Polarisation labelling spectroscopy technique was employed to study the double minimum 6(1)Sigma(+) state of NaRb molecule. Six lines of an argon ion laser were used to label rovibronic levels in the electronic ground state and the excitation spectra in the 6(1)Sigma(+) <-X(1)Sigma(+) band system were recorded in the spectral range 24 100-26 500 cm(-1). Energies of 901 rovibronic levels of the inner well of the 6(1)Sigma(+) state were determined for two isotopomers of NaRb. The potential curve of the 6(1)Sigma(+) state was constructed using the inverted perturbation approach method. (c) 2005 Elsevier B.V. All rights reserved.