-line NMR study of silicon-nitrogen compounds

(Me 3Si) 2NSi(Me) N Cl 3− N ( N = 0, 1, 2, 2, 3), (Me 3Si) 2NH, [(Me) 2ClSi] 2NH, and [(Me) 2ClSiNSiMe 2] 2 were studied by wide line NMR over the temperature range 77–300°K. The experimental line widths and second moments were calculated from the NMR absorption spectra. Theoretical line shapes and second moments were calculated for various motions to determine the actual inter-molecular and intra-molecular motions present. In particular, rotation of Me N Cl 3− N Si ( N = 1, 2, 3) groups about the SiN bond was studied. Powder X-rays were taken for the five crystalline compounds over the same temperature range to study possible phase changes. Activation energies were calculated for the various motions found to be present, and an attempt was made to correlate the barriers to rotation of the Me N Cl 3− N Si- groups about their SiN bond with changes in the symmetry of dπ- pπ bonding about the SiN bond. However, it was not possible to show any definite correlation and the differences in the barriers were best explained by differences in the intermolecular barriers.

Mössbauer effect in 157Gd

The Mossbauer effect of the 64 keV γ ray in 157Gd has been observed, using Gd2O3 as source as well as absorber material at 4.2°K. The hyperfine structure splitting is large compared to the experimental line width.

Parity ofBe11

Two measurements have been made which are relevant to the parity of ${\mathrm{Be}}^{11}$. In one experiment the gamma-ray spectrum from a mixed source of 13.6-sec ${\mathrm{Be}}^{11}$ and 7.4-sec ${\mathrm{N}}^{16}$ activities was examined with a 3-mm-thick lithium-drifted germanium detector having an experimental linewidth of 13 keV for 6-MeV gamma rays. Based on a calibration from lines due to the 6.132- and 7.116-MeV gamma rays in the decay of ${\mathrm{N}}^{16}$, the 6.8-MeV gamma ray in the decay of ${\mathrm{Be}}^{11}$ has an energy of 6.792\ifmmode\pm\else\textpm\fi{}0.006 MeV. This is in agreement with the excitation energy of the ${\mathrm{\textonehalf{}}}^{+}$ or ${\frac{3}{2}}^{+}$ upper member of the 6.752-6.804-MeV doublet in ${\mathrm{B}}^{11}$. In the other experiment a magnetic pair spectrometer measurement was made on the ground-state and first-excited-state transitions from the ${\mathrm{B}}^{11}$ 7.99-MeV level as excited in the ${\mathrm{Be}}^{9}({\mathrm{He}}^{3}, p){\mathrm{B}}^{11}$ reaction. It was shown that these transitions are $E1$, thereby requiring even parity for the 7.99-MeV state, and that the spin of the state is $\frac{3}{2}$. Since the beta-ray transitions of ${\mathrm{Be}}^{11}$ to the 6.804- and 7.99-MeV levels of ${\mathrm{B}}^{11}$ are both known to have allowed $log\mathrm{ft}$ values, the parity of ${\mathrm{Be}}^{11}$ is even.

Anisotropic Rotational Diffusion and Electron Spin Resonance Linewidths

The recent general theory of ESR linewidths for dilute solutions of free radicals developed by Freed and Fraenkel is extended to cover the possibility that the rotational motions of the free radicals are anisotropic. The analysis is greatly simplified by expressing the solution of the rotational-diffusion equation as an expansion in Wigner rotation matrices. Only the spectral-density functions in the linewidth expressions given by Freed and Fraenkel are changed by the inclusion of anisotropic rotational diffusion. These spectral-density functions are found to depend upon five different relaxation times which are expressible in terms of the three principal values of the diffusion tensor ${ \germ{ R \germ} $}. It is shown how an analysis of the ESR linewidth effects, which vary from one hyperfine component to another, may be used in conjunction with calculations of the anisotropic electron—nuclear dipolar interactions to estimate the principal values of ${ \germ{ R \germ} $}. The experimental linewidth study on the para-dinitrobenzene anion radical is reanalyzed, and it is shown that an anisotropic rotational-diffusion mechanism cannot by itself explain the anomalous absence of any significant quadratic linewidth dependence on the total Z component of spin of the ring protons. While an order of magnitude estimate of ${ \germ{ R \germ} $} was made, further information permitting a quantitative understanding of the anomalous result (presumably due to fluctuations in isotropic hyperfine interactions) would be necessary to obtain a meaningful estimate of the principal values of ${ \germ{ R \germ} $} for this radical. The physical significance of ${ \germ{ R \germ} $} in the approximation of a Brownian particle as well as in terms of intermolecular potentials is discussed.

