Nuclear quadrupole resonance of an electronically excited state from high-resolution hole-burning spectroscopy

Hole-burning spectroscopy can eliminate inhomogeneous broadening and thereby resolve the fine structure of optical transitions. In the case of rare-earth ions at low temperatures, the homogeneous linewidth is often small compared to the splittings due to nuclear-spin interactions. Hole-burning spectra can then be used to measure, e.g., nuclear quadrupole couplings. We have used this technique to study the hyperfine interaction of Pr in Pr³⁺:YAlO₃ in the electronic ground state as well as in an electronically excited state. Using a stabilized ring dye laser system (linewidth <30 kHz), we obtained hole-burning spectra that clearly resolved the excited-state interactions also. We show that the spectra depend sensitively on the relative orientation of the nuclear-spin quantization axes of the two electronic states. This allows us to decide between different values for the tensor orientation that have been published before.