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 las er 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 w avelength. The excitation and fragmentation mechanism was elucidated b y the determination of the number of absorbed photons for the ion form ation and the analysis of the metastable ion decay. On biphotonic exci tation at 248 nm extensive fragmentation of the neutral parent molecul e to metal atoms is observed, followed by the ionization of these meta l atoms after absorption of two further photons. In contrast, at 193 n m the molecular ion is formed after absorption of two photons and domi nates the fragmentation pattern up to moderate laser intensities. At h igh 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 in tensity for ion detection. Metal ion formation prevails at higher lase r intensities involving the absorption of five photons. Most likely th is absorption mechanism is resonant.