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

The vibrational level structure of the S0→S1x transition (the Qx band) and of the S0→S1y transition (the Qy band) of free‐base porphine in supersonic expansions of He was interrogated by laser‐induced fluorescence excitation spectroscopy. Electronic relaxation in the S1x 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 Ev=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 S1x manifold originates from inhomogeneous unresolved rotational structure, while the large linewidth Δ=11.9 cm−1 of the electronic origin of the S1y state is due to homogeneous electronic relaxation broadening in the statistical limit. The line shape of the electronic origin of S1y was found to be Lorentzian, providing a quantitative determination of the lifetime τ=5×10−13 s for interstate S1y–S1x electronic relaxation within a bound level structure of...

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