FT-EPR with a nonresonant probe: use of a truncated coaxial line

Abstract

A truncated transmission line probe (TLP) has been utilized to excite and detect time domain responses after pulsed excitation in electron paramagnetic resonance (EPR) spectroscopic experiments in the frequency range 200-400 MHz. The TLP device is a modified short-circuited coaxial line, which allows the irradiation of the sample by the traveling waveB₁fields in the frequency range of kilohertz to 30 GHz. In EPR studies at 300 MHz carrier frequency, with 10 W incident power, a 45° pulse is 45 ns in duration. This corresponds to a 0.9-GB₁field. Using the TLP, time-domain responses from the solidN-methyl pyridiniumtetra-cyanoquinodimethane (TCNQ) were collected at 200, 250, 300, and 350 MHz, with the range limited by the amplifiers. In addition two tubes containing TCNQ placed side-by-side vertically along the axis of the probe were used to collect time domain responses in the presence of magnetic field gradients to test the feasibility of two-dimensional imaging using a TLP. The magnetic field gradient was steered in thexzplane and 36 projections were collected at 5° intervals. Using filtered back-projection image reconstruction, the two-dimensional spatial image in thexzplane was obtained at good resolution.