Energies and relative cross sections of singlet–triplet transitions in methyl-substituted 1,3-butadienes by ion-impact spectroscopy

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Abstract
The energies of the 1 3Bu←? 1Ag and 1 3Ag←? 1Ag transitions of a number of methyl-substituted 1,3-butadienes have been determined by ion (He+ and H2+) -impact energy-loss spectroscopy. The observed values were relatively invariant at 3.2 and 4.9 eV, respectively, except in the case of 2,4-dimethyl-1,3-pentadiene where triplet states at 3.5 and 4.4 eV indicate a nonplanar conformation or conformations. The relative cross section for the 1 3Bu←? 1Ag transition in the He+-impact spectrum of trans-1,3-pentadiene was observed to increase slightly relative to that for the 1 3Ag←? 1Ag transition and substantially relative to that for the 1 1Bu←? 1Ag transition on decreasing the ion energy from 3.5 to 1.5 keV. Cross sections for all three transitions decreased on changing the projectile from 3.0 keV H+ to 3.0 keV H2+ and from the latter to 3.0 keV He+. Finally, the relative cross section for the 1 1Bu,2 1Ag←? 1Ag transitions is decreased only slightly (by up to a factor of 2) on adding up to four methyl groups whereas the 1 3Bu←? 1Ag and 1 3Ag←? 1Ag relative cross sections suffer a decrease of about a factor of 10 for He+ impact and a bit less for H2+ impact.