Evolution of the vibrational spectrum of ammonia from single molecule to bulk

Ammonia clusters (NH3)(n) (n=2-10(4)) have been assembled inside helium droplets and studied via infrared laser spectroscopy. The studied spectral range of 3100-3500 cm(-1) covers the nu(1) and nu(3) fundamental stretching bands as well as the 2 nu(4) overtone of the bend of ammonia molecules. The results show strong coupling of the 2 nu(4) overtone with the fundamental vibrations for all cluster sizes except dimers. The intensity of the nu(3) band relative to the total intensity in the spectrum increases from about 30% to about 80% upon increase of the average cluster size from < n >=5 to < n >=10(4). We attributed this effect to the concomitant decrease in the fraction of the surface molecules. The results indicate that ammonia clusters obtained in He droplets have a compact structure and that inner molecules in the clusters have similar hydrogen-bonded coordination as in the crystalline form of ammonia. This surprising result is ascribed to a directionality of the hydrogen bond, which guides the low temperature growth of the cluster in He droplets.