The branching of nonradiative processes in isoquinoline

We have measured the absolute rates of internal conversion and of intersystem crossing in isoquinoline excited to its S₂ vibronic states (3131-3062 Å) in supersonic beams. The combination of direct detection of the triplets, absorption and excitation spectra allowed us to extract these rates. The intersystem crossing rate barely depends on the excess vibrational energy in the S₂ manifold, being about 2×10 ⁹s^-1. The internal conversion rates grow by more than two orders of magnitude over the range of 0-800 cm^-1 of excess vibrational energy in the S₂ state, up to 5×10 ¹⁰s^-1. At the S₂ electronic origin intersystem crossing is the dominant nonradiative channel, while at vibrational energy exceeding 400 cm^-1 internal conversion becomes dominant.