A study of the singlet–triplet perturbations in the A 1A u state of acetylene by high resolution ultraviolet spectroscopy

Laser-induced fluorescence spectra of the 330 K10 and 340 K10 vibronic bands of the à 1Au←X̃ 1Σ+g transition in acetylene have been recorded with a resolution of 18 MHz. Each rotational transition consists of a group of lines due to coupling of the electronically excited singlet state with isoenergetic triplet states. Using the standard deconvolution procedure the singlet–triplet coupling elements and the density of coupled triplet states are derived for rotational levels up to J=4 in both bands. From the density of coupled triplet states it is concluded that the à 1Au state is perturbed by the T1 3B2 state. Magnetic field measurements have shown that the predissociation of acetylene in the 4ν3′ vibrational level of the à state is caused by a coupling via the T1 3B2 state with predissociating vibrational levels of the electronic ground state.