Electron-spin double resonance by longitudinal detection: Line shape and many-quantum transitions

A system of electronic spins in double-paramagnetic-resonance condition is examined. The impinging waves are both nearly resonant and with the magnetic field perpendicular to the static field. The nonlinearity of the sample induces a longitudinal magnetization oscillating at multiples of the difference of the frequencies of the two waves. The line shape shows only one peak for low-power levels and two for higher wave intensities. Measurements have been performed by an ad hoc built $X$-band spectrometer with remarkably high relative precision in the difference betwen the frequencies. Transitions involving up to 22 photons have been detected. Experimental and theoretical results are closely consistent.