Very strong Rydberg atom scattering in K(12p)–CH3NO2collisions: Role of transient ion pair formation

Collisions between K(12p) Rydberg atoms and CH3NO2 target molecules are studied. Whereas CH3NO2 can form long-lived valence-bound CH3NO−2NO2− ions, the data provide no evidence for production of long-lived K+⋯⋯CH3NO−2NO2− ion pair states. Rather, the data show that collisions result in unusually strong Rydberg atom scattering. This behavior is attributed to ion-ion scattering resulting from formation of transient ion pair states through transitions between the covalent K(12p) + CH3NO2 and ionic K+ + (dipole bound) CH3NO−2NO2− terms in the quasimolecule formed during collisions. The ion-pair states are destroyed through rapid dissociation of the CH3NO−2NO2− ions induced by the field of the K+core ion, the detached electron remaining bound to the K+ ion in a Rydberg state. Analysis of the experimental data shows that ion pair lifetimes ≳10 ps are sufficient to account for the present observations. The present results are consistent with recent theoretical predictions that Rydberg collisions with CH3NO2 will result in strong collisional quenching. The work highlights a new mechanism for Rydberg atom scattering that could be important for collisions with other polar targets. For purposes of comparison, results obtained following K(12p)–SF6 collisions are also included