Multiple-quantum electron-electron double resonance

Abstract

Continuous wave electron-electron double resonance involves the irradiation of one line in an EPR spectrum with an intense “pumping” microwave source and the observation of the effect of that irradiation on another line of the spectrum using a weaker “observing” microwave source. In fluid samples, three ELDOR mechanisms that can be responsible for the transfer of saturation from the pumped transition to the observed transition have been identified. These are Heisenberg exchange between colliding paramagnetic molecules ( 1)) nuclear relaxation induced by rotational modulation of electron nuclear dipolar interaction ( 1)) and, in samples undergoing very slow rotational diffusion, spectral diffusion because of rotational diffusion (2, 3). The ELDOR technique and also relevant theory were introduced in Ref. (1). ELDOR is the subject of a monograph (4) and has been recently reviewed (5). The longitudinal detection of electron paramagnetic resonance involves the irradiation of a single homogeneous line by two closely spaced equally intense microwave frequencies. It was shown by workers at the University of Pisa (6, 7) that under this irradiation multiquantum transitions are induced. The M, component of magnetization oscillates at the difference between the microwave frequencies as well as at multiples of the difference. These workers directly detected these Zeeman oscillations with a pick-up coil oriented along B,,. In this paper, we report the transfer of Zeeman oscillations as induced by two closely spaced microwave frequencies that irradiate one EPR transition to another EPR transition because of liquid-phase Heisenberg exchange. Thus, three CW microwave frequencies are incident on the spin system: two closely spaced frequencies (nominally 10 kHz apart at X band) that serve together to pump a transition and a weaker third frequency used to observe another transition. All frequencies are derived from a single klystron by use of a single sideband mixer (SSM) in an unusual configuration (Fig. 1). The two inputs of the SSM are supplied by the two outputs of the Hewlett-Packard 3326A two-channel synthesizer set to two