VUV Spectroscopy of Atoms, Molecules and Surfaces

56 High-resolution VUV spectroscopy of H −
et al. in a hyperspherical close-coupling calculation [10]. The inadequacy of
this approach might have been expected on the basis of a comparison of the
calculated cross section with the experimental results of Bryant et al. where
the measured strength appeared to be much larger than predicted [10]. The
widths obtained by Sadeghpour et al. [14], and Cortés and Martin [29], applying
R-matrix- and L 2 methods, respectively, are outside the uncertainty of
the present value. The calculation of Sadeghpour et al. in addition predicts a
resonance energy that deviates from the experimental value which, within the
quoted uncertainty, agrees with those of all the other approaches. It should
be noted that the improved resolution of the present experiment does not
allow a more accurate determination of the resonance energy than previously
reported. The uncertainty of this parameter is limited by the uncertainty
of the D − beam orbit length [20]. The complex-rotation method applied in
different variants by Ho [27], Lindroth et al. [16, 17], Kuan et al. [32], Chen
[33] and Bylicki and Nicolaides [15] seems to provide a good description of the
structure of the negative hydrogen ion. The measured magnitude of q agrees
very well with the few theoretical predictions for this parameter [16, 29, 30]
but this is to be expected since the q value couples to the width.
3.4 Conclusion
The natural width Γ and the asymmetry parameter q of the 1P o 2{0} − 3 resonance
of D− have been determined to a precision that, for the first time, has
allowed a critical test of the various theoretical models applied to this system.
The measurement was performed with the technique of Doppler-tuned VUV
spectroscopy, for which the resolution was improved by a factor of ∼4with
respect to previous experiments by electron cooling the D− beam. The energy
resolution obtained in the present experiment cannot be improved much
further, given the machine properties of the storage ring and the electron
cooler, and will make an observation of the last member of the 2{0} − m series,
the 1 P o 2 {0}− 5
resonance, difficult. Figure 3.6 (a) shows the cross section in
the vicinity of this resonance as predicted by Lindroth, with the 2p1/2, 2sand 2p3/2 thresholds of D(n=2) indicated by vertical lines. In figure 3.6 (b) this
cross section has been convolved with the present experimental resolution,
and the ratio of the resonant to off-resonant contribution is seen to be ∼1.4
with the 2p1/2 threshold being completely buried in the high-energy shoulder
of the 2{0} − 5 resonance. Given the decrease in strength by a factor of ∼700
when going from the 2 {0} − 3 to the 2 {0}− 5
state [16], this structure can only
be verified beyond statistical uncertainty by increasing the data-collection
time by at least a factor of 50, assuming a grating efficiency similar to that
of the 1996–97 experiments. Since the collection time for each data point is