A quasioptical transient electron spin resonance spectrometer operating at 120 and 240 GHz

TLDR

High‑frequency EPR at 120–240 GHz enables precise measurement of small g‑tensor anisotropies in excited triplet states of organic molecules such as porphyrins and fullerenes. The authors present a new multifrequency quasioptical EPR spectrometer. Its superheterodyne design with Schottky diode mixer/detectors provides sub‑nanosecond time resolution, and optical access makes it suitable for transient EPR at 120 and 240 GHz. The spectrometer achieves a continuous‑wave sensitivity of 10^13 spins cm^−3 (15 nM) at 240 GHz without a cavity, improves to ~3×10^9 spins with a Fabry‑Perot cavity and 30 nl sample, and its performance is demonstrated on transient EPR of organic triplet states, cw EPR of nitroxide references, and a colossal magnetoresistance thin film.

Abstract

A new multifrequency quasioptical electron paramagnetic resonance (EPR) spectrometer is described. The superheterodyne design with Schottky diode mixer/detectors enables fast detection with subnanosecond time resolution. Optical access makes it suitable for transient EPR (TR-EPR) at 120 and 240 GHz. These high frequencies allow for an accurate determination of small g-tensor anisotropies as are encountered in excited triplet states of organic molecules like porphyrins and fullerenes. The measured concentration sensitivity for continuous-wave (cw) EPR at 240 GHz and at room temperature without cavity is 1013spins∕cm3 (15 nM) for a 1 mT linewidth and a 1 Hz bandwidth. With a Fabry-Perot cavity and a sample volume of 30 nl, the sensitivity at 240 GHz corresponds to ≈3×109 spins for a 1 mT linewidth. The spectrometer’s performance is illustrated with applications of transient EPR of excited triplet states of organic molecules, as well as cw EPR of nitroxide reference systems and a thin film of a colossal magnetoresistance material.