15min:

P. C. GÓMEZ, Dep. Qu\'\i mica F\'\i sica I, Facultad Qu\'\i mica, U. Complutense de Madrid, E-28040 Madrid, Spain; L. F. PACIOS, Dep. Qu\'\i mica y Bioqu\'\i mica, E.T.S.I. Montes, U. Politécnica de Madrid, E-28040 Madrid, Spain; PER JENSEN, FB 9 -- Theoretische Chemie, Bergische Universität -- Gesamthochschule Wuppertal, D-42097 Wuppertal, Germany.

\def\cmcm^-1 An interesting novel pattern in rotation-vibration spectra is the cluster effect (i. e., the formation of four-member groups of nearly degenerate rotation-vibration energy levels at higher J and K_a values ) in the molecules H₂S, H₂Se, H₂Te, and H₂Po. In the cluster states, local mode behaviour is induced by rotational excitation so that it occurs in the lowest vibrational states. We report here investigations of the cluster effect in the vibrational ground states of the H₂Te and H₂Po molecules. For H₂Te, we have obtained a new potential energy function by fitting to the available spectroscopic data, and for H₂Po, we have calculated the potential energy function \sl ab initio by means of the CCSD(T) method, using an Averaged Relativistic Effective Potential for Po. With the new potential energy functions, the rotation-vibration energies of H₂¹³⁰Te and H₂²⁰⁹Po 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 \leq 40 in the vibrational ground states.

An interesting aspect of the cluster effect is the change in vibrational ground state cluster formation taking place for the sequence of molecules H₂O -> H₂S -> H₂Se -> H₂Te -> H₂Po. The H₂O molecule does not exhibit any discernible cluster formation at J \leq 40, but for all the other molecules the cluster size decreases rapidly with increasing J. For H₂Te and H₂Po, in which the central mass is effectively infinite compared with the hydrogen mass, a limiting behaviour is reached which would not change further if the central mass were made heavier.

This work was supported by the European Commission through contract no. CHRX-CT94-0665 ``Highly Excited Rovibrational States''. One of us (P. C. G.) would also like to acknowledge support from the Spanish DGICYT (Project PB94-1526).