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The dynamics of the liquid state of two organic free-radical solutions has been studied by means of EPR, NMR, and DNP (dynamic nuclear polarization). In our spectrometer these effects can be measured simultaneously and over a wide range of texnperatures.The aixn was to study the gradual transition froxn the ordinary liquid state to a greatly supercooled liquid state, and in particular the change from the Overhauser-type DNP to the solid-type DNP. The samples were a 0. 02-mole/liter solution of tanone in isopropanol and a 0. 03mole/liter solution of tetrachlorosexniquinone in tetraethyleneglycol.The EPR can be explained in terms of random rotational modulation of the anisotropic g factor of the radical, with a correlation time which is an exponential function of reciprocal temperature. At the temperatures of interest, T«and T2 differ by at least one order of magnitude. The NMR follows from the Torrey model, the protons relaxing through dipole interaction with the electron spin while their molecule is temporarily adsorbed in the solvation layer of a radical. The data can be fitted by a model with a dipole-dipole correlation function based on a rather broad distribution of correlation times, the mean value and temperature dependence being similar as in EPR. The DNP shows a typical gradual transition from the (even) Overhauser effect to the (odd) solid effect as the temperature is lowered. This happens when the correlation time and T2 cross. A theoretical analysis of the DNP on the basis of the liquid structure as revealed by EPR and NMR is given in the following paper.

32, av. de 1'Observatoire -25000 Elesanqon -France and P. Handel Physics Department, University of Missouri, St Louis, MO 63121 Sumnary The ultimate s t a b i l i t y of q u a r t z c r y s t a l o s c i l l a t o r s is given by t h e frequency n o i s e of t h e quartz resonator, which corresponds i n t h e tine domain t o a f l o o r . The measurement of t h e resonance frequency fluctuations of the resonator itself can be made only i f the influence of t h e e l e c t r onics noise is reduced. By u s i n g a r( transmission network and a balanced phase bridge the driving source phase fluctuations can be rejected by 50 dB t o 60 dB. This measurement system w a s used a t r m temperature with a l a r g e number of p a i r s of resonat o r s , and a correlation between l/f frequency noise P l s and Q-factors was found, following a l / Q law. In order t o extend t h e measurements t o very l o w temperatures, a new measurement system was studied, which allows t o o p e r a t e w i t h p a i r s of non identical resonators. T h u s l/f frequency noise was measured a t 4K and a t 1K. A large decrease of t h e noise is observed corresponding t o t h e i n c r e a s e of the Q-factors.A theoretical explanation of t h e I/Q' law is attempted. It is based on three-phonon interaction processes, which a r e a t t h e o r i g i n of the acoustic attenuation and of variations of velocity. Both phenomena a r e r e l a t e d by means of the thermal phonon relaxation time.

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