EXCITED STATE DYNAMICS AND SPECTROSCOPY OF AZABENZENES (PYRIDINE, PYRIDAZINE, TRANSIENT SPECIES)

Relaxation processes in pyridine and pyridazine have been investigated using time resolved spectroscopic methods. In order to investigate the photophysics and dynamics of molecular excited states a versatile transient absorption spectrophotometer has been designed and constructed to study low pressure gas phase samples on the nanosecond time scale. The sensitivity of this apparatus is high enough to allow investigation of samples with a (DELTA)(epsilon)p > 0.05 M('-1)cm('-1)Torr('-1), and has an rms noise level of (TURN)0.0002 absolute absorbance units. With this spectrophotometer a T(,n)()T(,1) intersystem crossing, and has an estimated oscillator strength of (TURN)0.02. An internal calibration method has enabled reliable excited state extinction coefficients to be evaluated. The intersystem crossing process in pyridine has been shown to be in or very near the statistical limit. Pressure-dependent and nonexponential nonradiative decay is exhibited by triplet pyridine. These two forms have intrinsic lifetimes of > 10 (mu)s and 200 ns. A weak complex is found to be formed between triplet pyridine and oxygen in the quenching process. Gas phase ground state repopulation experiments show that S(,o) pyridine is the resulting state of this quenching process. Total repopulation of vapor phase pyridine has also been observed in the presence of 10 Torr cyclohexane and 100 Torr O(,2) on the (mu)s time scale. A 4% (+OR-)2% photochemical yield has been calculated for pyridazine following excitation to the S(,1) state. A dual component transient of pyridazine is found to absorb in the region of 26,640 cm('-1) to 27,200 cm('-1). This is either vibrationally excited ground elctronic state molecules formed from two channels one of which is S(,1), the other of which has a lifetime of 280 ns, or it is vibrationally excited ground electronic state pyridazine and an intermediate in the photochemical process. Ground state repopulation experiments indicate the formation of a transient having (epsilon) > 590 M('-1)cm('-1). This transient is present even in the presence of 400 Torr of added SF(,6)