New insights into the Jahn-Teller effect in NO3 via the dark A2E" state

The recent cavity ringdown (CRD) measurement of the forbidden A E-2 '' <- X (2)A(2) ' transition of the nitrate radical NO3 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 NO3. This paper reviews the current experimental evidence for vibronic coupling in the A 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 A E-2 '' state of NO3 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 X, A and B states.