VUV Spectroscopy of Atoms, Molecules and Surfaces

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132 Chapter 6. Two-colour pump-probe experiments on He ... of the dye solution. The output wavelength of the DFDL oscillator, λDFDL, can thus be tuned by adjusting θ (by turning the mirrors) or—on a finer scale—by tuning the refractive index. In order to obtain tunability around the ∼759 nm required for the present experiment while keeping the mirrors parallel, the angle of incidence θ was increased from that of the +1st and −1st order diffractions by means of a prism inserted in front of the dye cell. The refractive index was tuned by tuning the temperature of the dye solution, resulting in a wavelength change of −0.217 nm/ ◦C. In the experiment, the absolute wavelength could be measured to an accuracy of ∼0.01 nm with a commercial spectrometer. Only a fraction of the ∼25 mJ, 532 nm, 70 ps output from the Nd:YAlG laser was used to pump the DFDL oscillator, the rest being distributed to two dye-cell amplifiers for amplification of the DFDL output to ∼1.5 mJ/pulse. In addition, a few mJ were split off and mixed with the fundamental 1064nm output from the Nd:YAlG laser in order to generate ∼0.5 mJ, 355 nm probe pulses. Further amplification of the DFDL output to ∼35 mJ/pulse was obtained by five passages through a Ti:Sapphire butterfly amplifier pumped with a total of ∼400 mJ/pulse from a 532 nm, 10 Hz, 10 ns Nd:YAlG laser. The DFDL pulses are expected to be approximately Fourier-transform limited (time-bandwidth product 0.41) with a pulse duration similar to that of the 70 ps pump laser, implying a bandwidth of ∼0.05 nm [50]. The resulting DFDL laser beam, consisting of 35 mJ, ∼759 nm, ∼70 ps pulses, was focused to an intensity of ∼5×1014 W/cm2 in a Kr gas jet with a lens of 12 cm focal length. Harmonic orders were generated up to the 15th with the 13th of 21.22 eV (58.4nm) photon energy being selected by a 1200 lines/mm holographic grating. The harmonic beam was intersected by the 355 nm probe beam at a 45◦ angle in a vacuum chamber equipped with a gas jet for He inlet and an Electron-Multiplier Tube (EMT) for detection of the harmonic signal. A flight tube extending from the top of the vacuum chamber and terminated with an EMT, was used for time-of-flight detection of He + ions repelled from the interaction region by a 1 kV potential. The probe beam was focused with a 30 cm lens to a ∼17 µm diameter focal spot ∼7 cm before the interaction region, corresponding to a maximum in the detected He + signal. The corresponding beam diameter in the interaction region was ∼1 mm. With a probe energy of ∼350 µJ in the chamber this corresponds to an intensity of ∼5×108 W/cm2 in the interaction region— very close to the intensity which can be estimated to saturate the transition (using the cross-section value stated in [36]). The probe beam passed through a delay line on its way to the interaction region, and a halfwave plate was positioned in front of the 30 cm lens for rotation of the polarization vector of the (initially) vertically polarized probe beam with respect to the (vertically
6.2 Experimental approach 133 Delay line Grating (f = 50 cm) 200 mJ Nd:YAG 10 ns 355 nm Pump Probe Kr 200 mJ Nd:YAG 70 ps f = 30 cm Slit Ti:Sapphire Spatial filter 25 mJ 532 nm EMT Dye amplifiers Figure 6.2: A schematic of the experimental setup used for the He 1s2p lifetime and cross-section measurements f = 12 cm He + EMT Power meter 70 ps, 35 mJ, ∼759 nm Grating
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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 110 and 111: 100 Chapter 5. Femtosecond VUV core
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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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