The two-spin solid effect (2SSE) is one of the established continuous wave dynamic nuclear polarization mechanisms that enables enhancement of nuclear magnetic resonance signals. It functions via a state-mixing mechanism that mediates the excitation of forbidden transitions in an electron-nuclear spin system. Specifically, microwave irradiation at frequencies omega(mu w) ~& nbsp; omega(0S) +/- omega(0I), where omega(0S) and omega(0I) are electron and nuclear Larmor frequencies, respectively, yields enhanced nuclear spin polarization. Following the recent rediscovery of the three-spin solid effect (3SSE) [Tan et al., Sci. Adv. 5, eaax2743 (2019)], where the matching condition is given by omega(mu w) = omega(0S) +/- 2 omega(0I), we report here the first direct observation of the four-spin solid effect (4SSE) at omega(mu w) = omega(0S) +/-& nbsp;3 omega(0I). The forbidden double- and quadruple-quantum transitions were observed in samples containing trityl radicals dispersed in a glycerol-water mixture at 0.35 T/15 MHz/9.8 GHz and 80 K. We present a derivation of the 4SSE effective Hamiltonian, matching conditions, and transition probabilities. Finally, we show that the experimental observations agree with the results from numerical simulations and analytical theory. (C) 2022 Author(s).
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