Laser-induced fluorescence study of the S₁ state of doubly-substituted ¹³C acetylene and harmonic force field determination

In the first half of this study, rotational and vibrational constants of six Franck-Condon bright vibrational levels of S₁ doubly-substituted ¹³C acetylene are determined from laser-induced fluorescence spectra and an updated geometry of the trans conformer of S₁ acetylene is obtained. In the second half, we determine the quadratic force field of S ₁ acetylene on the basis of the harmonic frequencies of four isotopologues of acetylene. The effects of both diagonal and off-diagonal x _ij anharmonicities are removed from the input harmonic frequencies. Results from both experimental and theoretical studies of various isotopologues of acetylene (including those from the first half of this paper) are used to obtain a set of force constants that agrees well with ab initio calculations. Our set of force constants for S₁ acetylene is an improvement over previous work by Tobiason et al., which did not include off-diagonal anharmonicities.