DOIRR spectrum was recorded in C 2 H 2 for triplet levels coupled by intramolecular interactions to levels of an adjacent A 1 A u singlet state, excited into individual rotational level of the 3 2 vibronic state. The spectrum was recorded for the V12K12, J′=4 level. The spectrum fitting and line assignment were done using a statistical model. The rotational constant B determined as a spectral fitting parameter is ca. 0.43cm −1 . Time-resolved measurements were carried out for DOIRR spectral line located at 1538G. It was found that at intermediate IR power levels, the fluorescence decay is biexponential, due to reversibility of the S–T conversion.