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The electronic spectrum of tetrahydrofuran has been observed between 180 and 210nm. Two electronic transitions have been identified; a system of sharp bands with an origin at 199nm (50188 cm-l) and a system of diffuse bands beginning at about 195 nm (51400 cm-I). A vibrational analysis of the sharp system has been carried out, and the transition has been shown to be from the non-planar ground state to an upper state in which the ring is planar. From hot bands, six ground state ring puckering intervals have been identified. The corresponding upper state intervals are small and very unharmonic, ranging from 65 to 124 cm-'. The latter levels have been fitted to a potential function which is almost a two dimensional quartic oscillator. TABLE 1 .-FREQUENCIES AND ASSIGNMENTS * FOR THE 199 nm SYSTEM wavenumber/cm-l assignment t