Electronic spectroscopy of s-trans 1,3-butadiene by electron impact

Abstract

The electron impact excitation of 1,3-butadiene has been studied experimentally at impact energies of 20, 35, and 55 eV and scattering angles from 10° to 85°. The energy and angular dependences of the cross section ratios are used to identify the nature of the excited states. Two transitions with maxima at 3.22 and 4.91 eV are identified as singlet-->triplet transitions. Comparison with theoretical calculations indicates that these are due to the 13Bu and 13Ag states, respectively. Their significance for the photochemistry of this molecule is discussed. The optically allowed X-tilde 1Ag --> 11Bu(N --> V1) transition is observed with a maximum at 5.92 eV. An additional transition appears between 6.9 and 7.8 eV with vibrational features at 7.09, 7.28, and 7.46 eV. The optical absorption in this region was originally attributed to a 1A1 state of the s -cis molecule and subsequently to a Rydberg state or to a 1Ag state of the s -trans molecule. On the basis of intensity arguments and the angular dependence of the cross section ratios, we suggest that it may instead be due to the X-tilde 1Ag --> 21Bu transition of the s -trans molecule. Rydberg transitions are observed at 8.00 and 8.18 eV. Two broad transitions are also seen beyond the first ionization potential with maxima at 9.50 and 11.00 eV. The results of this study are in good agreement with recent ab initio configuration interaction (CI) calculations, and give support to the analysis of the valence excited states in terms of a "molecules-in-molecules" approach. This is consistent with recent interpretations of the resonance energy and reactivity of this molecule and differs from the older classic model of extensive delocalization in the pi electron system.