Direct absorption spectroscopy of water clusters formed in a continuous slit nozzle expansion

In this article, we report on a Fourier transform infrared study of absorption bands belonging to small-sized water clusters formed in a continuous slit nozzle expansion of water vapor seeded in argon carrier gas. Clear signatures of free and H-bonded OH vibrations in water aggregates from dimer to pentamer are seen in our spectra. Following an increase in argon backing pressure, the position of the cluster absorption bands varies from those characteristics of isolated water aggregates in the gas phase to those known for clusters trapped in a static argon matrix. These variations can be interpreted in terms of sequential solvation of the water clusters by an increasing number of argon atoms attached to water clusters. Our measured spectra are in good agreement with those obtained previously either for free or Ar coated small-sized water clusters using pulsed slit-jet expansions. Our results are equally in accord with those originating from a variety of tunable laser based techniques using molecular beams or free jets or from the study of water aggregates embedded in rare gas matrices. Distinctions are reported, however, and discussed. Ab initio calculations have made it possible to speculate on the average size of an argon solvation shell around individual clusters as well as on the development of the OH stretch vibrational shifts in mixed (H2O)(m)Ar-n clusters having different compositions and architectures.