Why high field EPR?

Most commercial EPR spectrometers operate at fields of around 0.3 Tesla, corresponding to microwave frequencies of about 10GHz (X band). Moving to higher fields and frequencies can offer increased sensitivity and resolution.

minnb0ofspins.gif

 

The equation shows the frequency and power dependence of the minimum detectable number of spins in a one Hz bandwidth, assuming that the system obeys the Bloch equations and the Curie law. The separation in field of two lines with different g factors will scale with the field, so if the line widths remain constant, then the lines will eventually be resolved as the field is increased.

At St Andrews we have a CW EPR spectrometer operating at 90, 180 and 270 GHz and are currently developing a 90 GHz pulsed system for the HIPER project. Our systems use quasi-optical techniques to avoid the losses in conventional waveguide at high frequencies. We will shortly be acquiring a commercial pulsed X band system.