The dynamics of the interaction of the Rydberg E((1)Sigma(+)) state and the valence state V((1)Sigma(+)) of HBr and DBr were investigated using a reflectron time-of-flight mass spectrometer coupled with a femtosecond laser system. Interrogation of the state formed by the avoided crossing of the Rydberg and valence states revealed dynamic behavior that was different for the two isotopes. Pump-probe experiments on HBr showed no change in the lifetime over the range of pump wavelengths of 256.7-254.7 nm. However, the lifetime of DBr decreased as the pump wavelength was tuned bluer. Tunneling is involved in the evolution of the population in photoexcited Rydberg state to the ion-pair state. (C) 2000 American Institute of Physics. [S0021-9606(00)02210-8].