H2O Clusters Research Articles

Identification of tropospheric ions

Mass spectrometric measurements of naturally occurring atmospheric ions were performed near ground level at Sapelo Island, Georgia. The sampled ions were then passed through a chamber in which they could be dissociated by multiple collisions with a buffer gas. At low‐collision energies, H2O clusters were removed, allowing positive core ions to be identified at masses 18, 60, 80, 94, and 108. At high‐collision energies, positive core ions were dissociated, and the mass of their fragments was used to help determine their identity. Mass 18 is believed to be NH4+. Methylpyridine and dimethylpyridine were tentatively identified as the parent neutral species responsible for the core ions observed at mass 94 and 108.

First mass spectrometric measurements of atmospheric ions at ground level

Naturally occurring positive and negative ions in the troposphere have been mass identified at Sapelo Island, Georgia, and Mt. Evans, Colorado. These measurements were made at ground level under a variety of weather conditions. Over the course of the experiment, positive ions were seen at masses 54, 60, 80, and 94 AMU. Mass 54 AMU is believed to be NH4 + · 2(H2O). In the negative spectrum, ions were observed at masses 62 and 125 AMU and are believed to be NO3− and NO3− · (HNO3). Several ions that appear to be H2O clusters of the above ions were also observed. These are the first measurements of the mass identity (1‐AMU resolution) of naturally occurring ions in the troposphere with the exception of a previous measurement made in 1982 over a limited mass range in an urban environment (Eisele, 1983). Ions that were artificially produced were also studied and compared to the much longer lived natural ions.

Vibrational predissociation spectra and dynamics of small molecular clusters of H2O and HF

Experimental results are presented for the vibrational predissociation spectra in the frequency range 3000–4000 cm–1 for the species (HF)n and (H2O)n, n= 2–6, using molecular-beam techniques and a tunable infrared laser. The observed spectra show a dramatic change between the dimer and larger clusters which is thought to be a result of the cyclic structure of the trimer and larger clusters. The spectra are compared with calculated harmonic force constants of available intermolecular potentials to understand how these small, gas-phase clusters relate to the liquid and solid phases of HF and H2O. Additionally, the angular distributions of the predissociation products show that little energy appears as translational motion of the fragment molecules. This conclusion is consistent with recent theoretical models of the predissociation process. An upper limit of ca. 2 µs is observed for the lifetime of the vibrationally excited clusters.

Heteromolecular clusters of H2O, SO2, CO2, CO and NO

HOMOGENOUS clusters of gas molecules in isentropically expanding jets, first observed in 19611,2, have been the subject of several experimental studies3–7. Mixed or heteromolecular clusters have also been observed in mixtures of two or more gases expanded from a high pressure reservoir. Leckenby et al.8 have identified the species Ar2·CO2, Ar·(CO2)2 and larger clusters, and N2·O2, SF6(CO2)2 and SF6·CO2·Ar. Milne et al.9 have observed mixed H2O–Ar clusters and the heteromolecular dimers O2·H2O and NO·H2O.