Anion Photoelectron Spectroscopy of Chromium Trimer

The 488 nm partially vibrationally-resolved anion photoelectron spectrum of gas phase Cr₃ˉ displays two active modes with frequencies of 485 and 130 cm^-1 (± 15 cm^-1) in the lowest energy state of neutral Cr₃ observed here. If this is its ground state, then the Cr₃ electron affinity is 1.394 ± 0.010 eV. The vibrational progressions are short, indicating that the geometry in this Cr₃ state is very similar to that in the Cr₃ˉ ground state, which was prepared in a liquid nitrogen cooled flow tube. A second transition from the Cr₃ˉ ground state accesses a Cr₃ state lying 2650 ± 200 cm^-1 (T₀) higher in energy, and shows a more extended vibrational progression in the lower frequency mode, whose frequency in this excited state is 165 ± 15 cm^-1. Weaker transitions to two higher energy Cr₃ electronic states are also observed. At least 20 computational studies have been reported for gas phase Cr₃ or its ions. Their diverse predictions have spanned various spin states, and have included equilateral triangular (D_3h), isosceles (C_2v), scalene (C_S), asymmetric linear (C_∞v), and intrinsically floppy structures. While the vibrational frequencies measured here suggest structures that incorporate a Cr₂-like high order multiple d-d bond and do not appear consistent with D_3h or obtuse C_2v geometries, definitive assignments await the application of improved theoretical methods to this computationally challenging homonuclear triatomic cluster.