Continuous wave multiquantum electron paramagnetic resonance spectroscopy. II. Spin-system generated intermodulation sidebands

Abstract

Irradiation of a single homogeneous electron paramagnetic resonance (EPR) transition by two microwave sources separated by I:l.f, where I:l.f~ the linewidth, results in the production of intermodulation sidebands atfo ± (k + 1/2)1:l.f, wherefo is the mean of the two irradiating frequencies and k is an integer> I. These sidebands can be seen directly with a microwave spectrum analyzer. Anyone of the sidebands can be observed by suitable phase sensitive detection and displayed as a function of the polarizing magnetic field Ho, resulting in a multi quantum EPR spectrum. The widths of the EPR lines decrease as k increases, resulting in improved resolution. The two k = 1 transitions can be superimposed, resulting in a signal of double intensity. At saturating microwave power, this double-intensity line is two times less intense than an ordinary EPR line. No magnetic field modulation needs to be used when detecting multiquantum transitions, and pure absorption or pure dispersion signals are obtained with good baseline stability. Considerable attention is paid to the technical problem of irradiating the sample with just two microwave frequencies. In the apparatus described, spurious instrumental sidebands are reduced by 60 dB or more.

Contributions to the Gaussian Line Broadening of the Proxyl Spin Probe EPR Spectrum Due to Magnetic-Field Modulation and Unresolved Proton Hyperfine Structure

“…Similarly to the doxyl-labeled alkyl chains (8,9), large values of are encountered even though the spectral line remains unresolved; therefore, detailed knowledge of the proton hyperfine coupling constants cannot be obtained from EPR measurements alone. 2 Failure to correct the observed linewidth to find ⌬H pp L would result in errors of about 400% for either Hepes buffer or the deoxygenated CCl 4 solutions and 25% for airsaturated CCl 4 solutions. Now that the proxyl moiety has been studied, the Gaussian linewidths of all of the major spin probes are now available (8,9) and the methods of Ref.…”

“…The second moment of Eq. [4] is given by H m 2 /8; thus, we write the modulation-induced Gaussian linewidth as H m , the constant to be determined. The behavior of a Voigt line may be deduced by recalling that Gaussian linewidths add as the sum of their squares (8):…”

“…We combine the following two facts: First, only the Gaussian component is broadened when a Voigt line is subjected to the modulation given by Eq. [4] (7); and second, the Gaussian linewidth is proportional to the root-mean-square moment of the inhomogeneous broadening (8). The second moment of Eq.…”

“…Modulation distorts the signal, leading to modulation broadening, which has traditionally led the experimentalist to seek a modulation amplitude which is a compromise between noise and resolution. Alternative approaches avoiding field modulation have also been proposed (2)(3)(4)(5). In most nitroxide spin-probe studies, resolution is not an issue 1 ; however, in many studies, determination of the correct intrinsic (Lorentzian) linewidth is important.…”

“…To achieve this end and to test the modulation-broadening theory experimentally, we studied the nitroxide spin probe 2,2,5,5-tetramethylpyrrolidin-1-oxyl-3-carboxylic acid (3-carboxy proxyl) as a function of modulation amplitude and temperature in a buffered aqueous solution and in CCl 4 .…”