Luminescence and radiationless transitions from single vibronic levels of the isolated pyrazine molecule in the S 1(n,π*) state

Abstract

Luminescencespectra,fluorescence, and phosphorescence yields were measured for pyrazine excited to six different vibronic levels of the S 1(n,π*) state in pure pyrazine vapor and in gaseous pyrazine‐SF6 mixtures. The vibrational analysis of the fluorescencespectrum is given. As the total gas pressure increases from 10−2 to several torr, the fluorescence yield decreases and reaches a constant pressure‐independent value almost identical with that observed in condensed phases. The amount of this effect depends on the energy of the excited vibronic level. The phosphorescence yield, equal to zero at the low‐pressure limit, increases with gas pressure. The deviation from the Stern‐Volmer law for the fluorescence quenching is explained by the nonexponential decay of the fluorescence, evidenced by direct measurements of the decay curves. The results are discussed in terms of the kinetic and quantum‐mechanical models of radiationless transitions in intermediate‐size molecules.