Electron - impact spectroscopy of the alkynes : A comparison of propyne and 1 - butyne with acetylene • >

The electronic excitation spectra of propyne and 1-butyne have been investigated by the method of variable angle electron-impact spectroscopy. In both molecules, two singlet--;triplet transitions were observed with maximum intensities at 5.2 and 5.85 eV. No evidence was found for the existence of any transitions with vertical excitation energy below 5 eV. A number of previously unreported transitions to superexcited states in both propyne and 1-butyne were detected. The relationship of these spectra to those of acetylene is discussed, and the different behavior of valence and Rydberg transition energies in response to alkyl substitution is explained qualitatively in terms of the different excited state charge distributions.