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 ne gative 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 co mparatively high resolution in this photon-energy range. The ions are accel erated in thr ASTRID storage ring, which allows an accurate velocity measur ement and further enables the application of momentum-spread reduction tech niques. The fixed-frequency vacuum-ultraviolet laser beam (118 nm) is gener ated by sum-frequency mixing in a xenon gas cell. The photodetachment cross section exhibits pronounced resonances that correspond to the rich spectru m 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 predic t the behavior of a dipole series below H(n = 2). The measurement on both h ydrogen and deuterium facilitates a study of isotope effects. By means of m omentum-spread-reduction techniques it should be possible to resolve the na tural linewidth of the narrow dipole resonances. Preliminary studies show t hat improvements in electron cooling of the H- beam promise to reach this l imit. Finally, the feasibility of extending the high-resolution work to hig her-lying (n = 3) resonances is briefly discussed.