Electronic Spectroscopy by the Electron Impact Method

A previously constructed variable angle electron-impact spectrometer has been rebuilt and used to study electronic excitation spectra in the energy-loss range from 0 eV to beyond 15 eV for 13 molecules. Spectra were obtained in the impact energy range from 20 eV to 70 eV and for scattering angles from 0° to 80°. The differential scattering cross sections were obtained for several low-lying electronic transitions and used to determine whether a transition was spin-allowed or spin-forbidden. Further assignments of some transitions were made by comparison of the present results with earlier theoretical and experimental studies. In the 1, 3-conjugated polyenes the three lowest energy excited states are the 1 3Bu, the 1 3Ag and the 1 3Bu. These states have been studied here and produce peaks in the energy-loss spectrum at 3.22 eV, 4.91 eV, and 5.92 eV in s-trans-1, 3-butadiene, at 3.14 eV, 4.87 eV, and 5.80 eV in 1-trans-3-pentadiene, at 3.20 eV, 4.93 eV, and 5.77 eV in 1, 3-hexadiene, and at 3.11 eV, 4.85 eV, and 5.69 eV in cis- 2-trans-4-hexadiene. The energies of the low-lying transitions in the 1, 3 and 2, 4 non-cyclic dienes are nearly invariant with respect to alkyl substitution. This indicates that these are π → π* transitions and that the σ,π separability approximation is applicable to these molecules. In 1, 3-cyclohexadiene the state which is equivalent to the 1 3Ag is not seen but transitions to the states which are equivalent to the 1 3Bu and the 1 1Bu states produce peaks at 2.94 eV and 4.94 eV, respectively. The shifts in the transition energies from the noncyclic diene values are probably associated with the increased closeness of the two π bonds. In 1, 3, 5-hexatriene the energies of the transitions to the 3Bu, 1 3Ag, and 1 1Bu states are 2.61 eV, 4.11 eV, and 5.13 eV, respectively. There was no evidence for a weak singlet → singlet transition below the X1 Ag → 1 1Bu transition in contrast to the results of recent studies on larger substituted polyenes. Some non 1,3-conjugated polyenes were studied. The π → π*, singlet → triplet transition produced a peak at 4.25 eV in 1, 4-hexa- diene, 4.25 eV in 1,5-hexadiene and 4.29 eV in 1, 4-cyclohexadiene. The strong π → π*, singlet → singlet transition peaks at 6.84 eV, 7.00 eV, and 7.95 eV in these molecules. The results for the non-cyclic dienes were quite similar to the results in the alkyl substituted ethylenes and again suggest that the σ, π separation is valid. For 1,4- cyclohexadiene, however, the upward shift in the singlet-singlet transition energy may reflect the interaction of the two π orbitals by hyperconjugation with the σCH orbitals. Such an interaction implies a breakdown of the σ,π separability approximation. Two azo compounds (R-N=N-R) have been studied. The first three excited states are the 1 3Bg, the 1 1Bg, and a second triplet state. In azomethane transitions to these states produce maxima at 2.75 eV, 3.50 eV, and 4.84 eV. The corresponding values in azo-t-butane are 2.67 eV, 3.37 eV, and 4.9 eV. The near equality of the three lowest transition energies of these two azo compounds suggests that these excitations are primarily due to the azo group electrons. In propadiene (allene) the 1,2 interaction of the double bonds produces two singlet → triplet transitions with maxima at 4.28 eV and 4.89 eV. The separation of 0.61 eV between these triplet states is substantially smaller than the corresponding splitting of 1.69 eV in 1,3-butadiene indicating a much smaller π - π interaction due to the perpendicularity of the π molecular orbitals. In thiophosgene (Cl2CS) a previously unreported singlet → triplet transition was seen at 3.1 eV. These results illustrate the power of low energy variable angle electron-impact spectroscopy in the elucidation of the electronic structure of π-electron molecules.