A PRECISION IRON-FREE π√2 BETA-RAY SPECTROMETER OF 35-CM OPTIC CIRCLE RADIUS

An air-core high-resolving-power, high-luminosity double-focusing spectrometer of the high-aperture type is described. Focusing up to the fourth order in the axial departure angle is achieved with the use of three pairs of magnet coils. A compact coil structure was devised which enables electrons of up to 2.5 Mev in energy to be focused without undue rise in conductor temperature, thus preserving the feature of high precision in the measurement of relative electron momenta. For point sources the instrument is designed to achieve resolutions of 0.01%, 0.1%, and 1.0% in momentum, at transmission values of 0.08%, 0.38%, and 1.2% respectively. The use of a curved exit slit enhances the luminosity of the instrument by permitting the use of long sources.The narrowest experimental line width attained has been 0.014% in momentum at half the maximum intensity, at a transmission of 0.06% and using a source 0.1 mm in width.

Ferromagnetic Relaxation by the Exchange Interaction between Ferromagnetic Electrons and Conduction Electrons

The Hamiltonian for the exchange interaction between the ferromagnetic $d$ electrons and the conduction $s$ electrons is derived. The ferromagnetic relaxation time caused by the $s\ensuremath{-}d$ exchange interaction is calculated in a spin wave approximation. When one uses a screened value of the exchange integral ($\frac{{J}_{\mathrm{atomic}}}{30}$), the calculated relaxation time for nickel at room temperature is 5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}9}$ sec as compared to the time 2.5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}10}$ sec needed to account for the experimental line width. The exchange relaxation may be dominant in materials such as alloys which have narrower lines than nickel.

Structural Investigations by Means of Nuclear Magnetism. II. Hindered Rotation in Solids

The experimental absorption line widths, for nuclei with spin 1/2, at nuclear magnetic resonance are given as a function of temperature for a number of molecular crystals. Temperatures ranged from 90°K to the melting points of the compounds. In some cases it has been possible to relate observed line structure and transitions in the line width to the existence and frequency of certain types of hindered rotational motion in the solid state. These deductions are based on mathematical considerations of the quantitative effect of such motions on the structure and second moment of an absorption line. It is emphasized that relatively low frequency motion of the order 105 cycles/second suffices to narrow the width of an absorption line from its value in the absence of that motion. 1,2-dichloroethane, 1,1,1-trichloroethane, and perfluoroethane were found to have line-width transitions coinciding with changes in crystal form and anomalies in the heat capacity. In 1,2-dichloroethane and perfluoroethane these transitions correspond to rotational motion about the long axis of the molecule. 1,1,1-trichloroethane has a line-width transition corresponding to this type of motion, but the heat capacity anomaly and change in crystal form coincides with a further small decrease in the line width to that characteristic of the liquid. A number of molecules including acetonitrile, methyl iodide, nitromethane, dimethyl mercury, and ammonia have absorption lines at 90°K corresponding to molecular rotation about the C3 figure axis. Various other data and interpretations are presented for 2,2-dimethyl propane, methanol, ethanol, acetone, methylamine, and the ethyl halides. The possibility of estimating the potential barriers hindering rotation in solids from the line-width transitions is discussed.