A general spectroscopic method is described by which samples prepared
in supersonic molecular beams can be characterized independently in th
eir ground and excited electronic states. The centerpiece of this expe
riment is a Fourier transform spectrometer operating at high resolutio
n in the infrared, visible, and ultraviolet. This method is illustrate
d with studies of the NO2 and OClO radicals prepared in supersonic mol
ecular beams at temperatures and pressures characteristic of the Earth
's atmosphere. This general spectroscopic method is discussed by analo
gy with low-and high-resolution studies of electronic states which in
limited cases have been used to characterize sample conditions in supe
rsonic molecular beam experiments.