VUV Spectroscopy of Atoms, Molecules and Surfaces

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134 Chapter 6. Two-colour pump-probe experiments on He ... polarized) harmonic beam. The grating of 50 cm focal length created an image of the DFDL beam spot from the Kr gas jet ∼2 cm before the interaction region, corresponding to a ∼0.2 mm diameter harmonic beam in the interaction region. This value was confirmed by focusing the probe beam to a ∼17 µm diameterspotin the center of the interaction region and scanning the 30 cm lens vertically while measuring the He + signal. Due to the physical construction of the experimental setup it was not possible to move the vacuum chamber, i.e. the interaction region, closer towards the grating. The base pressure in the chamber was ∼1×10 −6 mB with the gas jets turned off and could be kept around this value with the Kr jet on due to a ∼2 mm slit positioned at the entrance to the chamber. The He gas jet could be operated either in a pulsed mode or with a continuous flow with pressures in the 2×10 −5 –6×10 −4 mB range. For the 13th harmonic, on the order of ∼5×10 6 photons/pulse were present after the grating and the ∼2 mm slit, as estimated with a calibrated VUV diode placed after an aluminum foil. The bandwidth of the harmonic was determined by measuring the He + signal as a function of the DFDL wavelength (cf. figure 6.5), giving a value of 0.0035 nm (1 meV) for a 0.23 ns delay of the probe with respect to the probe (in order to avoid any influence from the spike). For comparison, a value of 0.005 nm was found in the 1999 experiment [36] while 0.01 nm was measured in 1995 [37]. The spectral overlap with the Doppler-broadened width of 0.2 meV of the He 1s2p level is reasonable. 6.3 Results and discussion 6.3.1 Lifetime measurements and the role of the spike From the beginning of the experiment the spike was present at zero time delay in the exponential decay and it turned out that getting rid of it was not so easy (unfortunately, the exact conditions under which the spike-free decay curves had been recorded in 1995 could not be recalled)! Running the He gas jet in pulsed mode and varying the probe-pulse energy and the focusing conditions of the harmonic as well as the probe did not help. The focusing of the harmonic could only be changed by moving the chamber, and the harmonic could only be further defocused since the chamber could only be moved further away from the Kr gas jet. In the end it was decided to reduce the pressure in the interaction region by running the He gas jet with a continuous, static flow, and, in fact, the spike disappeared for pressures below 6×10 −5 mB. This is illustrated in figure 6.3 which shows the monitored He + signal on a semilogarithmic scale as a function of pump-probe delay for
6.3 Results and discussion 135 Ion signal (arb. units) 10 1 0.1 6x10 -5 mB 1x10 -4 mB 3x10 -4 mB 6x10 -4 mB 0.0 0.5 1.0 1.5 2.0 Delay (ns) Figure 6.3: Semilogarithmic plot of the He + ion signal as a function of pump-probe delay for four different He pressures. four different pressures. For all of the measurements presented, a probepulse energy of ∼350 µJ/pulse was used. The point of zero delay has been (arbitrarily) chosen as the delay corresponding to a maximum in the ion signal. From figure 6.3 the spike is seen to develop when the pressure is increased from 6×10 −5 mB to 6×10 −4 mB, the latter being the maximum pressure applied considering the presence of the EMT’s. The development of the spike is seen to be accompanied by an almost one order-of-magnitude decrease in the ion signal at long delays, implying a much reduced statistics in the tails of the decay curves. Abstracting from the consequent scatter in the data points it is evident that the slopes of the curves are constant, and ”only” the uncertainty on the measured lifetime is affected by the presence of the spike. The influence of the pressure is further illustrated in figure 6.4where the magnitude of the ion signal at zero (black) and 0.23 ns (grey) delays, respectively, is depicted as a function of pressure. The off-spike ion signal increases up to ∼2×10 −5 mB and then decreases, while the on-spike signal increases up to ∼6×10 −4 mB and then saturates. The presence of the spike is also signified in the harmonic spectral profile which is broadened by almost a factor of two when the pump- and probe pulses overlap in time. This is illustrated in figure 6.5, showing the apparent harmonic profile at zero (black) and 0.23 ns (grey) delays, respectively, for a pressure of 6×10 −4 mB. Subsequent fits of the profiles to a Gaussian function yielded widths (FWHM) of 0.0057 and 0.0035 nm for the on- and off-spike conditions, respectively, and corresponding mean wavelengths of 58.4292 and 58.4295 nm.
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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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82 Chapter 5. Femtosecond VUV core-
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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
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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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