We attributed these nominally forbidden peaks to pseudo-Jahn-Teller coupling with the NO3 B state, based on similar reasoning by Cederbaum and co-workers used to explain the photoelectron spectrum of isoelectronic BF3, an interpretation supported by subsequent theoretical work on NO3. In 2007, Stanton analyzed the spectroscopic data involving the ground state of NO3 and proposed that the previously observed feature at 1492 cm -1 in its infrared spectrum was not the 3=1 fundamental of the degenerate antisymmetric stretch, but instead was the 1+4 combination band. Subsequent work by Stanton suggested that the 1 and 3 frequencies are both just above 1000 cm , and that the feature assigned to the 1 0 1 transition in Figure was predominantly the Franck-Condon forbidden 3 0 1 transition. A paper in 2018 by Hirota questioned the existence of strong vibronic coupling between the X and B states, and proposed that peak a in the photoelectron spectrum was the Franck-Condon allowed 4 1 1 hot band transition rather than the F-C forbidden 4 0 1 transition.