Laser Spectroscopy of Large Polyatomic Molecules in Supersonic Jets

Abstract

Spectroscopic study of large polyatomic molecules had long been pre­ vented by spectral complexity. The complexity arises from the fact that a large molecule has a large number of vibrational degrees of freedom. Another complication comes from the thermal distribution of molecules over many ground-state vibrational levels, which causes many hot bands to appear in the spectrum. Molecular collisions in gas and condensed phases also contribute to broadening of spectral bands. All these com­ plicating factors become increasingly significant as a molecule becomes larger. Eventually, the spectrum becomes a continuous feature, even in the gas phase, owing to heavy spectral band congestion. A typical example of this spectral congestion is seen in the electronic absorption spectrum of biphenyl in the gas phase. The S 1 <So absorption spectrum of benzene is well known to exhibit prominent vibrational structure. However, in bi­ phenyl, which is composed of two benzene rings, the corresponding spec­ trum is completely structureless (1). We often encounter such a broad and structureless absorption spectrum when a molecule is larger than benzene. Clearly, detailed information on the energy levels cannot be obtained from such a broad spectrum. The recent development of the supersonic jet technique (2-5) has greatly improved the above situation. Supersonic expansion of sample molecules, seeded in high-pressure rare gas, into vacuum through a small nozzle