TIME-DELAYED 2-COLOR PHOTOELECTRON-SPECTRA OF ANILINE, 2-AMINOPYRIDINE, AND 3-AMINOPYRIDINE - SNAPSHOTS OF THE NONADIABATIC CURVE CROSSINGS

We present time-delayed two-color photoionization photoelectron spectr a of aniline, 2-aminopyridine, and 3-aminopyridine seeded in a cold mo lecular beam. The molecules are prepared in their S-1 electronic state s by a picosecond UV laser pulse and ionized by a time-delayed 200 nm probe pulse. The photoelectron spectrum is observed with a time-of fli ght spectrometer. All time-delayed spectra reveal only one product of the nonradiative relaxation process. Careful considerations of electro nic and vibrational overlap propensity rules for the ionization step l ead to the, conclusion that the dominant nonradiative decay mechanism in these molecules is the intersystem crossing to a bath of vibrationa lly excited levels of the T-1 electronic state. Our observations revea l no admixtures of T-2 or higher triplet levels. The pathway of the no nradiative electronic relaxation in 2-aminopyridine is found to be ind ependent of the initially prepared vibrational states up to 1000 cm(-1 ) of vibrational energy. We find no evidence of intramolecular vibrati onal relaxation preceding the electronic curve crossing. (C) 1995 Amer ican Institute of Physics.