VUV Spectroscopy of Atoms, Molecules and Surfaces

58 High-resolution VUV spectroscopy of H −
in figure 3.1) [19]. The shape resonance has been investigated a couple of
times by Bryant et al. yielding values for the width in the 20–30 meV range
while recent theoretical predictions agree on a value around 20 meV [15].
The beam-positioning system appeared to work well but a further evaluation
of the success of this experiment will have to avait the data analysis which
remains to be done.
Considering the series converging to the H(n=3) threshold, the 3 {0} + 3
and 3{0} + 4 Feshbach resonances have been observed by Bryant et al. at
12.650(1) eV and 12.837(4) eV, respectively, applying the doubtfull energy
calibration method mentioned previously [7, 35]. With widths of ∼30 meV
and ∼2 meV and strengths comparable to those of the 2{0} − 3 and 2{0} − 4 resonances,
these states should be easily observable at ASTRID. The intervening
3{0} − 4 and 3{0} − 5 resonances of widths ∼20 µeV and ∼6 µeV [16, 17] have
never been observed, but even the lowest-lying one represents an experimental
challenge since the strength is a factor of ∼3 lower than that of the 2 {0} − 5
resonance.
As an alternative to—or extension of—the present technique, one could
consider the use of a tunable VUV light source in an ordinary collinear beamline
setup with a low-energy (∼keV) H− beam.Withsuchasetuptheresolution
would be limited by the spectral bandwidth of the VUV light source
since the contribution from the velocity spread of the accelerated ion beam
would be much smaller [36, 37]. The resolution could thus be improved by at
least another factor of ∼4by the application of VUV light generated by, e.g.,
the 5th or 7th harmonic of a nanosecond dye laser. The one- or two-order-ofmagnitude
reduction in VUV flux implied by the off-resonant phase-matching
conditions would likely be compensated by the improved resolution.