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 infrared absorption bands of acetylene dimers have been investigated with long path low temperature Fourier transform infrared spectroscopy. Five bands were found, three of (HCCH)2 and two of (DCCD)2. Four of these bands can be assigned as coupled monomer modes. Normal mode analysis based on a theoretical intermolecular potential confirms these assignments. One band shows evidence of fast vibrational predissociation, and a symmetry based argument is made to support the claim that the predissociation fragments are rotationally excited.

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