Rydberg Progressions in cis- and trans-Butene

The far- and vacuum-ultraviolet absorption spectra of cis- and trans-butene and trans-butene-d8 have been recorded at low resolution. The more prominent bands are shown to be either the members of Rydberg series, ̄νn = 73 550 − R / (n − 1.03)2 (cis-butene), ̄νn = 73 650 − R / (n − 0.88)2 and ̄νn = 73 750 − R / (n − 0.24)2 (trans-butene), ̄νn = 73 900 − R / (n − 0.86)2 and ̄νn = 74 000 − R / (n − 0.23)2 (trans-butene-d8), or their vibrational subbands. The band observed at 49 690 cm−1 in trans-butene-d8 is seen, at higher resolution, to be polarized along the Z molecular axis, in agreement with its 3sR(Au) ← N(Ag) assignment. Finally, from a partial vibrational analysis of the trans-butene spectrum, the C=C and C–C bonds of the excited states are deduced to be slightly longer than in the ground state.