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 spectra of aniline, 2-aminopyridine, and 3-aminopyridine seeded in a cold molecular beam. The molecules are prepared in their S-1 electronic states 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 flight spectrometer. All time-delayed spectra reveal only one product of the nonradiative relaxation process. Careful considerations of electronic and vibrational overlap propensity rules for the ionization step lead to the, conclusion that the dominant nonradiative decay mechanism in these molecules is the intersystem crossing to a bath of vibrationally excited levels of the T-1 electronic state. Our observations reveal no admixtures of T-2 or higher triplet levels. The pathway of the nonradiative electronic relaxation in 2-aminopyridine is found to be independent of the initially prepared vibrational states up to 1000 cm(-1) of vibrational energy. We find no evidence of intramolecular vibrational relaxation preceding the electronic curve crossing. (C) 1995 American Institute of Physics.