15min:

MIKHAIL N. SLIPCHENKO, 1605 Gilman Hall, Department of Chemistry, Iowa State University, Ames, IA 50011-3111; BORIS G. SARTAKOV, on leave from: General Physics Institute, Russian Academy of Sciences, Moscow, 119991, Russia; ANDREY F. VILESOV, Department of Chemistry, University of Southern California, Los Angeles, CA 90089, USA.

Infrared spectra and intensities of the ₁, ₃ H-stretching modes of (NH₃)_n clusters (n = 2-4) have been obtained using the helium droplet isolation technique. Formation of a hydrogen bond in ammonia dimers leads to an increase of the infrared intensity by about a factor of four. In the larger clusters the infrared intensity per bond is close to that of dimers and approaches the value in the NH3 crystal. Intensity of the overtone 2 ₄ band in trimers increases by a factor of 10 relative to that in single molecules and in dimers, and is comparable to the intensity of the ₁, ₃ fundamental bands in larger clusters. These results provide new insights into the hydrogen bonding in ammonia clusters.