Structure and spectroscopy of the He2Cl2 van der Waals cluster

The rovibrational structure of the He2Cl2 van der Waals cluster in the X an d B electronic states is studied by means of full dimensional quantum-mecha nical calculations. He2Cl2 is the smallest cluster containing helium for wh ich rotationally state-resolved data are available and for which the effect s of Bose statistics are important. The He2Cl2 wave functions exhibit quite large amplitude motions, particularly for the He-He bending mode [associat ed with the angle formed between the two He-(center of mass of Cl-2) bonds] . The preferred geometry of the ground van der Waals state is planar, with the He-He axis perpendicular to the Cl-2 axis. It is shown that a reduced d imension model for the He-He bending vibration together with a rigid struct ural model reproduces well the low-lying energy levels of the complex and a llows us to assign proper statistical weights to the asymmetric top transit ions of the B <--X spectra. In particular, the symmetry under He exchange o f the rigid rotor levels is shown to depend on the He-He bending level. The observed excitation spectra are successfully simulated using the proposed model. The effective rigid structure that best reproduces the rotationally excited levels with a rigid rotor analysis is a distorted tetrahedron where the He-He angle is approximately 130 degrees. This difference from the 180 degrees most probable configuration is due to the complex dependence of th e moments of inertia on the internal degrees of freedom for such a floppy m olecule. Therefore, structural information obtained from observed spectra o f this or similar clusters should be carefully analyzed to avoid reaching m isleading conclusions. Fragmentation rates for He2Cl2(B,upsilon =10-13) are , for the first time, extracted from experimental data, confirming that the dissociation process is mainly sequential. (C) 2000 American Institute of Physics. [S0021-9606(00)02141-3].