ROTATIONALLY RESOLVED IR SPECTROSCOPY OF AMMONIA TRAPPED IN COLD HELIUM CLUSTERS

Infrared molecular beam depletion spectroscopy of ammonia molecules em bedded in large He clusters has been studied in the spectral region of the ammonia umbrella mode from 940 to 1060 cm(-1). The transitions be tween the lowest rotational energy levels could be resolved. By fittin g the spectrum with the Hamiltonian of a free rotor, we observe a larg e shift of the vibrational frequency of 17.4 cm(-1) to the blue, an ef fective decrease of the rotational constant B by 24%, and a reduction of the tunneling splitting by 31% in the excited and 75% in the ground state. This implies an unusually large interaction between the host c luster and the umbrella motion of the ammonia molecule, an intermediat e influence on the rotation and also a large perturbation of the inver sion by the helium environment. (C) 1998 American Institute of Physics .