EXPERIMENTAL EVIDENCE FOR THE FORMATION OF FOURFOLD ROVIBRATIONAL ENERGY CLUSTERS IN THE $\{nu_1}/{\nu_3}$ VIBRATIONAL STATES OF ${H_{2}}^{80}Se$

$^{1}$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. $^{2}$K. K. LEHMANN, J. Chem. Phys. 95, 2361-2370 (1991) $^{3}$I. N. KOZIN AND P. JENSEN, J. Mol. Spectrosc. 161, 186-207 (1993) $^{4}$J.-M. FLAUD, C. CAMY-PEYRET, H. BÜRGER, AND H. WILLNER, J. Mol. Spectrosc. 161, 157-169 (1993)Author Institution: Université Pierre et Marie Curie, Bte 76, 4, Place Jussieu, F-75252 Paris Cedex 05, France.; Université Pierre et Marie Curie, Bte 76, 4, Place Jussieu, F-75252 Paris Cedex 05, France.; Universität - Gesamthochschule Wuppertal, FB 9 - Chemie, Bergische, D-42097 Wuppertal, Germany.; Russian Academy of Science, Uljanov Street 46, 603 600 Nizhnii Novgorod, Russia.The present work 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 $\{nu_1}/{\nu_3}$ vibrational states of the ${H_{2}}^{80}Se$ molecule. The formation of such clusters in the vibrational ground state of ${H_{2}}^{80}Se$ has been experimentally $verified^{1}$. $Lehmann^{2}$ and Kozin and $Jensen^{3}$ independently predicted the existence of a new type of fourfold clusters (referred to by Kozin and Jensen as Type II clusters) in the ${\nu_1}/{\nu_3}$ vibrational states of ${H_{2}}^{80}Se$. These clusters form by coalescence of two energy doublets, one belonging to the $\nu_{1}$ vibrational state and one belonging to the $\nu_{3}$ state. However, from the experimental data available at the $time^{4}$ the term values of the levels predicted to form clusters could be obtained for $J \leq 13$ only. At this J value, the term values are still far from cluster formation. In the present work, we follow the doublets at highest energy in the $\nu_{1}$ and $\nu_{3}$ 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 ${\nu_1}/{\nu_3}$ vibrational states of ${H_{2}}^{80}Se$