Nonradiative relaxation processes of pyridazinersquo;sS1origin level have been investigated using several varieties of gas phase timehyphen;resolved spectroscopy. Measurements of ground state repopulation kinetics under vibrationally relaxing conditions showed rapid and nearly complete return of thermalizedS0, implying a quantum yield forS1thinsp;rarr;thinsp;S0internal 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 energizedS0, rather thanS1. The collisionhyphen;free lifetime of theS1origin level was measured as 3ndash;4 ns on the basis of near ultravioletSnthinsp;larr;thinsp;S1transient absorption andS1thinsp;rarr;thinsp;S0stimulated emission kinetics. HotS0molecules formed as the product ofS1internal conversion showed a broad, structureless transient absorption spectrum. Apart from its collisionhyphen;free radiationless decay, theS1origin level was also found to undergo an unusual collisional quenching process induced by electronically inert collision partners at rates between 5percnt; and 10percnt; of gas kinetic. This collisional channel is suggested to involve promotion to the anomalous level 373 cmminus;1above theS1origin, which had been tentatively assigned as the origin of pyridazinersquo;sS2state.
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