Unusual photofragmentation dynamics in the multiphoton ionization of Cr(CO)6 /methanol van der Waals heteroclusters

Abstract

Mixed van der Waals clusters containing Cr(CO)6 and methanol are generated in the free-jet expansion of a pulsed beam of seeded helium and subjected to 248 nm multiphoton ionization (MPI) at moderate laser fluence, and the product ions are analyzed by time-of-flight mass spectrometry. We find that the multiphoton dissociation and ionization dynamics of solvated Cr(CO)6 are strikingly different from those of the naked molecule. Two principal sequences of heterocluster ions are identified in the mass spectrum. A major sequence with the empirical formula SnCr(CO)+x (x=0,1,2), where S is a methanol molecule, first appears in the mass spectrum at n+x=6. A minor sequence with the empirical formula SnCr(CO)+x (x=5,6), first appears in the mass spectrum at n+x=7. We discuss two possible dynamical schemes for MPI of Cr(CO)6 /methanol heteroclusters: One scheme, in which initial photoionization of neutral Cr(CO)6 -containing clusters gives rise to solvated Cr(CO)+6 primary photoions, which subsequently photodissociate to yield the observed coordinatively unsaturated daughter fragments; and an alternative scheme, in which initial photodissociation in the neutral manifold gives rise to one or more coordinatively unsaturated primary photoproducts, each of which subsequently undergoes photoionization. We consider, in a qualitative fashion, the mass spectral fragmentation patterns predicted by these two alternative schemes under conditions of extremely high laser fluence (where essentially all one-photon processes are saturated), and compare these predictions with the observed MPI mass spectra following high-fluence irradiation at both 248 and 350 nm. Assuming that any secondary photodissociation in the ionic manifold occurs statistically, our high-fluence results are inconsistent with a dynamical scheme in which MPI precedes photodissociation. We suggest that the dynamics are correctly described by initial photodissociation in the neutral manifold, followed by MPI of the coordinatively unsaturated primary photoproduct clusters. Intracluster stabilization of nascent photoion internal energy, either through evaporative cooling or collisional V–V energy transfer, is suggested to strongly influence subsequent metal–ligand bond dissociation.