Multiphoton resonances in pulse EPR

Two- and three-photon electron spin echoes of a two-level system are observ ed using, in addition to the microwave excitation, a linearly polarized rad io-frequency field oriented along the static magnetic field B-0. Such multi photon echoes are detected when the sum of the energies of one microwave an d one or two radio-frequency photons are equal to the difference between en ergies of two spin states. The multiphoton character of the echoes is confi rmed by measuring the spin nutation frequency as a function of the angle be tween the radio-frequency field and B-0, and monitoring the echo amplitude as a function of the radiation field strengths. Floquet theory, usually app lied for the description of multiphoton resonances with an odd number of ph otons, is extended to the case where an even number of photons participate in the transition. (C) 2000 Academic Press.