As described in Chapter 11, laser-induced fluorescence (LIF) spectroscopy is one of the simplest and yet most powerful tools for obtaining high resolution spectra. Its high sensitivity is particularly convenient for the investigation of extremely reactive molecules, such as free radicals and ions. In this Case Study we illustrate how LIF spectroscopy can be used to obtain important information on a small carbon cluster, the C3 molecule. The spectra presented were originally obtained by Rohlfing [1], who produced C3 by pulsed laser ablation of graphite. This is a violent method for vaporizing a solid and the plasma formed above the graphite surface will undoubtedly contain carbon atoms, clusters such as C2, C3, and various cations and anions. To reduce spectral congestion, the laser ablation source was combined with a supersonic nozzle to produce a cooled sample for spectroscopic probing.

The LIF spectrum was obtained by crossing the supersonic jet with a tunable pulsed laser beam and measuring the intensity of fluorescence as a function of laser wavelength. As discussed in Section 11.2, an LIF excitation spectrum is similar to an absorption spectrum but the signal intensity depends not only on the absorption probability, but also the fluorescence quantum yield of the upper state.