High-resolution VUV spectroscopy of H- in the region near the H(n=2) threshold -: art. no. 022702

Experimental investigations of the photodetachment cross section for the negative hydrogen ion in the region near the H(n = 2) threshold are discussed. Doppler-tuned spectroscopy in a collinear geometry is used to obtain a comparatively high resolution in this photon-energy range. The ions are accelerated in thr ASTRID storage ring, which allows an accurate velocity measurement and further enables the application of momentum-spread reduction techniques. The fixed-frequency vacuum-ultraviolet laser beam (118 nm) is generated by sum-frequency mixing in a xenon gas cell. The photodetachment cross section exhibits pronounced resonances that correspond to the rich spectrum of doubly excited P-1 states near the H(n = 2) threshold. The position of the resonances is determined with an accuracy that challenges the current theoretical developtnents. The experimental observations are used to predict the behavior of a dipole series below H(n = 2). The measurement on both hydrogen and deuterium facilitates a study of isotope effects. By means of momentum-spread-reduction techniques it should be possible to resolve the natural linewidth of the narrow dipole resonances. Preliminary studies show that improvements in electron cooling of the H- beam promise to reach this limit. Finally, the feasibility of extending the high-resolution work to higher-lying (n = 3) resonances is briefly discussed.