A FAST 35-GHZ TIME-RESOLVED EPR APPARATUS

A time-resolved Q-band (35-GHz) electron paramagnetic resonance (EPR) apparatus with a 25-ns rise time and 10-ns time resolution is described. Built around the Varian E110 microwave bridge, a detailed description of the resonator, flow system, optical alignment, and preamplifier electronics is given. The resonator is a TE103 rectangular cavity with front and back cutoff flanges attached to allow maximum laser light access. An adjustable Gordon coupler was constructed to achieve critical coupling to the cavity. Two separate designs of quartz sample flow cell are described, one for low dielectric organic solvents and one for aqueous samples. The standard diode detection system as shipped by Varian was used, but the signal from the detector diode was branched into two separate amplifiers, one the standard preamplifier for automatic frequency control lock-in operation, the other based on a Comlinear op-amp which gave fast rise times (9 ns) before the signal was trapped by a boxcar signal averager. Test systems clearly showed that the instrument response is no longer limited by the quality factor of the cavity but by the laser pulse width and jitter. Strong, spin-polarized EPR signals from the photolysis of dimethoxyphenylacetophenone, which were well separated due to their large g-factor difference, were distinguished at sampling delay times as early as 10 ns after the laser flash producing the radicals. A sensitivity comparison to X-and is made using the acetone/2-propanol system.