Dissociation Dynamics and Electronic Structures of Highly Excited Ferrocenium Ions Studied by Femtosecond XUV Absorption Spectroscopy.

The dissociation dynamics of ferrocene are explored following strong field ionization using femtosecond time-resolved extreme ultraviolet (XUV) transient absorption spectroscopy. Employing transitions in the vicinity of the iron 3p (M2,3) edge, the dissociation is monitored from the point of view of the iron atom. With low strong field pump intensities (≈2 × 1013 W cm-2), only ferrocenium cations are produced, and their iron 3p absorption spectrum is reported. It very closely resembles the 3p spectrum of atomic Fe+ ions but is red-shifted by 0.8 eV. With the aid of time-dependent density functional theory calculations, the spectrum is assigned to a combination of doublet and quartet spin states of ferrocenium ions. Ionization with more intense strong field pump pulses (≥6 × 1013 W cm-2) leads predominantly to the prompt production of ferrocenium ions that dissociate to give the spectral signature of bare Fe+ ions within 240 ± 80 fs. Within the temporal resolution of the experiment (≈40 fs), no spectral intermediates are observed, suggesting that the dissociation process occurs directly from the excited ferrocenium ion and that the bonds between the iron center and both cyclopentadienyl rings are broken almost simultaneously in an asynchronous concerted decay process. No evidence of slower dissociation channels is observed at a pump-probe delay of 250 ps, suggesting that all energy is very rapidly routed into dissociative states.