VUV Spectroscopy of Atoms, Molecules and Surfaces

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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
3.4 Conclusion 57 Cross section (Mbarn) Cross section (Mbarn) 40 30 20 10 40 30 20 10 (a) (b) 2p 1/2 2s 2p 3/2 10.95620 10.95623 10.95626 10.95629 Photon energy (eV) Figure 3.6: (a) Theoretically predicted photodetachment cross section of D− in the vicinity of the 1P o 2 {0}− 5 resonance of D− ,withthe2p1/2, 2p3/2 and 2s thresholds of D(n=2) indicated by the vertical lines [16]. (b) A convolution of the cross section from (a) with the experimental resolution of ∼44 µeV(FWHM). a few minutes with a favourable photon flux, this seems a somewhat unrealistic task considering the demands to experimental stability and the limited outcome expected from such an experiment. Although a grating with an improved 118 nm reflection efficiency had been provided for the November 2000 beamtime, a search for the 2{0} − 5 resonance was therefore not attempted. Instead, a re-investigation of the 2 {0} + 2 shape resonance was performed, uti- lizing a newly developed, improved ion-beam positioning system which was expected to eliminate the drift in the laser- and ion-beam overlap when scanning the ion-beam energy (cf. the apparent strange shape of this resonance
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VUV Spectroscopy of Atoms, Molecule
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ii CONTENTS 4 .6 Conclusion . . . .
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iv Experimental and theoretical stu
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vi me to the principle of negative-
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viii
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2 VUV light generation: possibiliti
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4 VUV light generation: possibiliti
Page 16 and 17: 6 VUV light generation: possibiliti
Page 18 and 19: 8 VUV light generation: possibiliti
Page 20 and 21: 10 VUV light generation: possibilit
Page 28 and 29: 18 REFERENCES [14 ] C. R. Vidal, in
Page 30 and 31: 20 REFERENCES [55] C. J. G. Uiterwa
Page 32 and 33: 22 REFERENCES [98] Parameters from
Page 34 and 35: 24 Negative ions ing the binding en
Page 36 and 37: 26 Negative ions located energetica
Page 38 and 39: 28 Negative ions available has been
Page 40 and 41: 30 Negative ions assignment will th
Page 42 and 43: 32 Negative ions feel the repulsion
Page 44 and 45: 34 Negative ions Figure 2.1: Adiaba
Page 46 and 47: 36 Negative ions
Page 48 and 49: 38 REFERENCES [16] E. Kopp, Adv. Sp
Page 50 and 51: 40 REFERENCES [63] P. Balling et al
Page 52 and 53: 42 REFERENCES
Page 54 and 55: 44 High-resolution VUV spectroscopy
Page 70 and 71: 60 REFERENCES [19] J. S. Nielsen an
Page 72 and 73: 62 Lifetimes of molecular negative
Page 86 and 87: 76 REFERENCES [19] V. V. Petrunin e
Page 88 and 89: 78 REFERENCES [66] F. Robicheaux, P
Page 90 and 91: 80 Chapter 5. Femtosecond VUV core-
Page 110 and 111: 100 Chapter 5. Femtosecond VUV core
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106 Chapter 5. Femtosecond VUV core
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120 REFERENCES [17] A. Cavalleri et
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122 REFERENCES [61] R. J. Finlay et
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124 REFERENCES [104]R.W.Joyner,inTh
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126 REFERENCES [152] T.-C. Chiang,
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128 Chapter 6. Two-colour pump-prob
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144 REFERENCES [16] Information ava
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146 REFERENCES [62] S. Baier et al.
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148 The following three chapters ar
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