Exchange-coupled chromium ion pairs in ruby revisited

A full understanding of the complicated spectra of the “N lines” in ruby that result from exchange-coupled pairs of chromium ions has remained a challenging but daunting task, especially for the nearest neighbour pair. Various inventive techniques were employed by spectroscopists in their studies of the problem. A major advance was made in 1962 by Kaplyanskii and Przhevuskii where they showed how the piezospectroscopic technique (stressing the ruby crystal in various directions) could be employed to help associate specific N lines with specific types of pair centres. In this work, using the methods of selective excitation combined with phase-sensitive detection of luminescence, we have succeeded in identifying all four transitions between the lowest level of the excited state of the nearest neighbour pair centre and the ground state, thereby confirming the energy level structure in the ground state. The low-temperature emission from this centre is found to be dominated by an exchange-induced electric dipole process, but the emission is almost all contained in the vibrational sideband. A similar behaviour is found for second nearest neighbour pair centres.