Subsequent detailed studies of concentration and speed of cooling behavior of Shpolskii systems by L. A. Nakhimovsky and coauthors led to a hypothesis that these systems are
metastable segregational solid solutions formed when one or more chromophores replace two or more molecules in the host crystalline lattice.
The original observation of the Shpolskii effect was made at liquid nitrogen temperature (77 kelvins), but using temperatures close to that of liquid helium (4.2 K) yields
much sharper spectral lines and is the usual practice.
Shpolskii systems are low-temperature host–guest systems – they are typically rapidly frozen solutions of polycyclic aromatic hydrocarbons in suitable low molecular weight
The narrow lines characteristic of the Shpolskii systems are only observed at cryogenic temperatures because at higher temperatures many phonons are active in the lattice
and all of the amplitude of the transition shifts to the broad phonon sideband.
In addition to the weak inhomogeneous broadening of the transitions, the quasi-lines observed at very low temperatures are phonon-less transitions.
The emission and absorption spectra of lowest energy electronic transitions in the Shpolskii systems exhibit narrow lines instead of the inhomogeneously broadened features
normally associated with spectra of chromophores in liquids and amorphous solids.
[‘1. E. V. Shpolskii, A. A. Ilina and L. A. Klimova, 1952, Transactions Doklady of the U.S.S.R. Academy of Sciences, volume 87, page 935
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