Different photo-dissociation mechanisms in Fe(CO)5 and Cr(CO)6 evidenced with femtosecond valence photoelectron spectroscopy and excited-state molecular dynamics simulations

Measured and calculated time-resolved photoelectron spectra and excited-state molecular dynamics simulations of photoexcited gas phase Fe(CO)5 and Cr(CO)6 are presented. Samples were excited with 266 nm pump pulses and probed with 23 eV photons from a femtosecond high- order harmonic generation source. Photoelectron intensities are seen to blue-shift as a function of time from binding energies characteristic of bound electronic excited states via dissociated-state energies towards the energies of the dissociated species for both Fe(CO)5 and Cr(CO)6 but differences are apparent. The excited-state and dissociation dynamics are found to be faster in Cr(CO)6 because repopulation from bound excited to dissociative excited states is faster. This may be due to a stronger coupling between bound and dissociative states in Cr(CO)6, a notion supported by the observation that the manifolds of bound and dissociative states overlap in a narrow energy range in this system.