Submillimeter wave ESR measurement for Cr3+ in ruby crystal using a gyrotron as a radiation source

Fine structure constant of ruby has been measured using an ESR spectrometer with a pulse magnetic for high fields and a gyrotron as a radiation source in a millimeter to submillimeter wave range. The measurement was carried out at room temperature. The Zeeman energy in this frequency range is large enough compared with the fine structure constant, The higher order term in the effective spin Hamiltonian can explain the dependence of fine structure constant on the frequency, The observed fine structure constants depend on the field intensity.