The absorption spectrum of diacetylene in the vacuum ultraviolet

The spectra of diacetylene-h2 and diacetylene-d2 have been photographed in the vacuum ultraviolet region as far as 1000 Å. Comparison of the spectra of the normal and deuterated molecules has led to an extensive vibrational analysis. A revised assignment of the Rydberg origin bands is proposed. This gives a new spectroscopic value of the first ionization potential of 10.180 ± 0.003 eV, in agreement with the electron impact value. Some bands at shorter wavelengths are arranged in a new Rydberg series leading to the second ionization potential, for which a value of 12.62 ± 0.04 eV is found. The results of the analysis correlate in a satis­factory way with the 'T' spectrum of the diacetylene positive ion. The mean of the first and second ionization potentials of diacetylene is found to be equal within experimental error to the first ionization potential of acetylene. Some anomalous features of the spectrum are noted and discussed. The Fermi resonance in the ‘T’ spectrum between v3 and 2v7 is also discussed, and it is concluded that the current assignment should be reversed.