New insights into the Jahn–Teller effect in NO 3 via the dark à 2 E ″ state

The recent cavity ringdown (CRD) measurement of the forbidden transition of the nitrate radical NO 3 reveals a rich, well-resolved spectrum in the near-infrared. The spectroscopic detail provides a new window onto the Jahn–Teller (JT) and pseudo-Jahn–Teller (PJT) effects in NO 3 . This paper reviews the current experimental evidence for vibronic coupling in the à state and discusses the theoretical issues in the context of new preliminary EOMIP/CCSD and CCSD(T) calculations. The theoretical results to date indicate that the à 2 E '' state of NO 3 undergoes a relatively strong JT distortion which may require inclusion of higher order vibronic couplings. The intensity of this transition may involve multiple intensity borrowing mechanisms via PJT coupling among the , and states.