Using Infrared-Infrared Double Resonance (IIDR) high resolution spectroscopy, we have observed the 20200-21600 cm (1) energy range of the 4(1)Sigma(+)(g) electronic state of Rb-2. The absolute vibrational quantum number assignment is based on the isotope effect using 824 rovibrational levels assigned to the (RbRb)-Rb-85-Rb-85 and (RbRb)-Rb-85-Rb-87 isotopomers. In addition to perturbations between the 4(1)Sigma(+)(g) and 3(1)Sigma(+)(g) state, strong couplings between the 4(1)Sigma(+)(g) and 2(3) Pi(g) states were observed. Preliminary molecular constants and an RKR potential energy curve are given. (C) 2014 Elsevier B.V. All rights reserved.