Isolated ultracold porphyrins in supersonic expansions. III. Free-base porphine

The vibrational level structure of the S₀→S ₁^x transition (the Q_x band) and of the S ₀→S₁^y transition (the Q_y band) of free-base porphine in supersonic expansions of He was interrogated by laser-induced fluorescence excitation spectroscopy. Electronic relaxation in the S₁^x manifold was explored by time-resolved spectroscopy, revealing a constant value of the decay lifetime τ = 9.5±1.0 ns for excess vibrational energies in the range E_v = 0-5000 cm^-1. The line broadening (FWHM)Δ = 1.0-1.5 cm^-1 of the electronic origin and of low-lying vibrational excitations in the S₁^x manifold originates from inhomogeneous unresolved rotational structure, while the large linewidth Δ = 11.9 cm^-1 of the electronic origin of the S₁^y state is due to homogeneous electronic relaxation broadening in the statistical limit. The line shape of the electronic origin of S₁^y was found to be Lorentzian, providing a quantitative determination of the lifetime τ = 5 × 10^-13 s for interstate S₁^y-S₁^x electronic relaxation within a bound level structure of a large isolated molecule.