A general spectroscopic method is described by which samples prepared in supersonic molecular beams can be characterized independently in their ground and excited electronic states. The centerpiece of this experiment is a Fourier transform spectrometer operating at high resolution in the infrared, visible, and ultraviolet. This method is illustrated with studies of the NO, and OClO radicals prepared in supersonic molecular beams at temperatures and pressures characteristic of the Earth's atmosphere.This general spectroscopic method is discussed by analogy with lowand highresolution studies of electronic states which in limited cases have been used to characterize sample conditions in supersonic molecular beam experiments. stratospheric condition^.^^^ This technique is made pos-*