Photodissociation Spectroscopy and Photofragment Imaging of the Fe⁺(Acetylene) Complex

Tunable laser photodissociation spectroscopy in the 700–400 nm region and photofragment imaging experiments are employed to investigate the Fe⁺(acetylene) ion–molecule complex. At energies above a threshold at 679 nm, continuous dissociation is detected throughout the visible wavelength region, with regions of broad structure. Comparison to the spectrum predicted by time-dependent density functional theory (TD-DFT) indicates that the complex has a quartet ground state. The dissociation threshold for Fe⁺(acetylene) at 679 nm provides the dissociation energy on the quartet potential energy surface. Correction for the atomic quartet–sextet spin state energy difference provides an adiabatic dissociation energy of 36.8 ± 0.2 kcal/mol. Photofragment imaging of the Fe⁺ photoproduct produced at 603.5 nm produces significant kinetic energy release (KER). The photon energy and the maximum value of the KER provide an upper limit on the dissociation energy of D₀ ≤ 34.6 ± 3.2 kcal/mol. The dissociation energies determined from the spectroscopy and photofragment imaging experiments agree nicely with the value determined previously by collision-induced dissociation (38.0 ± 2.6 kcal/mol). However, both values are significantly lower than those produced by computational chemistry at the DFT level using different functionals recommended for transition-metal chemistry.