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Many students today are profoundly interested in the sustainability of their world. With growing public concern over global warming and greenhouse gases, students want to understand how human actions affect the health of our planet. Students are deeply concerned about pollution. They practice recycling. Moreover, they want to secure a healthy Earth for future generations. As students of chemistry, they have a unique opportunity to start at the ground floor of the exciting and expanding field of green chemistry.

A new molecular beam photodissociation spectrometer has been constructed with which it has been possible to measure, for the first time, the vibrational spectrum of a molecule attached to liquid helium clusters. The clusters are produced in a free jet expansion from a cold nozzle. They are then doped with SF6 by passing them in a scattering (pickup) cell containing this gas at a pressure of the order of 10-4 Torr, and finally they are interrogated spectroscopically by crossing them with the output of one of four line tunable lasers. Four absorptions have been detected, located at 932.9,945.8,946.1, and 954.1 cm-I. Monitoring the signal dependence on the density of the SF6 pickup gas, the two central absorptions, found to grow linear with that density, have been assigned as due to the SF6 monomer. The two outer absorptions at 932.9 and 954.7 cm-I, because of the quadratic dependence from the SFs pickup density, have been assigned to the dimer. The pressure of two absorptions due to the monomer indicates for the latter a nonsymmetric location near the surface of the cluster.This assignment is critically evaluated in the light of several preliminary theoretical calculations and a recent electron bombardment ionization experiment, all of which indicate that the more likely location for the SF6 molecule is in the cluster's interior. On the other hand, in the discussion, it is also pointed out that all the calculations performed so far miss at least one of the essential ingredients for a correct estimate of the "impurity" position within the cluster. The paper concludes with a discussion of the possible influence of the cluster's superfluidity on the observed spectra and with a critical assessment of the experimental challenges which stand between us and the possibility of measuring, in the same environment, rotationally resolved vibrational spectra.

It is demonstrated that matrix-like spectroscopy may be carried out in the gas phase using molecular beams of clusters and the conditions under which bulk-matrix-like behavior is achieved are illustrated. At the same time, we obtain information on the structural evolution of noble gas clusters as a function of their size. Infrared spectra for SF6 attached to noble gas clusters of argon, krypton, and xenon were recorded using a free jet cluster source and a laser photofragmentation detection technique. When a dilute mixture of the chromophore in Ar and Kr is expanded at relatively low pressures, the clusters spectra show a feature characteristic of the SF6 solvated in a defective, unannealed matrix. This feature disappears at higher source pressures (larger sizes) at which the chromophore prefers to reside on the surface of the cluster. This can be demonstrated by producing neat clusters and depositing the chromophore on them. However, on producing still larger clusters, a different absorption appears which is accurately located at the same position as the main absorption in a well-annealed matrix of Ar or Kr. This behavior is related to the transition of clusters from a Mackay icosahedral structure, shown to be the most stable for smaller clusters, to the face-centered-cubic (fcc) structure which is observed in the bulk phase. This structural transition occurs at a nozzle stagnation pressure which corresponds to an average cluster size of about 2000 atoms for both Ar and Kr. Scattering studies performed on argon clusters suggest that the fcc-type clusters correspond to the largest sizes in the cluster size distribution present in the beams. A similar structural transition for xenon cluster was not established as the SF6 appears to solvate only slightly in Xe in the size range studied here.

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