Conflicting observations resolved by a far IR and UV/Vis study of the NO3 radical.

By codeposition of NO/Ne and O(2)/Ne mixtures at 6 K, weakly bound complexes between O(2) and NO are formed. They exhibit a strong, structured charge transfer UV band at lambda(max)=275 nm. The UV band disappears during UV irradiation of the neon matrix, while the visible spectrum of the NO(3) radical appears. Simultaneously, the fundamental nu(4) of the NO(3) radical in the X (2)A(2)' ground state is observed in infrared absorption for the first time at 365.6 cm(-1). Its (14/15)N and (16/18)O isotopic shifts reveal strong couplings between the two e'-type modes of NO(3), which are both active in a pseudo-Jahn-Teller interaction with the excited B (2)E' electronic state. The dispute on the vibrational fundamentals of the NO(3) radical is now concluded by the unambiguous assignment of combination bands associated with the fundamental nu(4). Taking into account the observed isotopic shifts and estimated anharmonicities for nu(4) and the most intense IR band of NO(3) at 1492 cm(-1) (nu(3)+nu(4)), the frequency of the so far not observed fundamental nu(3) is estimated to be 1100+/-10 cm(-1). A tentative assignment of the vibronic levels in the IR spectrum in the range from 1000 to 3000 cm(-1) is given.