Nonadiabatic Electron Dynamics by Direct Excitation of Collision Pairs

Abstract

The potential interactions in collisional systems are reflected in the observed spectral line shapes. Optical collisions corresponding to laser excitation of a collision pair are the underlying events in the wing of a spectral transition. For atomic colliders a complete quantum description is possible using the close-coupling approach [1]. Recently these methods have also been applied to collisions in highly-ionized plasmas with potential applications to diagnostic tools for plasma-and astrophysics [2]. Optical collisions have been extensively studied under thermal gas ceU conditions leading to a wealth of information on potentials, disalignment mechanisms and nonadiabatic transitions in optical coUisions. [3]. These findings are the basis for the interest in optical collisions as a method to investigate details of the collision process on a subcolhsional time scale. The principle is shown in Fig. la for the case of optical collisions in Na with the rare gas Kr. The optical coUision method is rather unique since a direct investigation of a collision by ultrashort pulse techniques does not seem feasible or requires more complex excitation schemes that have not been realized, at present. Rydberg detector pulsed nozzle excitation laser Na oven Na-Kr distance (bohr) , K^»^ detection laser Figure 1: a) Potential scheme for an optical colhsion in Na + Kr. b) The setup in a crossedbeam optical collision experimentThe study of optical collisions in a crossed-beam setup has provided new insights in the collisional process [4]. The basic experimental setup is shown in Fig. lb. Rather then using the emitted fluorescence as in thermal optical collisions, the detection of the short-lived excited alkali atoms is made by converting them into more stable Rydberg states. These are measured with high sensitivity by field-ionization.The differential detection scheme used in the crossed-beam experiments is capable to reveal the quantum nature of the scattering process. This is seen by the Stueckelberg oscillations appearing in the differential cross sections of atom-atom optical collisions. The effect is understood by the interferences of a few (mainly 2) contributing pathes to a given scattering angle.Excitation of collision pairs with polarized hght gives access to the collision geometry ("collision photography") [5,6]. Control of the coUisional process has also been achieved [7].