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, ${\mathrm{̄\nu }}_n{\text{\hspace{0.17em}=\hspace{0.17em}73 550\hspace{0.17em}−\hspace{0.17em}R\hspace{0.17em}/\hspace{0.17em}(n\hspace{0.17em}−\hspace{0.17em}1.03)}}^2$ (cis‐butene), ${\mathrm{̄\nu }}_n{\text{\hspace{0.17em}=\hspace{0.17em}73 650\hspace{0.17em}−\hspace{0.17em}R\hspace{0.17em}/\hspace{0.17em}(n\hspace{0.17em}−\hspace{0.17em}0.88)}}^2$ and ${\mathrm{̄\nu }}_n{\text{\hspace{0.17em}=\hspace{0.17em}73 750\hspace{0.17em}−\hspace{0.17em}R\hspace{0.17em}/\hspace{0.17em}(n\hspace{0.17em}−\hspace{0.17em}0.24)}}^2$ (trans‐butene), ${\mathrm{̄\nu }}_n{\text{\hspace{0.17em}=\hspace{0.17em}73 900\hspace{0.17em}−\hspace{0.17em}R\hspace{0.17em}/\hspace{0.17em}(n\hspace{0.17em}−\hspace{0.17em}0.86)}}^2$ and ${\mathrm{̄\nu }}_n{\text{\hspace{0.17em}=\hspace{0.17em}74 000\hspace{0.17em}−\hspace{0.17em}R\hspace{0.17em}/\hspace{0.17em}(n\hspace{0.17em}−\hspace{0.17em}0.23)}}^2$ (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 ${\text{3sR(A}}_u{\mathrm{)\hspace{0.17em}\leftarrow \hspace{0.17em}N(A}}_g)$ 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.