HCN clusters in molecular beams and cryogenic matrices

Cluster formation of hydrogen cyanide (HCN) was traced by mass spectroscopy in molecular beams and by FTIR spectroscopy in cryogenic matrices in which the clusters were embedded. Both experiments provided evidence for the presence of linear (n greater than or equal to 2) and cyclic structures (n greater than or equal to 3) which were predicted by ab initio calculations as comparably stable. The mass spectra clearly reflect a fundamentally different behaviour of the two types of clusters within the expansion zone: the linear species exhibit the cabability of cluster growth, while the cyclic ones do not. This leads to a characteristic size distribution with two marked maxima. During matrix deposition, considerable distortions of the clusters are created which may be healed out by annealing. The resulting sharp bands in the CH stretching region were assigned in agreement with ab initio calculations and previous IR studies aiming at small linear and cyclic species (n less than or equal to 3) in diverse liquid and solid environments.