Fourfold clusters of rovibrational energies in H2Te studied with an ab initio potential energy function Original Research Article

We report here an ab initio investigation of the cluster effect (i.e., the formation of four-member groups of nearly degenerate rotation-vibration energy levels at higher J and Ka values) in the H2Te molecule. The potential energy function has been calculated ab initio at a total of 334 molecular geometries by means of the CCSD (T) method where the (1s-4f) core electrons of the Te atom were described by an effective core potential. The values of the potential energy function obtained cover the region up to around 10 000 cm−1 above the equilibrium energy. On the basis of the ab initio potential, the rotation-vibration energy spectra of H2 130Te and its deuterated isotopomers have been calculated with the MORBID (Morse oscillator rigid bender internal dynamics) Hamiltonian and computer program. In particular, we have calculated the rotational energy manifolds for J⪕40 in the vibrational ground state, the ν2 state, the “first triad” (the ν1/ν3/2 ν2 interacting vibrational states), and the “second triad” (the (ν1 + ν2)/(ν2 + ν3)/3 ν2 states) of H2130Te. We have also investigated the cluster formation in the vibrational ground state of H2 130Te by first fitting the rotational data available from experiment with a modified Watson-type effective Hamiltonian and then using the optimized ground state constants to extrapolate the rotational structure to higher J values. Both the ab initio calculation and the prediction with the effective Hamiltonian show that the cluster formation in H2Te is very similar to that in H2Se and H2S, which we have studied previously. However, contrary to semiclassical predictions, we do not determine any significant displacement of the clusters towards lower J values relative to H2Se. Hence the experimental observation of the cluster states in H2Te will be at least as difficult as in H2Se.