What is the most difficult thing of EPR measurement?
The system built for EPR measurement consists two parts — lockin unit and spin-flip unit. The lockin unit applies a RF field to the cell, lockin into the D2 fluorescence excited by this RF field, and track its frequency. RF field is necessary for measuring D2 light, but at the same time it causes a field gradient which will depolarize the target. Hence the most difficult part is to optimize the whole system so the D2 signal induced by RF field is strong enough to perform the measurement, at the same time the polarization loss caused by this RF field is minimized. To reduce the polarization loss, the crucial thing is to minimize the time of activation RF field, which means, to do the measurement as quick as possible. However, this is not easy since the D2 fluorence is usually very weak. A PIN photodiode is used to measured this fluorescence. A D2 filter is placed right before photodiode to block D1 light. Though most of D1 light can be blocked by the D2 filter, it still dominates the p
