The Far Ultraviolet Absorption Spectra and Ionization Potentials of C6H6 and C6D6

Abstract

The absorption spectra of C₆H₆ and C₆D₆ have been photographed under high dispersion in the region 2000–1000A. The spectra of both substances are very similar. A strong continuous absorption starts fairly sharply around 1840A and gradually weakens out to zero at 1600A. It is followed by a region extending down to about 1360A which contains very strong sharp bands. The strongest of these bands only suffer small shifts to the violet in going from C₆H₆ to C₆D₆. This shift which is due to the change in the difference of zero‐point energy between the normal and the excited states reveals the bands to be vibrationless electronic transitions. It was found that these bands could be arranged into two Rydberg series which had approximately the same limit corresponding to an ionization potential of 9.190±.005 volts for C₆H₆. The ionization potential of C₆D₆ is only about 3 × 10^—4 volts greater than that for C₆H₆. All the electronic states are split up into patterns which are believed to be caused by true electronic multiplicity. The excitation is apparently from a nonbonding electron more strongly attached to the carbon atom. The vibrations which accompany the bands are comparatively weak. However for C₆H₆ the following vibration frequencies associated with the excited states were observed. ω₁ = 677 cm^—1, ω₂ = 968 cm^—1. These become 630 and 926 cm^—1 for C₆D₆. Tentative assignments of their modes of vibration are made. The bands below 1360A are considerably more diffuse than those previously mentioned and the isotopic shifts to the violet are much larger for these bands. However they are still believed to be vibrationless electronic transitions though they are due to the excitation of an electron which is considerably more bonding than that corresponding to the first ionization potential. A consideration of them enables the approximate value 11.7±.3 volts to be predicted for the second ionization potential of benzene.