Rotational analysis of the òA₁-X^˜2A₁ band system of yttrium dicarbide, YC₂

The 000-000 and 3₀¹ bands of the 775-nm electronic transition of YC₂(òA₁←X̃²A ₁) have been studied at high resolution, using the laser-induced fluorescence from a supersonic jet expansion. Three types of experiment have been carried out. First, the complete rotational and hyperfine structures of the two bands were recorded. To measure the small asymmetry splittings in the K = 2 levels of the X̃²A₁ state, portions of the b-type 3₀¹ band were then recorded in the presence of a weak static electric field. Finally, a number of pure rotational transitions between the K = 0 levels of the ground state were recorded by pump/probe microwave optical double resonance. A few small rotational perturbations occur in the upper electronic state but, omitting the perturbed lines, the combined data sets could be modeled using an effective Hamiltonian operator appropriate for the rotation, electron spin, and hyperfine structure of a rigid asymmetric top molecule. The molecule is confirmed as being "T-shaped," where the Y atom is bonded to the side of a C₂ group; the rotational constants determined are for the òA₁, 3¹ level, A = 1.76128, B = 0.189949, C = 0.170056 cm^-1, and for the X̃²A₁, ν = 0 level, A = 1.742731, B = 0.201947, C = 0.181285 cm^-1. Allowing for electron orbital corrections to the rotational constants, the geometrical structures are found to be òA₁ state, r (Y-C) = 2.2795 Å, r (C-C) = 1.2630 Å, ∠C-Y-C = 32.17°; X̃²A₁ state, r (Y-C) = 2.1946 Å, r (C-C) = 1.2697 Å, ∠C-Y-C = 33.63°. A molecular orbital diagram is given for the states of YC₂ and the interpretation of the electron spin and hyperfine parameters is discussed.