The OODR mass spectroscopy is applied to investigate dynamical properties of Xe**Kr in high Rydberg states. It is found that two dissociation pathways producing both the Xe** and Kr** fragments compete, where the ion core of high Rydberg molecule switches. It is revealed that ion-core switching (ICS) occurs because of the interaction between bound potentials of high Rydberg states Xe**Kr and a repulsive potential of XeKr**. Moreover, the observed variation in ICS ratio is explained by the population transfer from high Rydberg states of Xe**Kr of the A ion core to those of the X ion-core.
The OODR mass spectroscopy is applied to investigate dynamical properties of XeKr in high Rydberg states. It is found that two dissociation pathways producing both the Xe and Kr** fragments compete, where the ion core of high Rydberg molecule switches. It is revealed that ion-core switching (ICS) occurs because of the interaction between bound potentials of high Rydberg states XeKr and a repulsive potential of XeKr. Moreover, the observed variation in ICS ratio is explained by the population transfer from high Rydberg states of Xe**Kr of the A ion core to those of the X ion-core.
Processes involving a change of orbital angular momentum l and the dissociation of Rydberg molecules when they collide with inert-gas atoms are considered, using a method based on analyzing the terms of the interacting systems. The proposed method makes it possible to take into account the perturbations in a large group of Rydberg states that weakly penetrate into the ion core and to go beyond the limits of the two-level approximations [R. F. Stebbings and F. B. Dunning, eds., Rydberg States of Atoms and Molecules (Cambridge University Press, Cambridge, 1983; Mir, Moscow, 1985)] widely used in the theory of l mixing with the participation of Rydberg atoms. Using the Na**( nd )+Xe reaction as an example, it is demonstrated that this method gives good results in a wide range of variation of the principal quantum number n of the Rydberg electron ( n ≈10–40). Features of the l mixing and dissociation of Rydberg molecules are studied for the H 2 **+Xe system. It is shown that collisional dissociation occurs by the formation of an intermediate Rydberg state of the molecules, followed by self-decay into a dissociative continuum.
Thermal collisions between laser excited Xe(nf) Rydberg atoms and a variety of neutral atomic and molecular targets have been investigated. In Xe(nf)-HF collisions, n-changing, 4-changing and collisional ionization were observed. For n = 26 and 27, absolute collisional cross sections for these processes have been determined using time-resolved selective field ionization. In addition, the first evidence of molecular rotational excitation through the transfer of energy of electronic excitation from a Rydberg atom is presented. In Xe(nf) - Xe, Kr, Ar, CO2, CO and N2 collisions the dominant mechanism was observed to be 4-changing: no evidence of n-changing or collisional ionization was apparent. Absolute collisional cross sections have been determined for a range of n between 24 and 37. Comparison of these cross sections with those obtained using a theoretical model in which effects due to the Rydberg core are ignored appears to lead to satisfactory results only for Xe, Kr and CO2, whereas when core effects are included satisfactory agreement between experiment and theory is found in all cases.
Scattering of K(np) Rydberg atoms by electron-attaching molecules can result in the formation of positive-negative ion pairs through electron transfer reactions. These reactions can involve the dissociation of a transient intermediate negative ion into a smaller negative ion and a neutral fragment, or they can result in the parent molecule forming a metastable negative ion, with the excess energy being distributed among its internal modes. For Rydberg atoms of intermediate n, it is possible for these ion pairs to orbit each other at large separation, weakly bound by their mutual electrostatic attraction, forming what are known as heavy-Rydberg states. Measurements of the binding energy and velocity distributions of the ion pairs are used to probe the dynamics of electron transfer reactions. The data are analyzed using a semiclassical Monte Carlo collision code to explore the lifetime of the intermediate, the nature of its dissociation, the distribution of the excess energy of reaction, and the branching ratio of the various reaction channels. Results obtained using a variety of attaching targets including CF3I, CCl4, SF6, CH2Br2, 1,1,1-C2Cl3F3, CCl3Br, Fe(CO)5, and CH3NO2 have demonstrated both the wide variety of different behaviors that can accompany electron capture as well as the different behaviors of the product ion-pair states themselves. Remarkably, despite previous work showing that it was possible for CH3NO2 to form long-lived valence-bound negative ions through Rydberg electron transfer, the present work showed that such collisions do not result in the formation of long-lived bound ion-pair states. Rather, the data show that collisions lead to very strong Rydberg atom scattering through formation of transient ion-pair states. These results are consistent with theoretical work that predicts significant collisional quenching of Rydberg atoms by highly polar molecules and highlight a new mechanism for Rydberg atom scattering.
Heavy-Rydberg ion-pair states, molecular quantum systems that are bound by their long-range pure Coulomb attraction, are formed through Rydberg atom collisions with molecules that attach low energy electrons. Collisions of potassium Rydberg atoms K(np) with the attaching species CCl4 and SF6 lead, respectively, to dissociative and non-dissociative electron attachment with subsequent formation of bound ion-pair states K+..Cl- and K+..SF6-.\nThe experimental apparatus used to produce and detect heavy-Rydberg ion-pair states consists of a reaction region and a separate analysis region. It separates a fraction of the product ion pairs from the parent Rydberg atoms, eliminating spurious background signals associated with blackbody-induced and electric-field ionization of Rydberg atoms. A position sensitive detector in the analysis region is employed to measure their velocity, angular, and binding energy distribution and lifetimes. Measurements of the spatial distribution of ion pairs provide information on their velocity and angular distributions, and their binding energy distribution is measured using electric field-induced dissociation. The lifetime of the ion-pair states is influenced by multiple processes including internal-to-translational energy transfer, autodetachment of the electron, and neutralization through charge transfer.\nThe experimental results are compared with the results of a Monte Carlo collision code that models both the initial Rydberg electron attachment and the subsequent evolution and molecular dynamics of the ion pairs. This model highlights the factors such as the kinematics of the Rydberg atom and the attaching particle and energy released in dissociation (in the case of dissociative attachment) that are important in governing ion pair formation. The model calculations are in good agreement with the experimental data.
Atoms in highly excited Rydberg states possess physical characteristics quite unlike those associated with atoms in the ground or low-lying excited states. In particular, they are physically very large and are only very weakly bound. In consequence, collisions can lead to a wide variety of reaction processes many of which are unique to Rydberg species and have very large collision cross sections. In collisions with neutral targets, Rydberg atoms behave not as an atom but rather as a pair of well-separated independent scatterers, namely the core ion and the excited Rydberg electron. In the present article we discuss many of the different reactions that can occur when Rydberg atoms collide with neutral targets, focusing principally on reactions that are dominated by (binary) Rydberg electron-target interactions and include collisions with molecules that attach free low-energy electrons and with polar targets. In certain situations, however, interactions involving the Rydberg core ion are important. This is illustrated using as an example the destruction of ultralong-range Rydberg molecules excited in a cold dense gas.