Transient spectroscopy of pyridazine vapor: Photophysics of the S1 origin

Abstract

Nonradiative relaxation processes of pyridazine’s S1 origin level have been investigated using several varieties of gas phase time-resolved spectroscopy. Measurements of ground state repopulation kinetics under vibrationally relaxing conditions showed rapid and nearly complete return of thermalized S0, implying a quantum yield for S1 → S0 internal conversion of at least 0.95. The room temperature photochemical quantum yield was found to be higher in the low pressure gas than in solution by a factor of at least 600. From these results the photochemically active state was deduced to be vibrationally energized S0, rather than S1. The collision-free lifetime of the S1 origin level was measured as 3–4 ns on the basis of near ultraviolet Sn ← S1 transient absorption and S1 → S0 stimulated emission kinetics. Hot S0 molecules formed as the product of S1 internal conversion showed a broad, structureless transient absorption spectrum. Apart from its collision-free radiationless decay, the S1 origin level was also found to undergo an unusual collisional quenching process induced by electronically inert collision partners at rates between 5% and 10% of gas kinetic. This collisional channel is suggested to involve promotion to the anomalous level 373 cm−1 above the S1 origin, which had been tentatively assigned as the origin of pyridazine’s S2 state.