VUV Spectroscopy of Atoms, Molecules and Surfaces

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138 Chapter 6. Two-colour pump-probe experiments on He ... Ion signal (arb. units) 10 1 0.0 0.5 1.0 1.5 Delay (ns) Figure 6.6: The two low-pressure decays from figure 6.3 together with theoretical predictions from a rate-equation model (solid curves). and where Γ UV r dNr dt = Ω(t)Ng − Γ UV r Nr − ArNr (6.4) dNi dt = ΓUV r Nr, (6.5) is the intensity- and thus time-dependent ionization width of the resonant level, Ar = 0.57 ns the radiative lifetime and Ng the ground state population. As indicated above, Ω(t) is calculated as the ”area” of the Starkbroadened, ”hole-burned” Lorentzian atomic transition profile, multiplied by the Gaussian time-varying intensity of the harmonic. The Ω(t) obtained this way is used as the input to equation 6.3 which is solved to get Ng(t). Ng(t) andΩ(t) are inserted into equation 6.4which is solved numerically with respect to Nr(t). Finally, Nr(t) is inserted into equation 6.5 from which Ni(t) is obtained. Preliminary results from the application of this procedure for pressures of 5×10 −8 and 1×10 −4 mB are shown as the black- and grey solid curves in figure 6.6, respectively. By normalizing the low-pressure theoretical curve to the decay curve measured at 6×10 −5 mB, the best agreement of the highpressure theoretical curve (in terms of the on- to off-spike ion signal level) was found with the decay curve measured at 1×10 −4 mB, as indicated in the figure. The development of the spike is predicted by the rate-equation model
6.3 Results and discussion 139 Ion signal (arb. units) 8 7 6 5 0 60 120 180 240 300 360 Polarization angle (degrees) Figure 6.7: The He + ion signal as a function of angle of polarization between the pump and the probe for a pressure of 6×10 −5 mB. The theoretically predicted cross section is indicated by the solid curve. to occur at somewhat lower pressure (∼10 −7 mB) than found experimentally. The model may need some fine adjustment—for example, the Stark shift of the 1s2p level has been assumed to be equal to the ponderomotive energy— but it is evidently capable of predicting the essential features in the decay. It should be noted that without taking the absorption, or hole-burning, into account the spike cannot be reproduced unless an artificially high probe intensity (several times saturation) is used in the model. Finally, it remains to be checked how closely the pressure dependence of the ion signal measured on- and off spike, as given in figure 6.4, is reproduced by the model. The influence of radiation trapping and pressure broadening [25, 26] can be considered insignificant in the present experiment due to the low harmonic photon flux and given the pressure-independent slopes of the decay curves in figure 6.3. 6.3.2 Photoionization cross-section measurements Concurrently with the spike investigations discussed above, experiments were performed to show that relative partial photoionization cross sections of the 1s2p state could be determined by varying the angle between the pump- and probe polarization vectors. The measurements presented below were obtained with the time delay fixed at 0.23 ns and at a pressure of 6×10 −5 mB in order to minimize an eventual influence from the spike. By conservation of angular
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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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10 VUV light generation: possibilit
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18 REFERENCES [14 ] C. R. Vidal, in
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20 REFERENCES [55] C. J. G. Uiterwa
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22 REFERENCES [98] Parameters from
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24 Negative ions ing the binding en
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26 Negative ions located energetica
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28 Negative ions available has been
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30 Negative ions assignment will th
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32 Negative ions feel the repulsion
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34 Negative ions Figure 2.1: Adiaba
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36 Negative ions
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38 REFERENCES [16] E. Kopp, Adv. Sp
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40 REFERENCES [63] P. Balling et al
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42 REFERENCES
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44 High-resolution VUV spectroscopy
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60 REFERENCES [19] J. S. Nielsen an
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62 Lifetimes of molecular negative
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76 REFERENCES [19] V. V. Petrunin e
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78 REFERENCES [66] F. Robicheaux, P
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80 Chapter 5. Femtosecond VUV core-
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Page 98 and 99: 88 Chapter 5. Femtosecond VUV core-
Page 110 and 111: 100 Chapter 5. Femtosecond VUV core
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Page 132 and 133: 122 REFERENCES [61] R. J. Finlay et
Page 134 and 135: 124 REFERENCES [104]R.W.Joyner,inTh
Page 136 and 137: 126 REFERENCES [152] T.-C. Chiang,
Page 138 and 139: 128 Chapter 6. Two-colour pump-prob
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Page 156 and 157: 146 REFERENCES [62] S. Baier et al.
Page 158 and 159: 148 The following three chapters ar
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