Reversible resonant hole burning in the fluorescence spectra of protochlorophyll

Hole burning at a resonance frequency wthin the S₁-S₀ fluorescence band of protochlorophyll in ether, ether-butanol glass and n-heptane was observed at temperatures of 1.8–30 K. Both triplet-state population and photochemical burning mechanisms were found. Besides narrow lines (0.3 cm^?1 wide), distinct broad phonon wings werc detected in the hole-burning spectra. The photochemical phase was shown to consist of light-induced reversible site interconversions.