As an Emeritus professor I have no lab or students, but I continue to be active in collaborative research. My principal collaborations are with Prof. R.W. Field (MIT) and with Dr. Y.-C. Hsu (Institute of Atomic and Molecular Sciences, Taipei, Taiwan).
My collaboration with Prof. Field is on the near ultraviolet electronic spectrum of acetylene. In this transition the molecule goes from its (linear) ground state to its first excited singlet state, S1, where it is trans-bent. The spectrum has been studied by many authors since the first successful analyses in 1953, with hundreds of papers now in the literature, but the story is by no means complete. My contribution has been to the detailed analysis. Working with new high resolution laser-induced fluorescence spectra of acetylene taken at MIT, I showed that the nu1 vibrational frequency of the S1 state had been mis-assigned previously. This re-assignment opened the way for a full analysis of the system from jet-cooled spectra, from which I showed that the bending overtones of the S1 state suffer from strong Darling-Dennison (anharmonic) resonance. It was then possible to give an essentially complete vibrational assignment up to a vibrational energy of 4300 cm-1. Theoretical work had predicted that the S1 potential surface should also support a cis-bent isomer but, since this transforms as A2 in the C2v point group, it was thought that it would never be observable because transitions to it from the ground state are forbidden by the electric dipole selection rules. Although every expected vibrational level of the trans isomer had been accounted for, a few bands still remained unassigned, and I was able to show that these represent transitions to levels of the cis isomer near the barrier to cis-trans isomerization, where the upper states pick up intensity by interaction with nearby levels of the trans isomer. A complication is that quantum mechanical tunnelling through the isomerization barrier takes place, which gives rise to an even-odd staggering ot the K-rotational structure of the vibrational levels. At present there is no simple theory to predict the magnitudes of these staggerings, so that the spectrum becomes increasingly irregular; work is continuing on the assignment of the levels near and above the isomerization barrier.
The triatomic carbon radical, C3, is important in combustion processes, and features also in the spectra of various astronomical objects. It has a strong electronic transition in the 330-410 nm wavelength region. Working with jet-cooled laser-induced fluorescence spectra taken at Taipei, I was able to clarify the rotational assignments of the astrophysically-important 405 nm band, and to assign various perturbations at higher energy. At very low temperatures C3 forms van der Waals complexes with the noble gases. My analyses of high resolution spectra of C3Ar have shown that the complex is T-shaped, with the argon atom lying 3.8 Å from the centre of mass of the C3 radical. The excited electronic state of the C3 radical is orbitally degenerate, but the degeneracy is lifted in the lower symmetry of the C3Ar complex; however the orbital angular momentum does not go away, but re-appears as Coriolis (rotational) coupling between the two close-lying component electronic states. The emission spectra of C3Ar are very complicated because there is strong intramolecular vibrational relaxation in the upper states; the energy cascades to the lowest vibrational level of each of the vibrational potentials.