Function Of Microwave Power Research Articles

Nuclear Dynamic Polarization and Spain-Lattice Relaxation in Plastic Cyclohexane Doped with a Nitroxide Free Radical in Low Concentration

In order to check the theory of spin diffusion with a new kind of sample, we present measurements of dynamic polarization and spin-lattice relaxation, at 3300 G, on the plastic phase of cyclohexane doped with a nitroxide free radical. The EPR lines are sufficiently narrow to allow an ideal ``solid effect.'' The enhancement of the proton resonance was measured as a function of microwave power for various temperatures. The decay rate of these enhanced signals both as a function of temperature and radical concentration was studied, in order to determine the proton spin-lattice relaxation time. The electronic spin-lattice relaxation time was measured by the saturation method. The data were found to be fairly consistent with a rapid diffusion regime between −10 and −60°C. The critical radius (i.e., the radius of a sphere, centered on the electronic spin, inside which the diffusion coefficient is zero) was found to be ≃ 1.7 Å. This value is much smaller than the distance between the free radical and the nearest cyclohexane molecules. The meaning of this fact is not yet clear.

Nuclear Spin-Lattice Relaxation and Dynamic Polarization inγ-Irradiated LiF

The spin-lattice relaxation at 13.6 MHz of ${\mathrm{F}}^{19}$ nuclei in $\ensuremath{\gamma}$-irradiated LiF is investigated at 300, 77, and 4.2-1.7 \ifmmode^\circ\else\textdegree\fi{}K. The crystals used have optically determined $F$-center concentrations of 7.0 \ifmmode\times\else\texttimes\fi{} ${10}^{17}$, 2.0 \ifmmode\times\else\texttimes\fi{} ${10}^{18}$, and 3.5 \ifmmode\times\else\texttimes\fi{} ${10}^{18}$ ${\mathrm{cm}}^{\ensuremath{-}3}$. The relaxation in the temperature range 4.2-1.7 \ifmmode^\circ\else\textdegree\fi{}K is found to be characterized by a spin-diffusion regime intermediate to the slow-and rapid-diffusion cases. At 300 \ifmmode^\circ\else\textdegree\fi{}K, the nuclear spin-lattice relaxation is found to be predominantly controlled by $F$-center spin-lattice relaxation. However, for the $F$-center concentrations used here, temperature-independent $F$-center spin-spin relaxation is found to control the nuclear spin-lattice relaxation below 10 \ifmmode^\circ\else\textdegree\fi{}K. The variation of the ${\mathrm{F}}^{19}$ dynamic polarization as a function of microwave power at 1.7 \ifmmode^\circ\else\textdegree\fi{}K is investigated for microwave magnetic field amplitudes up to 2.6 G. For this range of fields, the dynamic polarization process also is characterized by an intermediate spin-diffusion regime. Existing theory yields an expression giving the variation of the dynamic polarization time with microwave power, which is used to estimate the correlation time of the $F$-center-nucleus dipole-dipole interaction at 1.7 \ifmmode^\circ\else\textdegree\fi{}K. Estimates are also obtained for the $F$-center spin-lattice relaxation time at 300 \ifmmode^\circ\else\textdegree\fi{}K.

Measurement of the Level Shift inHe+,n=3

The level shift $\mathcal{S}(^{2}S_{\frac{1}{2}}\ensuremath{-}^{2}P_{\frac{1}{2}})$ and the fine-structure interval $\ensuremath{\Delta}E\ensuremath{-}\mathcal{S}(^{2}S_{\frac{1}{2}}\ensuremath{-}^{2}P_{\frac{3}{2}})$ have been measured in the $n=3$ level of the helium ion ${\mathrm{He}}^{+}$ with a microwave-optical technique. Electron bombardment of helium gas produces excited ions. The 1640-\AA{}A light from the $n=3 \mathrm{to} 2$ decay undergoes a decrease when a microwave field induces an $S$-to-$P$ transition. Extensive data were taken on three transitions as a function of microwave power, helium pressure, and bombarding current in a search for systematic effects. The major effect was a slight shift of the resonance curves with microwave power due to transitions in higher excited states. The high-frequency transitions $\ensuremath{\alpha}c$ and $\ensuremath{\beta}d$ yield $\ensuremath{\Delta}E\ensuremath{-}\mathcal{S}=47 844.05\ifmmode\pm\else\textpm\fi{}0.48$ MHz. Using the Taylor, Parker, and Langenberg value for the fine-structure constant to calculate $\ensuremath{\Delta}E$, we obtain an indirect measurement of $\mathcal{S}=4183.73\ifmmode\pm\else\textpm\fi{}0.51$ MHz. Measurements on $\ensuremath{\alpha}e$ yield a direct value of $\mathcal{S}=4183.17\ifmmode\pm\else\textpm\fi{}0.54$ MHz, which, combined with the indirect value, yields $\mathcal{S}=4183.45\ifmmode\pm\else\textpm\fi{}0.52$ MHz, in agreement with the theoretical value of 4184.4 \ifmmode\pm\else\textpm\fi{} 0.5 MHz of Applequist and Brodsky. (All uncertainties are expressed as 68% confidence values.)

