In the present paper, we report the direct absorption spectra of the 1 1B+u←1 1A−g transitions of gas phase butadiene, deuterated and methylated butadienes, and the cis and trans isomers of hexatriene cooled to low rotational and vibrational temperatures in supersonic molecular jets. These jet absorption spectra allow the more accurate determinations of Franck–Condon factors, upper state vibrational intervals and vibronic band homogeneous widths. We discuss the experimental constraints that the measurements reported here and in the previous paper of this series impose on theoretical models of the equilibrium structures and relaxation dynamics of the 1 1B+u excited states of the small linear polyenes.
The vapor phase absorption spectra of cyanogen, cyanoacetylene, dicyanoacetylene, and dicyanodiacetylene have been recorded between 2000 and 1050 Å. Substantial perturbations in the intensity and positions of particular bands upon the addition of helium at 1500 psi pressure served to identify several Rydberg series. These series are assigned on the basis of their quantum defects and symmetry considerations. CNDO/2-CI calculations were used to interpret the valence shell transitions. A very intense intravalence shell band, assigned as a Σu+1 ← 1Σg+ system arising from the lowest π→π* configuration, dominates the spectra of all the molecules. A moderately intense intravalence shell band, assigned as a Πu1 ← 1Σg+ transition arising from the lowest n→π* configuration further characterizes the spectra.
The gas-phase electronic absorption spectra of triethylenediamine and quinuclidine have been measured. Each compound shows two strong bands in the regions 1650–2500 Å and 1650–2300 Å, respectively, which have well-resolved vibrational structure, as well as very weak, structured absorption at longer wavelengths (2560–2700 Å for triethylenediamine and 2300–2500 Å for quinuclidine). An essentially complete vibrational analysis of all but one of these bands has been accomplished. The results indicate a substantial change in the equilibrium positions of the nuclei in the excited states relative to the positions in the ground state. For quinuclidine, the most prominent excited state vibrational mode can be correlated with the ground-state mode which corresponds to an approach to planarity of the nitrogen-bearing end of the molecule. For triethylenediamine, ambiguities in the ground-state assignments preclude conclusions about the upper state geometry. By comparisons with the spectra of other amines, the strong electronic bands of triethylenediamine and quinuclidine are assigned as n → p Rydberg transitions. Exciton model calculations of the shift of the first strong band of triethylenediamine relative to the corresponding band in quinculidine predict changes of about 3000 cm−1 to lower energy for an n → 3pz transition and about 1500 cm−1 to higher energy for an n → px y transition. The observed shift of the 0–0 band is 3920 cm−1 to lower energy, which is clearly in accord with z polarization. The weak, longer-wavelength absorption of the cage amines is tentatively considered to arise from sub-Rydberg transitions.
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