I have read that rather than holding the static magnetic field constant and varying the frequency of the oscillating field in Electron Paramagnetic Resonance/Electron Spin Resonance (EPR/ESR) spectroscopy, the oscillating field is held constant while the static field varies in strength. I understand why this might be convenient for the purposes of physical implementation, but it seems to be that it would make it much harder to perform theoretically calculations of EPR structure (which is necessary to derive physical meaning from the results of the experiment). Specifically, if the spin Hamiltonian with external magnetic field strength $B$ is $H(B)$, it seems to me that the following things are true:
- If the static field $B_0$ is held constant while the oscillating field $B_1$ is varied, it seems like we can just calculate the eigenvalues of $H(B_0)$. Then, the resonant frequencies at which $B_1$ is observed will correspond to transitions between these energy eigenvalues.
- If the oscillating field $B_1$ is held at constant frequency while the strength of the static field is varied, it seems that it would be necessary to find the eigenvalues of $H(B_0)$ for every possible field strength $B_0$, so that one can check when the frequency of the oscillating field happens to align with one of the transition energies. This seems like it would require a massive amount of additional computational overhead.
So are my instincts correct - does calculating the frequency of EPR spectra require much more computation as a result of the experimental conventions? Or can anyone explain to me where I am confused?