Nanosecond multiphoton ionization spectroscopy of transition-metal sandwich compounds: a comparative study of nickelocene, ferrocene and bis(η6-benzene)chromium

Efficient multiphoton ionization of nickelocene, ferrocene and bis(η6-benzene)chromium was achieved on excitation of jet-cooled sandwich molecules with nanosecond pulses of dye lasers via an intermediate low-lying Rydberg 4p x , y level. One-colour photoionization mass spectra revealing solely molecular ion signals were obtained. The signal intensity increases significantly when an intense pulse of a second dye laser is used for ionization of the Rydberg-state molecules. Two-colour resonance-enhanced multiphoton ionization spectra of Cp2Fe (Cp = η5−C5H5) and Bz2Cr (Bz = η6−C6H6) show vibronic structures of the a1g(3d z 2) → e1u(R4p x , y ) transition. The metal–ligand symmetric stretch ν 4 wavenumbers for the Rydberg-state molecules are 310 and 263 cm−1, respectively. Multiphoton mass-analysed threshold ionization (MATI) signals appear for Bz2Cr when the and levels of the Rydberg state are ionized. The Bz2Cr ionization potential was determined on the basis of the MATI spectrum as 44 087 and 17 333 cm−1 for the ground-state and Rydberg-state molecule, respectively. The ν 4 wavenumber for the ground-state Bz2Cr+ cation is 262 cm−1 as determined from the MATI spectrum. The influence of electronic structure on the photoionization behaviour of the sandwich molecules is discussed.