TIME-RESOLVED ELECTRON-PARAMAGNETIC RESONANCE SPECTROSCOPY IN THE Q-BAND MICROWAVE REGION

Time-resolved electron paramagnetic resonance (EPR) spectra from a Q-band (35 GHz) spectrometer are reported. Organic free radicals in liquid solution, created by laser flash photolysis within the microwave cavity, are observed at delay times as short as 300 ns after the light pulse. Chemically induced dynamic electron polarization (CIDEP) patterns at Q-band are unusual compared to those from a conventional X-band (9.5 GHz) spectrometer due to different spin relaxation times and large g-factor differences between the radicals. These effects are demonstrated using the photoreduction of acetone by 2-propanol and the Norrish I alpha-cleavage of alkyl and benzylic ketones as examples. Comparison of Q- to X-band spectra of a 1,11-bis(alkyl) biradical shows the utility of multiple-field EPR spectroscopy in the study of magnetic field effects and CIDEP phenomena of high-spin species on the submicrosecond time scale.