Spectroscopic Identification of Multiple Conformers of $o,p$-H$_{2}\cdot \cdot \cdot $ICl and $o,p$-H$_{2}\cdot \cdot \cdot $I$_{2}$ Complexes

Laser-induced fluorescence and action spectroscopy experiments have identified ro-vibronic transitions associated with multiple conformers of the $o,p$-H$_{2}\cdot \cdot \cdot $ICl(X,v²=0) and $o,p$-H$_{2}\cdot \cdot \cdot $I$_{2}$(X,v²=0) complexes. For each complex, the conformers with the hydrogen molecule localized at the end of the dihalogen, with a $C_{2v}$ symmetry, are more stable than the conformers with the hydrogen molecule localized in the T-shaped well, which lies orthogonally about the dihalogen bond axis. Furthermore, the conformers containing $o$-H$_{2}$(j=1) and $p$-D$_{2}$(j=1) are found to be more strongly bound than those containing $p$-H$_{2}$(j=0) and $o$-D$_{2}$(j=0). The role of multi-pole electrostatic interactions is elucidated by comparing the binding energies of the H$_{2}\cdot \cdot \cdot $ICl(X,v²=0) and H$_{2}\cdot \cdot \cdot $I$_{2}$(X,v²=0) complexes with $C_{2v}$ symmetries. The relative populations of the $C_{2v}$ and T-shaped conformers can be altered by changing the properties of the supersonic expansion used to stabilize the complexes. The relative populations of the $o$-H$_{2}\cdot \cdot \cdot $ICl(X,v²=0) and $p$-H$_{2}\cdot \cdot \cdot $ICl(X,v²=0) conformers can also be manipulated, with a population ratio of 3:1 approached by decreasing the hydrogen concentration in helium.