Using the Bloch equations, Ernst and Anderson made a study of the conditions under which Fourier transformation (FT) of the free induction decay (FID) could provide spectral information in a more efficient way than observation of a "normal" field or frequency sweep versus time (CW). They showed that for equivalent signal/noise the FT method provides a saving in time over the CW method of approximately {entire spectral range}/{typical spectrum linewidth}. Thus the time to record the spectrum is independent of the width of the spectrum. R.R.Ernst and W.A.Anderson, Reviews of Scientific Instruments, 1966, 37, 93

However, the pioneering work in pulsed EPR was carried out by Bill Mims, starting in 1958. This followed Erwin Hahn's first pulse NMR experiment in 1950.

W.B.Mims, Electron Spin Echoes, Chapter 4 in Electron Paramagnetic Resonance, Ed S.Gerschwind, Plenum, New York, 1972