Isolated ultracold porphyrins in supersonic expansions. I. Free-base tetraphenylporphyrin and Zn-tetraphenylporphyrin

In this paper we report the results of an experimental study of the fluorescence excitation spectra and of the time-resolved emission of Zn-tetraphenylporphyrin (ZnTPP) and of free-base tetraphenylporphyrin (H2TPP) seeded in pulsed supersonic expansions of He. We have studied the S0→S1 transition (the Q band) and the S0→S2 transition (the Soret, B band) of ZnTPP, as well as the S0→S1x transition (the Qx band), the S0→S1y transition (the Qy band), and the S0→S2x transition (the Bx band) of H2TPP. Information was obtained on the electronic energy levels, the vibrational level structure, the details of low-frequency nuclear motion, and some characteristics of electronic relaxation.