Spin-forbidden c [sup 3] Sigma[sub 1][sup +] <-- X [sup 1] Sigma[sup +] band system of YF

Optical spectra of jet-cooled diatomic YF have been recorded using resonant two-photon ionization spectroscopy. A vibrational progression corresponding to the c ←X  system has been identified. The vibrational frequency ωe′ and anharmonicity ωe′xe′ of the c  state are 546.70 and 2.45 cm−1, respectively. The 0-0, 1-0, and 2-0 bands of the c ←X  system were rotationally resolved and analyzed, allowing the v′ = 0, 1, and 2 levels of the c  substate to be characterized. From these studies, Be′ = 0.269 81(3) cm−1, αe′ = 0.001 72(3) cm−1, and re′ = 1.9979(1) Å were obtained (1σ error limits). For these levels the spin-spin coupling constant λv is identical within experimental error, as λ = −22.5 cm−1. The spin-forbidden c ←X  transition is made allowed by spin-orbit interaction between the c  and the B  states. Excited state lifetimes of the c  and the B  states have been measured as 7.11(41) and 0.133(15) μs, respectively. A spin-orbit analysis shows that the c  state is contaminated with 2% B  character, which is approximately sufficient to explain the 7 μs lifetime of the c  state.