Absorption units in simulation

[yausern]

I am having some difficulty understanding the units of the output spectra for pepper using different harmonic settings. I am using non-standard EPR setups (only continuous wave) and have little experience working with commercial EPR systems. I am using EasySpin to compare my results to theoretical predictions but I need to be able to reliably translate the outputs into spectra I can directly compare to the experiment.

As I understand it, in a continuous wave setup without modulation (harmonic 0), an EPR spectromenter measures the absorption of a sample placed in its cavity. It irradiates the system with a certain input power and measures changes in the reflected power off the cavity while sweeping the DC field. I assume pepper outputs spectra that are directly comparable to the output of most commercial EPR spectrometers.

My question is, what are the units for the spectra outputted by pepper? Is it dimensionless and analogous to that obtained by dividing the reflected power (in watts) at each field by the input (or maybe a background) power? Or does it measure wave amplitudes (volts) instead of power? In either case, why are the spectra then not bounded between 0 and 1? What defines the scale of this output or is it arbitrary? Is the scale linear or are dB scales used (the reference being the background or input) so that the spectra are in fact dB of absorption? Or something different?

In a modulated measurement (harmonic 1 and the default setting for EasySpin), I assume that the output is directly proportional to the derivative of whatever the output of the harmonic 0 spectra is. Am I correct?

Sorry if this seems too basic... I just can't seem to find a clear answer in the documentation.

Thank you in advance for any help

[Stefan Stoll]

The intensities of the cw spectra calculated by EasySpin don't take any instrumental factors into account. Microwave power, quality factor of the resonator, coupling coefficient, preamplifier gains, etc. are not included. The reason for this is that there are many different setups that are being used for field- or frequency-swept cw EPR, not only resonator reflection setups.

In pepper, the intensities are calculated as a product of three factors:

1. The quantum-mechanical transition rate between the two states involved in the transition (using Fermi's golden rule and dropping the microwave amplitude B1 from that expression).
2. The polarization factor that contains the relative population difference between the two states (set equal to 1 if you don't specify a temperature).
3. The Aasa-Vanngard 1/g factor that takes into account the fact that this is a field sweep and not a frequency sweep.

Check out section 3.4.3 of the 2014 review on the EasySpin publications website.