SIMPLE MODIFICATION OF VARIAN E-LINE MICROWAVE BRIDGES FOR FAST TIME-RESOLVED EPR SPECTROSCOPY

Modification of the popular Varian E-line series of electron paramagnetic resonance (EPR) spectrometers for fast direct detection EPR spectroscopy is described. The overall time response and signal reproducibility of the spectrometer is improved by changing the values of two capacitors in the microwave bridge preamplifier circuit, with no alteration of instrument performance in normal field-modulated experiments. The rise times are slew rate limited for large input signals. There is a propagation delay of 40 ns before any preamplifier response is observed. Signal-to-noise ratios for the direct detection EPR experiment with the modified and unmodified bridges are comparable. A test of the modified bridge using a biradical, formed within 20 ns by laser flash photolysis, shows a signal decay consistent with that measured by optical methods, in contrast to the unmodified circuit. Other factors influencing the time response of the experiment are discussed. While the preamplifier rise time is still the limiting factor, spectra as close as 70 ns to the laser flash can now be measured with adequate signal-to-noise ratios.