SOME PREDICTIONS RELEVANT TO FUTURE SPECTROSCOPIC OBSERVATION OF S-1 VINYLIDENE

The equation-of-motion coupled cluster method is used to obtain a prec ise estimate of the lowest adiabatic singlet-singlet excitation energy (S-1<--S-0) of vinylidene. As photodetachment spectroscopy offers a pr omising means for producing the first excited singlet state, harmonic vibrational frequencies of S-1 and the extent of mixing between the co rresponding normal modes and those of the anion are also calculated. T o calibrate the calculations and provide a basis for empirical correct ion, parallel studies of the previously observed T-1 and T-2 triplet s tates are reported. The adiabatic excitation energy [S-1<--S-0] is est imated to be 3.12+/-0.05 eV, suggesting a photodetachment threshold en ergy of 3.61+/-0.05 eV. Progressions in the totally symmetric C-C stre tch and H-C-H bending modes should be pronounced in the spectrum, with the extent of mode mixing similar to that found for the S-0<--anion a nd T-1<--anion processes, but differing in phase.