Some predictions relevant to future spectroscopic observation of S1 vinylidene

The equation-of-motion coupled cluster method is used to obtain a precise estimate of the lowest adiabatic singlet–singlet excitation energy (S1←S0) of vinylidene. As photodetachment spectroscopy offers a promising means for producing the first excited singlet state, harmonic vibrational frequencies of S1 and the extent of mixing between the corresponding normal modes and those of the anion are also calculated. To calibrate the calculations and provide a basis for empirical correction, parallel studies of the previously observed T1 and T2 triplet states are reported. The adiabatic excitation energy [S1←S0] is estimated to be 3.12±0.05 eV, suggesting a photodetachment threshold energy of 3.61±0.05 eV. Progressions in the totally symmetric C–C stretch and H–C–H bending modes should be pronounced in the spectrum, with the extent of mode mixing similar to that found for the S0←anion and T1←anion processes, but differing in phase.