Observation of the singlet-triplet pair of the 4p Rydberg state and assignment of the Rydberg series of SO2

The triplet (g) over tilde Rydberg state of SO2 with an origin at 78 245.8 cm(-1), 526 cm(-1) below the previously detected singlet (G) over tilde state origin, has been observed in a two-color resonantly enhanced multiphoton ionization experiment. The bending vibrational levels (v=0-5 with intervals of 400 +/- 10 cm(-1)) has been identified and reported. Based on the symmetry, bending frequencies, quantum defects, and term values, this (G) over tilde state singlet-triplet pair was assigned to the sulfur 4p state belonging to the series converging to the SO2+ (X) over tilde (2)A(1) state. The same procedure also allows the reassignment of the Rydberg state previously observed in the 111.5-115 nm region to the sulfur 4p orbital (converging to the SO2+ (A) over tilde (2)A(2) state) and the one in the 106.3-110.4 nm region to the 5p orbital (converging to the SO2+ (X) over tilde (2)A(1) state). The spectral patterns also indicate that the (G) over tilde states have a detectable increase in the dissociation rate above the v(2)=2 level. The triplet state near the 193 nm region is dissociative with a homogeneous width of similar to 13 cm(-1). (C) 2000 American Institute of Physics. [S0021-9606(00)00505-5].