Rotational energy cluster formation in XY3 molecules: Excited vibrational states of BiH3 and SbH3

Previous theoretical work on energy cluster formation at high rotational excitation in the vibrational ground state of PH3 [S.N. Yurchenko, W. Thiel, S. Patchkovskii, P. Jensen, Phys. Chem. Chem. Phys. 7 (2005) 573] is extended to BiH3 and SbH3. By means of variational calculations of the rotation-vibration energies based on ab initio potential energy surfaces, we analyze the rotational energy clustering of BiH3 and SbH3 at J ⩽ 70 for a number of vibrational states. We show that BiH3 and SbH3, with their pronounced local mode behaviour, exhibit cluster formation already at moderate rotational excitation. In addition, owing to its quasi-spherical-top character, BiH3 undergoes an imperfect bifurcation at high J. This gives rise to an energy cluster type not present in PH3 and SbH3. We present a semi-classical approach to the construction of the rotational energy surfaces for vibrationally excited states.