Spin-forbidden c??+??X??+ 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 3?1+?X 1?+ system has been identified. The vibrational frequency ?e' and anharmonicity ?'exe' of the c 3?+ state are 546.70 and 2.45 cm?1, respectively. The 0-0, 1-0, and 2-0 bands of the c 3?1 +?X 1?+ ystem were rotationally resolved and analyzed, allowing the v'=0, 1, and 2 levels of the c 3?1 + substate to be characterized. From these studies, B'e=0.269 81(3) cm?1, ?'e=0.001 72(3) cm?1, and r'e=1.9979(1) ? were obtained (1? error limits). For these levels the spin-spin coupling constant ?v is identical within experimental error, as ?=?22.5 cm?1. The spin-forbidden c 3?1 +?X 1?+ transition is made allowed by spin-orbit interaction between the c 3?1+ and the B 1II states. Excited state lifetimes of the c 3?1 + and the B 1II states have been measured as 7.11(41) and 0.133(15) ?s, respectively. A spin-orbit analysis shows that the c 3?1 + state is contaminated with 2% B 1II character, which is approximately sufficient to explain the 7 ?s lifetime of the c 3?1+ state.