Wavelength dependence of the multiphoton ionization and fragmentation dynamics of Cr(CO)₆/methanol van der Waals heteroclusters

Mixed van der Waals clusters of Cr(CO)₆ and methanol, generated in a pulsed free-jet expansion of seeded helium, are irradiated by strongly focused UV laser pulses, corresponding to intensities on the order of 10¹²-10¹³ W/cm². Photoions created via multiphoton dissociation (MPD) and ionization (MPI) of these neutral clusters are analyzed by time-of-flight mass spectrometry. We find that the multiphoton dissociation and ionization dynamics of Cr(CO)₆ within van der Waals clusters are quite unusual in comparison to those of the naked molecule, since the observed photoion yields are highly wavelength dependent and since complete ligand stripping does not appear to dominate the photophysics in the neutral ladder. The observed photoions are accounted for in terms of a dynamical scheme wherein the solvated Cr(CO)₆ first undergoes single-photon photodissociation, yielding a solvated coordinatively unsaturated photoproduct, S_nCr(CO)_x. The extent of unsaturation in the primary photoproduct is dependent on photon energy, analogous to gas-phase photodissociation of naked Cr(CO)₆. This neutral species subsequently undergoes MPI, giving rise to a nascent parent cluster ion, S_nCr(CO)_x⁺. If this parent cluster ion is sufficiently excited, it can relax to a distribution of daughter ions by at least three different mechanisms: further ligand loss; intracluster bimolecular reaction with an adjacent solvent molecule, leading to a more thermodynamically stable product; or intracluster V-V energy transfer to the solvent bath. The first of these three alternatives appears to be the dominant energy disposal channel following MPI at 248 nm, while the second of these three alternatives appears to dominate following MPI at 350 nm. CD₃OD is apparently more efficient than CH₃OH in cooling the various photoions via intracluster V-V energy transfer. This may be due to a more favorable correlation of vibrational frequencies and symmetries. Our ability to prepare and spectroscopically interrogate specific chromium carbonyl species within van der Waals clusters holds promise as a general technique for probing the electronic structures of coordinatively unsaturated organometallic complexes.