Strong ortho/para effect in the predissociation spectra of the ³⁵Cl^-(H₂) complex and its isotopologue ³⁵Cl^-(D₂) recently measured in an ionic trap at low temperature

The predissociation spectra of the ³⁵Cl^-(H₂) and ³⁵C^l-(D₂) complexes are determined within an accurate quantum approach and compared to those recently measured in an ionic trap at 8 K and 22 K. [1,2,3] The calculations are performed using an existing three-dimensional potential energy surface [4]. A variational approach is used for the accurate quantum calculations of the rovibrational bound states. Several methods are compared for the search and the characterization of the resonant states. A good agreement between the calculated and measured spectra is obtained, despite a slight shift to the red of the calculated spectra. The comparison shows that only the ortho or para contribution is observed in the measured ³⁵Cl^-(H₂) or ³⁵Cl^-(D₂) spectrum, respectively. This result is attributed to the rapid para <-> ortho conversion of the complexes by collision with H₂ or D₂ molecules inside the trap at low temperature. Quantum numbers are assigned to the rovibrational resonant states. It demonstrates that the main features observed in the measured predissociation spectra correspond to a progression in the intermonomer vibrational stretching mode.