Peter Kok, Edgar J. J. Groenen, Lex van der Meer and Wim van Amersfoort
IR spectra of the lowest triplet state T0 of pyrazine, pyrimidine and pyridine have been recorded by the use of the free-electron laser FELIX. Vibrational transitions have been observed as laser-induced changes in the intensity of the phosphorescence from T0. Five triplet vibrations have been recognised for pyrimidine and two for pyridine. For pyrazine, all seven fundamental IR-active triplet vibrations in the range 250–2000 cm-1 have been detected and the vibrational frequencies are found to be considerably lower than the corresponding ones in the ground state.