Chemical physics: Molecular clouds, clusters, and corrals

Vignettes of three frontier areas illustrate the eclectic scope of modern chemical physics. (1) Radioastronomy has revealed a profusion of organic molecules in interstellar clouds, now attributed to sequences of exoergic, bimolecular ion-molecule reactions proceeding far from thermodynamic equilibrium. The organic profusion occurs because ${\mathrm{He}}^{+}$ ions react far more readily with CO than with the much more abundant ${\mathrm{H}}_2$ molecules. (2) Molecular clusters, generated by supersonic expansions, have become a favorite medium for study of reactions and spectra. Exemplary episodes are the discovery of carbon-60 and kindred fullerene molecules and the observation of sharp rotational spectra of guest molecules in superfluid helium clusters. (3) New means to control molecular trajectories are being developed. These include spatial orientation or alignment by field-induced hybridization of rotational states, corralling (or trapping) molecules after collisional or mechanical quenching of translational kinetic energy, and laser control of photochemical reaction pathways.