With recent developments of sophisticated experimental techniques and advanced theoretical methods, the fields of molecular spectroscopy and chemical dynamics have reached to the point that theory‒experiment comparisons can be made at a quantitative level. Such comparisons are normally made in terms of some experimental observables, such as the spectrum, cross section, or the product distribution. Up to now, this is the standard and only route to assessing the accuracy of the potential energy surface (PES) used in the dynamical calculations. Yet, it is known that the connection between the interaction potential and the observables is complicated and involves considerable averaging―some of them are not yet experimentally controllable. In addition, full-dimensional quantum dynamics calculations for polyatomic systems remain a major challenge at present. It is therefore desirable to develop an alternative approach capable of retrieving the critical features of the PES directly from the experimental data, thus enabling a more stringent comparison with the theoretically calculated results. Needless to say, a direct determination of “experimental” potential requires an entirely new way of thinking and represents an important milestone, which could have significant impacts to the future development of the field of physical chemistry/chemical physics.
In this talk I shall present our recent attempt to meet the above challenge, using two different polyatomic reactions as examples. By devising a new experimental strategy , the “potential” deduced in Cl + CHD3(v1=1) shows excellent agreement with ab initio calculated potential, validating the experimental method as well as the accuracy of PES. The proposed and demonstrated method is believed to be generally applicable to many other direct chemical reactions with a collinear barrier. The second reaction of F + CH3D(v=0) exploits a newly discovered phenomenon, the reactive rainbow , to access the elusive vibrational-adiabatic well that traps the quasi-bound resonance states during the reaction. This work awaits theoretical confirmation.
If time allows, I may touch upon an on-going adventure: Will the vibrational phase affect the chemical reactivity? and how?
 H. Pan, F. Wang, G. Czako, and K. Liu, Nat. Chem. 9, 1175-1180 (2017).
 H. Pan and K. Liu, J. Phys. Chem. A 120, 6712-6718 (2016).