Knowledge Bank

Infrared spectra and intensities of the $\nu 1$, $\nu 3$ H-stretching modes of (NH$3$) 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$\nu 4$ 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 $\nu 1$, $\nu 3$ fundamental bands in larger clusters. These results provide new insights into the hydrogen bonding in ammonia clusters.