An Experimentalist's View of the H⁻ Ion

Abstract

We present a short survey of selected experimental studies of the negative ion of hydrogen over the past twenty‐three years at the 800 MeV linear accelerator at Los Alamos. Experimental techniques are described and heuristic models are used to explain and motivate experimental results. We discuss the first observations of the 1P resonances just below n = 2, the so‐called “Feshbach” and “shape” resonances, seen in photodetachment. As one experimental question is answered, another question becomes exposed, for example, the effects of electric fields on the spectroscopy of the ion. Techniques used to convert the H− ion into a dual‐beam interferometer moving near the speed of light are described. Experimental data taken near thresh old in an electric field are displayed on a universal plot. Multiphoton detachment extends the spectroscopy of the ion to singlet states that would be unreachable by single photodetachment processes. A thin foil also is used to probe the ion structure. Using ion beams in the 35 kV range, 2‐photon measurements of an isotope shift between Hand D− reveal two‐electron correlation consequences. Finally we ad dress the question, “Why has the shape resonance not been seen in astrophysical observations?

Isotope shift for the1Deautodetaching resonance in H−and D−

“…A recent experimental investigation on H − resonances has also been performed [6]. The experimental development on the measurements of the autodetaching resonance states in the hydrogen negative ions has been documented in recent reviews [7,8]. A search has been made recently for the far-ultraviolet autodetaching transitions from doubly excited states of H − in astrophysical environments such as planetary nebulae and translucent clouds [9].…”

Resonances in electron–hydrogen scattering in Debye plasmas

“…In view of the above calculations on the electron-hydrogen systems [2,6,7,9,[10][11][12]18], we have studied the S-, D-wave resonance states of the electron-hydrogen system under the influence of Debye plasmas. In free atomic cases, several theoretical studies [20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] and experimental investigations [35][36][37] have been performed to estimate resonance states in electron-hydrogen scattering. Our results are well conversed and comparable with the reported results in the unscreened case.…”

Double Photoexcitation of Helium in a Strong dc Electric Field

“…We have chosen to study states with N 2, n > 5, where the field strengths and resolution available are sufficient to observe the electric-field effects. Photoexcitation of H ÿ has previously been studied in an electric field both experimentally and theoretically; see, for example, Halka et al [14], Comtet et al [15], and the review of the experimental work of Bryant et al [16]. Despite being isoelectronic, the case of helium is very different from that of H ÿ , where the spectrum is dominated more by Feshbach resonances.…”