Very strong Rydberg atom scattering in K(12p)–CH3NO2 collisions: Role of transient ion pair formation

Abstract

Collisions between K(12p) Rydberg atoms and CH 3 NO 2 target molecules are studied. Whereas CH 3 NO 2 can form long-lived valence-bound CH 3 NO − 2 ions, the data provide no evidence for production of long-lived K + • • • CH 3 NO − 2 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) + CH 3 NO 2 and ionic K + + (dipole bound) CH 3 NO − 2 terms in the quasimolecule formed during collisions. The ion-pair states are destroyed through rapid dissociation of the CH 3 NO − 2 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 CH 3 NO 2 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)-SF 6 collisions are also included.

Dissociative electron attachment studies with hyperthermal Rydberg atoms

“…The velocity and angular distributions of the product ionpair states that result from Rydberg collisions are simulated using a semi-classical Monte Carlo collision code that models the detailed kinematics of electron transfer reactions. [2][3][4][5][6]15 The model is based on the independent particle picture, and attachment is viewed as resulting from a binary interaction between the Rydberg electron and target particle. The initial velocities of the Rydberg atom and target particle are selected at random from the appropriate distribution of these quantities.…”

“…In earlier experimental studies of the velocity and angular distributions of ion-pair states produced in Rydberg atom collisions, only those ion pairs that travel within a few degrees of a fixed (horizontal) plane, i.e., atom pairs that have only a small component of transverse (vertical) velocity, were measured. [2][3][4][5][6] Such "two-dimensional" velocity distributions reflect more directly the reaction dynamics and are more straight forward to interpret. Such ion pairs are therefore identified in the simulations and it is their velocity and angular distributions that are built up by considering the outcome of many collision events.…”

“…Since many of the properties of such ion-pair states parallel those of Rydberg atoms, they are also frequently termed heavy-Rydberg states. 1 Such states have been generated not only through Rydberg atom collisions [2][3][4][5][6] but also by direct photoexcitation of ground-state diatomic molecules. 1,[7][8][9][10] Earlier Rydberg atom studies undertaken using a target gas cell and K(np) Rydberg atoms with low-to-intermediate values of n, 10 n 15, excited in a tightly collimated thermal potassium atom beam have shown that measurements of the velocity and angular distributions of the product ion-pair states can provide a window into the properties of the excited intermediates formed upon initial electron capture, including their lifetimes and the energetics of their decay.…”

Slow Decay Processes of Electrostatically Trapped Rydberg NO Molecules

“…Their decay dynamics affect recombination in atmospheric and astrophysical plasmas [2,3], while in the laboratory they have been exploited in studies of ultracold plasmas [4,5]. Samples in these states are also of interest for low-energy inelastic scattering, including, e.g., studies of long-range dipole-dipole interactions that lead to resonant energy transfer [6][7][8], low-energy electron scattering that can allow the stabilization of weakly bound long-range Rydberg molecules [9,10] and result in electron transfer [11,12], and short-range ion-molecule reactions [13]. In these areas, advances are expected if the molecules are prepared in slowly moving, velocity-controlled beams or in traps.…”