FIGURE 2.

(a) The six-pulse DQC-ESR sequence designed to measure distances between electron spins in solids. The amplitude of the echo formed at 2(t_p + t₁ + t₂) is recorded as a function of t_ξ ≡ 2t_p−t_m, with t₁ and t_m ≡ t_p + t₂ being kept constant. The phase cycling leaves only the part of the echo modulated by the dipolar coupling a. For the dipolar coupling in DQC, wherein r is the distance between electron spins, and θ is the angle between the vector connecting the electron spins on the nitroxides and the magnetic field, B₀. (b) An illustrative temporal envelope of the signal recorded with the six-pulse sequence for a nitroxide biradical in an isotropic medium. (c) The dipolar spectrum, produced by Fourier transformation of b, has the shape of a Pake doublet with an inner splitting of 2ν_⊥ = 4D/3.