Infrared molecular beam depletion spectroscopy has been used to measure the spectrum of ammonia dimers embedded in large helium clusters after the excitation of the umbrella mode. The spectral region from 940 to 1010 cm(-1) was covered by a line-tunable CO2-laser. By measuring the dependence of the depletion signal on the pressure of the pick-up cell, we confirmed that all lines are caused by the dimer. The spectrum exhibits two groups of two lines at 978/983 and 992/997 cm(-1) separated by 14 cm(-1) with a line splitting of 5 cm(-1) each. The infrared transitions are only marginally shifted compared to the free ammonia dimer. Also the inversion tunneling splittings are equal to those of the free dimer. Differences in the amplitudes are attributed to relaxation effects of the different inversion components of the same symmetry at the low temperature of 0.4 K. The interchange tunneling splitting, however, which dominates the line splittings in the free dimer is considerably quenched in the helium environment. In addition, the difference in the excitation of the two non-equivalent monomers is remarkably reduced in support of a more cyclic structure in the helium environment. (C) 1997 American Institute of Physics.