Photodissociation Spectroscopy of the Chromium Trimer Ion

The electronic and geometric structures and photodissociation dynamics of the chromium trimer ion, Cr3+, were investigated by photodissociation spectroscopy in the photon-energy range from 1.32 to 5.52 eV. The branching fractions of the product ions, Cr+ and Cr2+, exhibit stepwise changes at the threshold energies for dissociation into Cr++Cr2, Cr+Cr2+, Cr++2Cr, and Cr*+Cr2+. It is noted that Cr2+ is produced even above the threshold for atomization; the excess energy is redistributed to produce a fragment atom, Cr*, in an excited state. The photodissociation action spectrum is well explained by a mixture of simulated spectra for two nearly-degenerate structural isomers identified by density functional calculations: those having a metastable C2v structure and the most stable structure slightly distorted from the C2v one. The barrier height between the two isomers which is lower than the zero-point energy suggests that Cr3+ has an intrinsically floppy structure.