Optical–Optical Double Resonance Spectroscopy of the 1 g( 3P 1)- A3Π (1 u)- X1Σ g+ Transition of I 2

Optical–optical double resonance spectroscopy was used to study the 1 g ( 3P 1) ion-pair state of I 2 correlating to I −( 1S)+I + 3P 1) at the dissociation limit. We gained access to the 1 g ( 3P 1) state though the A 3Π (1 u ) state in the (1+1) photon-excitation scheme. The pump laser excited the A 3Π (1 u )-X 1Σ g + transition at a fixed frequency for state selection. The probe laser was scanned to detect the 1 g ( 3P 1)- A 3Π (1 u ) resonance by monitoring the ultraviolet emission from the 1 g ( 3P 1) state at 278 nm. The 1 g ( 3P 1) state was observed in a vibrational progression consisting of P and R doublets. An energy level analysis was carried out for the 1 g ( 3P 1) state in the 0≤ v ≤ 14 and 12≤ J≤135 range, which led to a set of molecular parameters including the Ω-doubling constant. The Ω-doubling of the 1 g( 3P 1) state was discussed by the pure precession model and interpreted to occur through the heterogeneous coupling with the 0 g −( 3P 1) state correlating to the same ionic asymptote.