Rebane, K. K. (2003). Purely electronic zero-phonon line as the foundation stone for high-resolution matrix spectroscopy, single-impurity-molecule spectroscopy, and persistent spectral hole burning. Recent developments: Spectroscopy in Cryocrystals and Matrices. ФІЗИКА НИЗЬКИХ ТЕМПЕРАТУР, 29(9-10), 1101–1108. https://doi.org/10.1063/1.1619354