We propose a phenomenological model involving the cross-coupling of the allowed and forbidden epr transitions between electrons belonging to neighboring packets of an inhomogeneously broadened epr spectrum. This model yields all the observed qualitative features of the new effect in dynamic polarization previously reported.In a previous communication 1 we reported the empirical evidence of a new effect in the dymanic polarization of protons in chemically doped plastics at liquid-helium temperatures. 2 We there attributed this new effect to some mechanism of spin-spin interaction, which described the qualitative behavior of our results. This Letter presents a phenomenological model for a specific form of spin-spin interaction.We assume essentially the Portis 3 picture of the structure of inhomogeneously broadened epr due primarily to hfs. The epr line then consists of many narrow packets of electrons e$ (j = 1, 2,3, • • •), with populations iV-7, in proximity to each other. Each packet of electrons couples dipolarly with nJ protons. Consider three such packets whose resonant frequencies satisfywhere v n is the nmr frequency of the protons at a given applied external magnetic field H', ^\oc * s ^e l oca * magnetic field for thejth packet, and y e is the gyromagnetic ratio of the electron. Figure 1 shows the energy levels for representative electrons in weak dipolar coupling with a proton for each of these three packets together with the relevant stimulated transition rates Wj through W 3 J and the relaxation rates wj through w s J. The direct excitation of transitions WJ and W 3 J by an applied rf magnetic field gives rise to the "solid effect" discussed in great detail by many investigators. 4 " 6 For our phenomenological model, we propose here two additional transition rates Wj'and W 6 J which represent the crosscoupling between W^ and WJ-1, Wj and Wj-1 (WJ); or between Wj and WJ + 1 WJ and Wj + lThe energy levels and pertinent connecting rates for the packets of Eq. (1). The A z are total electron splittings; W^ couples transitions W^-1 to W£ and Wj> to w£-l, W Q i couples W^+l to W$ and wf> to W£+\ The proton rates W} and 40/ are not shown.(W Q l ). An essentially identical form of interaction was first implied by Kessenikh, Manenkov, 7 and Pyatnitskii in their treatment of dynamic polarization of protons in irradiated polyethylene. However, their results are not valid for the very high electron polarizations germane to our case. Moreover, our treatment of the microwave power dependence is simpler, allowing us to approximate the power dependence in closed form. We assume specifically that transition rates W 5 J and W Q J are effectively much larger than the forbidden transition rates W 2 j and W Z J 9 because of the spin-spin interaction among the electrons. The enhanced transition rates could be visualized as caused by the increased virtual photon density over the real applied photon density on account of the saturation of the allowed transitions Wj. This assumption is the essence of our phenomenological mode...

A new effect in dynamic polarization is found showing characteristics similar to those of the "solid effect" but with significant difference in the spacing between enhancement peaks. A possible explanation, ascribing it to the electron spin-spin interaction, is discussed.In the theory of dynamic polarization of nuclei by the "solid effect," 1>2 the spacing (D) between the positive and negative nuclear-polarization enhancement peaks (±E max ) is a well-defined quantity for a system with completely resolved forbidden transitions. For systems in which both the electronic and nuclear spins are |, one has D = 2«2c%. Withincreasing radical concentration we found D « 1.3co w as shown in curves c and d.The same spacing, D«1.3co w , and curves of E vs H 0 similar in shape to that of Fig. 1(d) were also observed for 5% Ley's radical in frozen toluene at 4.2°K and for 5% galvinoxyl in polystyrene at 4.2 and 1.2°K. A reduced!) is not limited to these two radicals since we also observed D » 1.5w w for 3.3% DPPH in polystyrene at 4.2°K. The absolute value of H 0 was monitored continuously during the experiment by a F 19 nmr spectrometer with the F 19 probe located outside the multimode microwave cavity. In addition, both a> and oo n were also monitored, and we found oo~g e (3Ho and oo n =gn^n^0 with H 0 given by the F 19 nmr spectrometer. Hence we believe that the error in D due to uncertainties in# 0 was negligible. 20 10 0 -10 -20 r r -r-i [ (a)

We report a result of (1 ±4) xlO" 7 for the parity-nonconserving component in the p-p nuclear cross section at 15 MeV. Our experiment uses rapid spin reversal of a longitudinally polarized proton beam and an unpolarized H 2 target. Sources of systematic error are discussed and found to be <10" 7 .

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