Different Photodissociation 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 molecules 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 toward 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 the repopulation from bound excited to dissociative excited states is faster. This may be due to 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.