Fluorescence of Li excited by laser light and simultaneous thermal Sr∗ collisions in the reaction Sr(5p)+Li(2s)+\ensuremath{\Elzxh}\ensuremath{\omega} (\ensuremath{\lambda}=669.8 nm)\ensuremath{\rightarrow}Sr(5s2)+Li(4d) shows an asymmetric excitation spectrum of the energy transfer versus \ensuremath{\lambda}. The peak variation with light intensity demonstrates the strong-field regime and verifies current models. A change in the slope of the quasistatic wing agrees with the result of a recent three-level model.
High Resolution Image Download MS PowerPoint Slide In the literature, the chemi-ionization of Li in the 2 2 S 1/2 ground level by He in a metastable state is typically described as an electron transfer process in which an electron from the 2s orbital of Li is transferred to the 1s orbital of He while an electron from the 2s orbital of He is ejected. Therefore, one would not assume that the orbital of the valence electron of He strongly influences the coupling strength of the collision complex to the ionization continuum. However, we observe that the chemi-ionization rate is decreased when He is laser-excited from the metastable 2 3 S 1 level to the 2 3 P J level (with J = 0, 1, 2). A semiclassical treatment of the reaction dynamics reveals a strong dependence of the ionization rate on the reaction-channel-specific ionization width functions to which the observed decrease of the rate coefficients can be related to. The results are relevant for the improved understanding and control of chemi-ionization processes in merged beams and in traps.
In the course of this work dynamics in ionization processes of lithium by photon and electron impact were studied. Due to its electronic structure, lithium is an ideal candidate to examine electronic correlation effects and collision dynamics of simple few-body systems. The lithium target is cooled in a magneto-optical trap (MOT) down to the sub-millikelvin regime, which in combination with a reaction microscope (ReMi) allows for the determination of the momentum information of all collision products with unprecedented resolution. An electron gun serving as a new projectile source was implemented in the MOTReMi experiment. From first measurements, which showed that the developed setup is principally operational, time-of-flight and position spectra were analyzed, determining the ratio of Li2+=Li+ ion creation. In a second series of experiments, photoionization of laser-excited lithium has been investigated and fully differential cross sections (FDCS) were examined. As the target and also the laser beam were polarized, orientational effects could be studied. In some cases the electron emission characteristics could directly be deduced from dipole selection rules.