Experimental Evidence for the Formation of Fourfold Rovibrational Energy Clusters in the ν 1/ν 3 Vibrational States of H 280Se

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The present paper reports experimental evidence for the "cluster effect" (i.e., the formation of nearly degenerate, four-member groups of rotation-vibration energy levels at high rotational excitation) in the ν 1/ν 3 vibrational states of the H 2 80Se molecule. The formation of such clusters in the vibrational ground state of H 2 80Se has been experimentally verified (see I. N. Kozin, S. Klee, P. Jensen, O. L. Polyansky, and I. M. Pavlichenkov, J. Mol. Spectrosc. 158, 409-422, 1993 and references therein). K. K. Lehmann ( J. Chem. Phys. 95, 2361-2370, 1991) and I. N. Kozin and P. Jensen ( J. Mol. Spectrosc. 161, 186-207, 1993) independently predicted the existence of a new type of fourfold clusters (referred to by Kozin and Jensen as Type II clusters) in the ν 1/ν 3 vibrational states of H 2 80Se. These clusters form by coalescence of two energy doublets, one belonging to the ν 1 vibrational state and one belonging to the ν 3 state. However, from the experimental data available at the time (J.-M. Flaud, C. Camy-Peyret, H. Bürger, and H. Willner, J. Mol. Spectrosc. 161, 157-169, 1993) the term values of the levels predicted to form clusters could be obtained for J ≤ 13 only. At this J value, the term values are still far from cluster formation. For the present work, we have recorded a new FTIR spectrum of H 2 80Se at high pressure × (path length) with the purpose of providing new information about the cluster formation. In this spectrum, we can follow the doublets at highest energy in the ν 1 and ν 3 states, respectively, to J = 18 where the energy difference between them is 0.94 cm −1. On the basis of this fact and on the basis of the trend shown by the experimentally derived term values, we conclude that the present work provides experimental verification of the formation of Type II clusters in the ν 1/ν 3 vibrational states of H 2 80Se.