Figure 1. Depletion spectra of NH3 panel a and H2O panel b single molecules and clusters in He droplets of about 3500 atoms with an average number of about 1.5 and 2 captured molecules per droplet, respectively. Vertical dashed lines give the frequencies of the vibrational band origins of monomers. Sharp rovibrational lines of monomers are assigned Refs. 25 and 26. Additional strong bands are assigned to the absorption of the (NH3)k and (H2O)k clusters, which have been enhanced by hydrogen bonding. Cluster size, k, is indicated by numbers. Spectral peaks a – d in panel b are assigned to the dangling OH bonds of water clusters of different size.