Photodissociation spectra of NH4+ (NH3)(n) were measured for n = 3 and 4 in the range of 1070-1680 cm(-1) by using an infrared free electron laser. Three vibrational bands observed for each species are assigned to the intramolecular vibrational modes: the v(2) and v(4) modes of NH3 and the v(4) mode of NH4+. When the first solvation shell is completed at n = 4, the v(2)(NH3) and v(4)(NH4+) wavenumbers are found to be higher by 182 and 36 cm(-1), respectively, than those in free molecules, whereas the v(4)(NH3) wavenumber is lower by 29 cm(-1). (c) 2007 Elsevier B.V. All rights reserved.