Background
The study of the transient species present during a reaction is an essential part of understanding and monitoring the reaction. Given the short lifetimes and reactivity of such species the method of study are necessarily limited, and spectroscopy is by far the most effective and versatile method as many spectroscopic techniques are not limited by the lifetime of the species involved.
Simple carbon containing radicals are a particularly important class of transient species; not only do they have an obvious relevance to combustion processes but they have are also of great interest to astronomers as the wide variety of small molecules detected in the interstellar medium by spectroscopy includes several carbon containing radicals. Furthermore, they are also of great interest from the point of view of understanding electronic structure and bonding. The molecule studied here, C3, has a very complicated electronic structure with several unusual features, and unravelling its electronic spectroscopy presents a significant challenge. Indeed, despite many studies of C3 (stretching back to a 19th century spectrum of a comet ) the UV absorption spectrum is very complicated and still only partially analysed.
The detailed analysis of the UV spectrum of C3 was started by Gausset et al in 1965 who assigned the longer wavelength part of the absorption as the A1Πu - X1Σg+ transition. They found the ground state bending frequency to be extremely low, only 63 cm–1, which explains some of the reasons for the complexity of the spectrum. In addition they found a large Renner-Teller interaction in the excited state; this is the the lifting of the π degeneracy by bending of the molecule. For C3 this causes a splitting of the bending levels by ~350 cm–1, comparable to the bending frequency of 307 cm–1. The system has thus become of considerable theoretical interest as a prototype for Renner-Teller interactions. Despite all the interest in the system, the initial analysis of the A – X transition was only completed in 1995 by Izuha and Yamanouchi when the asymmetric stretch, ν3, in the excited state was determined. This mode was found to be very anharmonic implying a double minimum in the asymmetric stretching co-ordinate. Furthermore the shorter wavelength part of the UV spectrum remains completely unassigned though an extensive system of structured bands is observed; these authors explained the complexity as a forbidden transition to a 1Δu state that is allowed by vibronic mixing.
L. Gausset, G. Herzberg, A. Lagerqvist and B. Rosen, Astrophys. J. 142, 45 (1965).
M. Izuha and K. Yamanouchi, Chem. Phys. Lett. 242, 435 (1995)