Vibrational excitation process in NH3 by a mid-infrared free electron laser at Tokyo University of Science (FEL-SUT) is reported. The population of the v(2) = 1 and 2 vibrationally excited states was monitored by the (2 + 1) resonance-enhanced multi-photon ionization (REMPI) technique via the (B) over tilde E-1" Rydberg state. Excitation efficiency of the v(2) = 1 <- v(2) = 0 transition was approximately 30%. The rotationally resolved pathway for the vibrational ladder climbing up to v2 = 2 was identified through the analyses of the REMPI spectra and the FEL excitation spectra. (c) 2006 Elsevier B.V. All rights reserved.