The singlet–triplet spectroscopy of 1,3-butadiene using cavity ring-down spectroscopy

Abstract

The T1←S0 absorption spectrum of gas-phase 1,3-butadiene (C4H6) has been investigated over the region from 20 500 to 23 000 cm−1 using cavity ring-down spectroscopy. Resolved vibrational structure and partially resolved rotational structure have been observed for the first time in the gas phase. The T1←S0 origin transition is located at 20 777 cm−1, with a peak absorption cross section of 2.5×10−26 cm2/molecule. Vibronic bands appear 249, 491, 1166, and 1617 cm−1 above the origin. This structure is observed on top of a rising background whose absolute magnitude and wavelength dependence is quantitatively accounted for as Rayleigh scattering. Using the recent calculations of Brink et al. [J. Phys. Chem. A 102, 6513 (1998)] as a guide, the bands 491, 1166, and 1617 cm−1 above the origin can be assigned as totally symmetric fundamentals, while the band 249 cm−1 above the origin is the first overtone of the bg symmetry CH2 torsion (calculated at 129.6 cm−1) of a planar T1 excited state. The rotational band contour of the origin transition shows several sharp band-heads that appear in doublets with a splitting of 2 cm−1. Only part of this structure can be accounted for as a single vibronic band. The possible explanations for the remaining band heads and the observed Franck–Condon intensities are discussed in terms of the shape of the T1 potential energy surface.