Supersonic jets molecular beam electronic spectroscopy

The supersonic jet molecular beam electronic spectroscopy experiments of Smalley, Levy and Wharton on He l2 were the first to reveal, from lineshape measurements, unambiguous vibrational predissociation lifetimes. 

A few years ago, a powerful version of molecular optical spectroscopy with supersonic beams and jets was developed by Smalley, Wharton and Levy . Supersonic expansion of molecules in an inert carrier gas yields an ideal spectroscopic sample. As a result of the expansion, the translational temperature of the carrier gas decreases to extremely low values (below O.I K). The flow is collisionless so that even extremely unstable species survive. Special attention was paid to fluorescence excitation spectroscopy but the technique is by no means limited to this type of spectroscopy. (Because of fundamental difficulties, however, direct measurement of light absorption in molecular beams is not easy.) Cooled molecules in the beam are electronically excited with a tunable dye laser. The emitted fluorescence is detected and plotted against the wavenumber of the exciting radiation. The obtained fluorescence excitation spectrum is generally very similar to the corresponding absorption spectrum. The technique was used for analysis of the spectra of interesting vdW molecules He. .. NOj, He... Ij, X. .. tetrazine and Xj. .. tetrazine (X = He, Ar, H ) complexes . 

The first gas-phase spectroscopy of nudeobases was reported by Levy and coworkers in 1988 [23]. They measured the electronic spectra of the pyrimidine bases uradl and thymine in a supersonic molecular beam both by LIF and REM PI. The observed spectra were very broad and featureless, in spite of eflSdent jet cooling. 

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