Femtosecond laser mass spectroscopy of ferrocenes: Photochemical stabilization by bridged cyclopentadienyl rings?

We present time-of-flight mass spectra of ferrocene measured with 200 fs UV laser pulses at various wavelengths. As a striking result, strong parent i on peaks and FeCp+ fragments are observed in the Femtosecond experiment whi le in the nanosecond mass spectra only Fe+ fragment peaks have been detecte d. The latter is ascribed to a fast dissociation of neutral ferrocene after absorption of the first photon. Excitation with femtosecond pulses favors further photon absorption and ionization. The measured fragmentation patter ns are explained by an energetic model assuming switching of absorption fro m the neutral molecule to the ion as soon as the ionization energy is reach ed. The bridging of the two Cp rings in [3]-ferrocenophan seems to stabiliz e the cation but does not suppress the fast production of Fe from electroni cally excited neutral molecules. (C) 2000 Elsevier Science B.V.