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J. K. G. (Jim) Watson was a world-leading authority on the basic theory of molecular rotation and vibration, especially renowned for his simplification of the rotational and rotation–vibration Hamiltonians (energy expressions). Other significant scientific achievements include the treatment of ‘forbidden’ rotational transitions in symmetric molecules and analysis of the spectrum of the H+3 molecular ion, both of which have important astrophysical implications. Jim was a reticent individual who carried out much of his most important research solo, while at the same time he had a wide circle of collaborators who were often experimentalists seeking theoretical guidance. Although his most profound contributions were highly mathematical, he always endeavoured to present the ultimate results in a widely useful form.

Extensive infrared spectra of the weakly-bound CO$_2$-CO dimer are observed in the carbon dioxide $��_3$ asymmetric stretch region (~2350 cm-1) using a tunable infrared OPO laser source to probe a pulsed slit jet supersonic expansion. Both C-bonded and O-bonded isomers are analyzed for the normal isotopologue as well as for 13CO$_2$-CO and $^{16}$O$^{13}$C$^{18}$O-CO, the latter being the first observation of an asymmetrically substituted form for which all values of Ka are allowed. Combination bands involving the lowest in-plane intermolecular bending modes are also studied for both isomers. Weak bands near 2337 cm-1 are assigned to CO$_2$ hot band transitions (v1, v2l2, v3) = (0111), 26 pages, 5 figures and 5 tables

We report observation of new infrared bands of (CS2)2 and (CS2)3 in the region of the CS2 {\nu}1+ {\nu}3 combination band (at 4.5 cm-1) using a quantum cascade laser. The complexes are formed in a pulsed supersonic slit-jet expansion of a gas mixture of carbon disulfide in helium. We have previously shown that the most stable isomer of (CS2)2 is a cross-shaped structure with D2d symmetry and that for (CS2)3 is a barrel-shaped structure with D3 symmetry. The dimer has one doubly degenerate infrared-active band in the {\nu}1+ {\nu}3 region of the CS2 monomer. This band is observed to have a rather small vibrational shift of -0.846 cm-1. We expect one parallel and one perpendicular infrared-active band for the trimer but observe two parallel and one perpendicular bands. Much larger vibrational shifts of -8.953 cm-1 for the perpendicular band and -8.845 cm-1 and +16.681 cm-1 for the parallel bands are observed. Vibrational shifts and possible vibrational assignments, in the case of the parallel bands of the trimer, are discussed using group theoretical arguments., Comment: 24 pages, 5 figures and 2 tables