Very high frequency (263 GHz) pulse EPR spectroscopy of high spin transition metal centers

Pulse Electron Paramagnetic Resonance (EPR) spectroscopy provides powerful tools for examining species with unpaired electrons, such as organic radicals and metal ions, in a wide variety of interesting systems. However, the lack of high power pulse amplifiers above approximately 100 GHz has limited its applicability at very high frequencies and magnetic fields. Here we describe the use of a new high frequency traveling wave vacuum tube amplifier employed in a pulse EPR spectrometer operating over a wide frequency bandwidth centered at 263 GHz. This novel instrument provides significant improvements in sensitivity and resolution when compared to lower frequency instruments. Very high frequency pulse EPR is of particular utility for studying spin S>1/2 species, given that large broadening effects of ``zero-field splitting'' interactions often dominate EPR spectra obtained at low frequency. Improvements obtained at 263 GHz are illustrated for a series of high spin (S=5/2) Mn(II) complexes.