Very High Pressure Effects upon the EPR Spectrum of Ruby

The variation in the electron paramagnetic resonance (EPR) spectrum of ruby as a function of pressure has been observed to beyond 70 kbar for a magnetic field orientation parallel to the crystalline c axis. The data can be interpreted in terms of the usual spin Hamiltonian: H=g||βHzSz−12δ(Sz2−13S2), where the spectroscopic splitting factor g‖ has the same value as at ambient pressures, but where the zero-field splitting δ increases in a linear fashion from ∼0.38 to ∼0.43 cm−1. The experimental apparatus has been described previously, although important improvements in the pressure seal and in the pressure calibration have been made and are described in an Appendix.