HFHF ESR: continuous-wave

The extension of ESR to sub-terahertz frequencies (the millimeter wavelength-end of the far-infrared region),
has been one of the most important instrumental advancements at ACERT:

• The primary advantage of going to sub-THz is increased signal-to-noise ratio (SNR) and improved spectral resolution in many cases.
• The second, higher frequency ESR acts as a faster "snapshot" of the motional dynamics. That is, for a given diffusional rate the spin label motion appears to become slower as one utilizes higher frequencies. For the same motional rate, at low or
  conventional frequencies (e.g. 9 GHz) one may observe motionally narrowed spectra, whereas at high frequencies (e.g. 250 GHz) the spectra may display very slow motion features, almost the rigid limit character. This "snapshot" feature enables a multi-frequency ESR
  approach to study the complex modes of motion of proteins, DNA, and other polymers, which leads to decomposition of the motion into modes according to their different time scales

• The third feature is the increase of the orientational resolution of the nitroxide spectrum, due to the dominant role of the g-tensor, as the ESR frequency increases. For rigid limit spectra at 250 GHz, one can clearly distinguish the well-separated spectral regions corresponding to those nitroxide spin labels with theirx-axes parallel to B₀, their y-axes parallel to B₀, and their z-axes parallel to B₀. Then as motion is introduced (e.g. by warming the sample) one can discern the axis (or axes) about which the motion occurs. Because of this enhanced resolution, the 250 GHz slow-motional spectra are much more sensitive to the details of the motional dynamics than are those at microwave frequencies.

The demonstrated benefits of a multifrequency approach to ESR for unravelling the complicated dynamics in biological systems provides
the impetus for upgrading and improving the sensitivity of ACERT existing spectrometers of 95, 170 and 250 GHz. Our extensive
accomplishments in HFHF-ESR summarized in seminal publications are indicative both of the commitment and success of our group in exploiting the rich interplay between quasioptical techniques and ESR instrumentation needs.

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