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)mArn clusters having different compositions and architectures.
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Infrared molecular beam depletion and fragment spectroscopy has been employed to study the absorption behavior of small water clusters [(H2O)n, n=2,3,4,5]. The spectral region between 3300 and 3800 cm−1 was covered with an injection‐seeded optical parametric oscillator. Size‐specific information has been obtained by dispersing the cluster beam with a secondary helium beam and measuring the depletion as a function of the scattering angle. Three absorption bands could be assigned to the water dimer (H2O)2, with the bonded OH stretch being localized at 3601 cm−1. For each of the larger water clusters (n=3,4,5), which have cyclic structures, two absorption bands could be identified, one belonging to the free OH stretch and the other being due to the excitation of the OH ring vibration. The measurements on free water clusters were complemented by studies on small water complexes formed on large argon clusters. The positions of the absorption bands observed in these spectra are close to those found for (H2O)n in argon matrices.
