MULTIPHOTON EXCITATION OF FERROCENE AND VANADOCENE AT 351-NM IN COMPARISON WITH 248-NM AND 193-NM - WAVELENGTH DEPENDENT COMPETITION BETWEEN IONIZATION AND DISSOCIATION

Multiphoton ionization of ferrocene and sanadocene upon nanosecond laser excitation at 351 nm is reported in comparison with results at 248 and 193 nm. A similar behaviour was found for both compounds at each wavelength. The excitation and fragmentation mechanism was elucidated by the determination of the number of absorbed photons for the ion formation and the analysis of the metastable ion decay. On biphotonic excitation at 248 nm extensive fragmentation of the neutral parent molecule to metal atoms is observed, followed by the ionization of these metal atoms after absorption of two further photons. In contrast, at 193 nm the molecular ion is formed after absorption of two photons and dominates the fragmentation pattern up to moderate laser intensities. At high laser intensities the metal ion, formed after resonant absorption of three photons, becomes prominent. At 351 nm the exclusive formation of molecular ions is observed only near the threshold of the laser intensity for ion detection. Metal ion formation prevails at higher laser intensities involving the absorption of five photons. Most likely this absorption mechanism is resonant.