A crossed laser-molecular beam study of the one and two photon dissociation dynamics of ferrocene at 193 and 248 nm

Abstract

A crossed laser-molecular beam study of the one and two photon dissociation mechanism of bis (cyclopentadienyl) iron (ferrocene, FeCp2) has been performed at 193 and 248 nm. By combining electron bombardment mass spectroscopy with time-of-flight (TOF) measurements, the photodissociation mechanism at 193 nm is shown to have two distinct mechanisms. (1) FeCp2+hν→FeCp*+Cp; (2) FeCp+2hν→FeCp+Cp, FeCp→Fe+Cp. For the first mechanism, which accounts for less than 5% of the photodissociation events, the FeCp* velocity distribution is quantitatively consistent with a statistical dissociation producing FeCp in an excited, ligand field electronic state. The velocity distributions of the Cp and Fe fragments produced by the second mechanism (FeCp is an unstable intermediate) are also in excellent agreement with microcanonical calculations for both Cp elimination steps using the known metal–ligand bond energies of ferrocene. For the second mechanism, dissociation occurs on the lowest potential energy surface for each Cp elimination. Although one photon is energetically sufficient to remove one Cp ligand from ferrocene, RRKM calculations of the lifetime indicate that Cp elimination is extremely slow for dissociation along the ground electronic state potential energy surface. Hence, after internal conversion to the ground electronic state, the large photon absorption cross section (∼4 Å2) for the experimental irradiation conditions allows additional photons to be absorbed until the dissociation rate exceeds the up pumping rate. The large photon energy causes the dissociation rate to increase by many orders of magnitude for each additional photon absorbed. Consequently, there is strong selectivity for the total number of photons absorbed. Both mechanisms, occurring on two different electronic potential energy surfaces, suggest that dissociation induced by excitation of the ligand-to-metal charge transfer states accessed at 193 nm can be quantitatively described as a statistical, unimolecular decomposition. At 248 nm, the measured product velocity distributions are qualitatively consistent with the mechanism deduced from the 193 nm results, but the energy available for translation at this wavelength is too small to extract quantitative product translational energy distributions which are required to independently test the applicability of the statistical dissociation model.

Dissociative Intramolecular Charge Transfer after Local Two-Photon Ionization of Ferrocene Derivatives with Aromatic Chromophores

“…In a series of multiphoton dissociation (MPD)/multiphoton ionization (MPI) studies of ferrocene − with nanosecond (ns) laser pulses at different wavelengths in the visible and near UV (UV/VIS) it has been shown that the cyclopentadienyl (cp) ligands are lost prior to ionization, and as a consequence, no intact ferrocene cations can be observed in the mass spectra. In the intensive laser radiation field the central iron atom loses its cp rings in one or more MP steps, and the uncaged iron atom can be ionized and detected by a MPI process when the laser wavelength is close to an atomic resonance.…”

Wavelength Dependence of Photooxidation vs Photofragmentation of Chromocene

“…[16][17][18][19][20][21][22][23][24][25][26][27] The wavelength dependence of the total ion current spectrum for ferrocene and cobaltocene in the ranges 366-370, 380-390, and 445-455 nm and for nickelocene in the range 375-520 nm was reported. 20,24,25 The features in the excitation spectrum were attributed solely to the metal ion.…”

Synthesis of magnetic alloy-filling carbon nanoparticles in super-critical benzene irradiated with an ultraviolet laser

“…It is also known that the dissociation energy of a second hydrogen atom is lower than that of the first one [38] and therefore six-membered rings of carbon atoms may be quite easily produced by irradiation of photons of 266 nm wavelength, noting that the possibility of multi-photon absorption increases as the state of benzene approaches its critical point due to the formation of large flexible molecular clusters [38] . The decomposition energy of Fe(cp) 2 into Fe + cp + cp is 6.8 eV [39] , whereas that of Co(cp) 2 into Co + cp + cp is 5.64 eV [40] and therefore the decomposition of ferrocene and cobaltocene into iron, cobalt and cp-rings was induced by two-photon absorption during the laser irradiation. Dissociated iron and cobalt atoms were cooled to form the core particles and the carbon layers covering the core particles were formed by cp-rings and six-membered rings of carbon atoms during the interval between two pulses of laser beams [41] .…”