Progress in electronic spectroscopy of large molecules

The use of lasers and, particularly, the supersonic jet or molecular beam has revolutionised the electronic spectroscopy of molecules of the size of benzene, or larger. With the extreme rotational and vibrational cooling in the jet much of the congestion observed in the spectrum at room temperature is removed. To resolve individual rotational lines it is necessary to remove most of the Doppler broadening. The techniques most often employed are either to use a skimmed supersonic jet, to leave only the core molecular beam, or to arrange to observe only the core. Fluorescence excitation, or resonant multiphoton ionisation, provides vibrational or high-resolution rotational information about the excited electronic state. Single vibronic level (or dispersed) fluorescence is used to obtain vibrational information about the ground state. Structural and conformational information about these molecules, in both their ground and excited states, much of it difficult to obtain in any other way, can be obtained by these techniques.