Cyclotron Resonance of Hot Electrons in n-Type Germanium

The cyclotron resonance of electrons heated by the microwave field in n-type germanium is investigated. The half-width of the resonance line is calculated as the function of microwave power on the basis of the classical model and the concept of the electron temperature. The relation between the number of electrons in a valley and their temperature is discussed.

The generation of microwave phonons for studying the spin-lattice interaction∗

We have produced microwave phonons and studied their interaction with paramagnetic impurity centers in quartz. Phonon packets were produced by the piezoelectric effect at the end surface of a quartz rod placed in the electric field of a re-entrant cavity. They propagated down the rod, were reflected from the opposite end, and were detected at the generating surface by the inverse effect. The amplitude envelope of the echoes consisted of a series of maxima and minima, or beats, superposed on an exponential decay. From the decay, the average lifetime of the coherent phonons was found to be 10 −5 sec. The t = 0 intercept of the envelope measured the square of the fraction of the microwave power converted into acoustic power, which was found to be smaller by a factor of 10 −3 than the computed value of 0.7 × 10 −6. This discrepancy, together with the departure from an exponential decay, is due, principally, to interference effects arising from a slight nonparallelism of the end surfaces. The absorption of phonons by spins produced in the natural quartz by gamma irradiation from a Co 60 source was detected as a partial saturation of the spin electron paramagnetic resonance. The partial saturation, S/ S 0, was found to be 0.75 at 1.7°K, 0.75 at 4.2°K, and 0.92 at 20°K,the error in S/ S 0 falling between 2 and 5 per cent. These values for S/ S 0 were independent of the difference between the microwave and phonon frequencies within 20 Mc/s, and at a given temperature were independent of the microwave power. From microwave saturation measurements, the order of magnitude of the spin-lattice relaxation time T 1 was found to be greater than 3 sec at 1.7°K, 0.3 sec at 4.2°K, 3 × 10 −4 sec at 35°K, and 3 × 10 −6 sec at 77°K. The spin-resonance absorption, which was measured as a function of microwave power, saturated in such a manner as to indicate partial inhomogeneous broadening, and cross relaxation took place between the closely spaced, narrow (approximately l G wide) lines of the spectrum. These two effects prevented quantitative determination of the spinphonon transition probability.

Dynamic Nuclear Polarization Studies of Charred Carbohydrates

The dynamic enhancement of proton spin polarization by pumping on ``forbidden'' electron-nuclear transitions is studied as a function of microwave power, O2 pressure, and magnetic field for a pair of low-temperature paramagnetic sucrose chars. The dependence of the electron spin-lattice relaxation time on O2 pressure is obtained for both samples. Comparison of the enhancement for a 405°C char with theory indicates that the electron spin resonance line is inhomogeneously broadened, and yields absolute values for the spin-spin and spin-lattice relaxation times of the spin packets. No definite fit could be made to the 605°C char data. The difference between the magnetic fields for maximum positive and negative polarization enhancement is anomalously small for the 405°C char.

Anomalous Relaxation of Hyperfine Components in Electron Spin Resonance. II

A detailed study has been made of the linewidth and peak height of each of the five components in the electron spin resonance spectrum of the p-benzosemiquinone ion as a function of microwave power at X-band frequencies. Studies were also made of the spectrum of the peroxylamine disulfonate ion as a function of power. A 20% variation in the linewidths from one hyperfine component to another was found in the p-benzosemiquinone ion spectrum, and a 13% variation in the saturation parameters (T1). The average value for the linewidth parameter at low power for the p-benzosemiquinone ion was found to be (1/T2) = 1.11×106 sec—1, and the reciprocal of the average value of the saturation parameter was found to be about (1/T1) = 0.74×106 sec—1. No variation in linewidths or saturation parameters among the three hyperfine components in the peroxylamine disulfonate ion spectrum was found, and the results were T2 = 2.5×10—7 sec, and T1 = 3.3×10—7 sec. The variation in the linewidth of the p-benzosemiquinone ion can be accurately represented by a quadratic equation in mJ, the field-direction component of the total nuclear spin angular momentum, and the form of this equation is in excellent agreement with the linewidth predicted for a relaxation mechanism arising from an anisotropic intramolecular dipolar interaction between the unpaired electron and the four protons in the molecule, as well as a rotational relaxation of the anisotropic g tensor. Good quantitative agreement between theory and experiment is obtained for the coefficient of mJ2 in the equation for (1/T2) if the rotational correlation time is taken to be τc = 7×10—11 sec, a value which is consistent with the molecular dimensions. The dependence of the saturation parameters on mJ is also in qualitative agreement with the theory, but quantitative discrepancies are found which have not been explained.

Detectors for Microwave Spectrometers

The sensitivity of detectors used in sample modulated microwave spectrometers is investigated. Characteristics of crystal diodes and barretters as functions of microwave power are compared with an ideal detector operating at room temperature. A spectrograph is described which was used to measure a CFCl3 line having a calculated absorption coefficient of 3.9×10−11 cm−1. The relationship between recorder deflection and absorption coefficient is discussed.