VUV Spectroscopy of Atoms, Molecules and Surfaces

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108 Chapter 5. Femtosecond VUV core-level spectroscopy ... limit is set by the absorption cut-off of the grating (probably due to pumpoil contamination) while the long-wavelength limit is given by the length of the bellow (the longer the wavelength the more stretched the bellow). The harmonics are generated with the 810 nm, 10 Hz, 100 fs pulsed output from a Ti:Sapphire laser system, consisting of an oscillator and a regenerative amplifier system from Clark-MXR in addition to a home-built butterfly amplifier. The Ti:Sapphire gain medium of the oscillator is pumped with the 5 W, continuous-wave output from an Ar + laser, and ∼3 nJ,∼800 nm, ∼100 fs pulses are generated by the mechanism of Kerr-lens modelocking [137] at a repetition rate of 91 MHz. The 3 nJ, ∼50 fs pulses are stretched to ∼100 ps by means of a number of reflections from a grating and subsequently regeneratively amplified to ∼1.5 mJ in another Ti:Sapphire crystal pumped with 7 mJ, 532 nm, 1 kHz, 50–100 ns pulses from a Nd:YAlG laser. The 1.5 mJ pulses are further amplified to ∼35 mJ by passing four times through a butterfly amplifier with a third Ti:Sapphire crystal, pumped by the ∼220 mJ, 532 nm, 10 Hz, ns pulsed output from a second Nd:YAlG laser. The ∼35 mJ are finally compressed to ∼100 fs by undergoing a number of reflections from another grating, compensating for the chirp and group-velocity dispersion introduced by the stretcher and the gain media. The commercial compressor is not designed for this last amplification step as reflected by the resulting modest output of ∼8 mJ/pulse which is limited by the damage threshold of the grating. The resulting ∼8 mJ, 810 nm, 100 fs pulses are focused with a 50 cm lens to an intensity of 3.4×1015 W/cm2 in the Ne gas jet. The generated, diverging VUV light is focused ∼10 cm in front of the entrance slit of the spectrometer by a toroidal gold mirror with estimated horizontal- and vertical radii of curvatures of 3.50 and 0.14m, respectively. Ideally, the VUV light should be focused onto the entrance slit but this was not possible given the focusing conditions of the toroidal mirror (which had been previously used with the spectrometer for another purpose). The focusing conditions could be improved by moving the gas jet closer to the spectrometer, but this would increase the laser fluence incident on the grating, implying an enhanced risk of damage. A large fraction of the VUV light is thus wasted on the entrance slit but also on the toroidal mirror due to the grazing-incidence configuration, the angle of incidence being ∼87◦ . With the present configuration the distance from the gas jet to the toroidal mirror is ∼1.5 m which also leaves space for two valves, an aperture and a ∼500 ˚A Zr foil (home-made) in the beam-path. The purpose of the aperture is to limit the pressure rise in the spectrometer from the Ne gas flow which, in addition, is directed into a pump sitting below the chamber. The Zr foil has a transmission of 80–90 % for photon energies above ∼50 eV and can be pushed into the beam path in order to
5.5 The VUV light source of high-order harmonics 109 prevent stray laser light from reaching the spectrometer. This is necessary for the detection of the highest harmonic orders for which the dispersion angle approaches that of the zeroth- order reflection of the grating. The VUV light is detected with a channeltron detector positioned either directly after the exit slit or following the reflection of the VUV light from the multilayer mirror mounted in a small chamber. The base pressures of the HHG chamber and the spectrometer are 1×10−7 and 1×10−6 mB, respectively, with the HHG chamber pressure increasing to ∼2×10−4 mB with the gas jet on. The harmonic yield for the highest orders is optimized by focusing 3 cm in front of the gas jet and by slightly aperturing the laser beam, resulting in a 10 mm beam diameter and a ∼7 mJ pulse energy. This indicates that the harmonic yield is limited by the phase-matching conditions rather than the (saturation) intensity and the use of a lens with a longer focal length could possibly be advantageous. The gas jet is operated with a Ne backing pressure of 1–2 Bar and a 750 µs opening time and the laser-beam focus is located at the closest possible vertical distance from the orifice of the 0.8 mm diameter tube extension from which the gas jet originates. Here, the Ne gas pressure is highest and on the order of 150–200 mB, depending on the backing pressure [138]. Under the specified conditions harmonics have been generated up to 118 eV (order 77th) as shown in figure 1.1 of chapter 1. This is in very good agreement with the cut-off energy of 120 eV calculated from the cut-off law Ecut = Ip +2Up (see chapter 1), assuming a saturation intensity of 8×1014 W/cm2 [135]. The spectrum was recorded with the Zr foil in the beam path, giving a low-energy cutoff at ∼55 eV. Assuming a slow variation of the spectrometer efficiency with wavelength, harmonics are seen to be effectively generated up to 112 eV (order 73th), in accordance with the previous experiments referred to above. Figure 5.11 shows a high-resolution (∼0.1 eV FWHM) spectrum of the 63rd harmonic which has been fitted to a Gaussian function to yield a width of 1.4eV (FWHM). This width is much larger than expected and much larger than the value of 0.43 eV recently reported for the 47th–55th harmonics generated with 2.5 mJ, 800 nm, 70 fs laser pulses focused to an intensity of 5×1014 W/cm2 in a Ne gas jet [139]. In that experiment the width was observed to increase gradually from 0.11 eV at the 7th to 0.37 eV at the 45th harmonic, then stabilizing at the 0.43 eV. The disagreement could be ascribed to the intensity of the laser beam in the present experiment being well above that of saturation. In a previous experiment the spectrum of the 15th harmonic generated in Xe was found to broaden by almost a factor of two, following an increase in the intensity by a factor of three from a value just below saturation [135]. With an estimated saturation intensity for Ne of ∼8×1014 W/cm2 [135] the intensity of the present experiment is likely
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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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Page 68 and 69: 58 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
Page 130 and 131: 120 REFERENCES [17] A. Cavalleri et
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
Page 154 and 155: 144 REFERENCES [16] Information ava
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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