Evidence for predissociation of Rb*2(C 1Πu) into Rb*(2P12) and Rb(2S12)

A nozzle beam of rubidium is irradiated by the 4765 A line of the argon ion laser, and the resulting fluorescence is spectroscopically analyzed. Under these collision-free conditions, the emission consists of molecular fluorescence from the Rb₂ C ¹Π - X ¹Σ⁺_g electronic system as well as atomic fluorescence from the Rb ²P_{$\text{3}\text{2}$} fine structure component (the Rb D₂ line) only. The latter transition is caused by partial predissociation (? 25%) of the Rb₂ C ¹Π_u state. Within experimental error, the atomic fluorescence is not polarized and its intensity does not change when an electric field of 14.7 kV/cm is applied across the excitation region. The molecular fluorescence is polarized and has the value P = 0.345 ± 0.0009. This is interpreted to be caused by the superposition of P and R as well as Q lines. Although it is not possible to identify which state causes the predissociation of the Rb₂ C ¹Π_u state, arguments are presented that only the Rb D₂ line would appear provided the predissociation is an adiabatic process.