VUV Spectroscopy of Atoms, Molecules and Surfaces

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48 High-resolution VUV spectroscopy of H − tensity of ∼2×1013W/cm2 in a gas cell containing ∼7 mB Xe. This pressure corresponds to optimum phase-matching conditions for the given focusing conditions, as verified by a maximum in the generated 118 nm signal. The conversion efficiency was resonantly enhanced by the proximity of the 118 nm wavelength to the 5p–5d electronic transition in Xe [20]. The 118 nm light was separated from the 355 nm beam by an astigmatically focusing spherical grating and subsequently collimated by a curved mirror before entering the ring through a LiF window as a ∼1 cm2diameter beam. In order to obtain the maximum possible signal in the cross section measurements, the 355 nm laser was operated at a pulse energy of 30–40 mJ which was on the edge of the damage threshold of the grating. In order to prevent an eventual damage implied by the presence of Xe gas, an additional LiF window could be installed in the beam path behind the laser focus to separate the grating from the Xe gas cell (not shown in the figure). Since the transmission of a LiF window, initially ∼30 %, was found to decrease as a function of exposure time to the 355 nm light, 2–3 windows were typically used during two weeks of beamtime, and for the final measurements the LiF window was removed. Although water-cooled, the grating also exhibited a decrease in efficiency as a function of time, following the eventual development of a burned spot, and a new grating had to be used for each beam-time. As it turned out, the gratings bought after the successful beam-times of 1996–97 were down in efficiency by at least an order of magnitude, providing a reflectivity of at most 3 %. Only very recently, just before our latest beam-time in November 2000, it was admitted to us that the coating procedure had been changed around 1997. The results presented in the present chapter, recorded in May 1999, have, accordingly, been obtained with an order-of-magnitude reduction in the number of 118 nm photons per pulse compared with the 1996–97 experiments but this was compensated by an one-order-magnitude increase in the repetition rate provided by a new laser. From an absolute measurement utilizing an NO ionization cell, the amount of 118 nm flux available immediately after the grating was estimated to ∼6×109 photons/pulse as verified by an orderof-magnitude reduction in the neutral-atom signal per pulse compared with the earlier experiments. The effective photon energy seen by the ions travelling with mean longitudinal velocity v is given by the Doppler formula as Eph � = γEL(1 + (v/c)cosα), where γ =1/ (1 − (v/c) 2 ), EL =10.48 eV is the photon energy in the laboratory system and α the angle of intersection, with α =0◦ corresponding to counter-propagating beams. With an ion-storage energy limited by the rigidity of the bending magnets to 160 MeV and 85 MeV, the effective photon energy can be varied up to 18.7 eV and 14.2 eV for H− and D− , respectively. The spread δE in E, equal to the energy resolution, is determined by the
3.2 Experimental approach 49 Electron gun Collector Cathode Toroid magnet Anode (grid) Solenoid magnets Electrostatic pick-up plates Figure 3.3: A schematic of the electron cooler mounted at ASTRID. Clearing electrodes C-168-193A spread in α, EL and v. Due to the almost collinear geometry, the contribution γ(v/c)EL sin αδα to δE from the spread δα ≈ 0.2 ◦ (FWHM) in α ≈ 0.5 ◦ amounts to only 13 µeVfora1.6MeVD − beam. With a bandwidth δEL < 1 µeV of the laser light, the contribution (ELγ/c)(cos α/γ 2 + v/c)δv to δE from the velocity spread δv is the resolution-limiting factor, amounting to ∼180 µeV for δv/v ∼ 4×10 −4 (typical of an uncooled beam, all values are FWHM). The energy spread is seen to increase with kinetic energy, thus attaining a value of 2.4meV at the maximum effective photon energy available for H − . The electron cooler is mounted on the straight section of ASTRID opposite to that of the laser beam. It consists of an electron gun, a 1 m interaction region, and an electron collector as shown in figure 3.3. A 3.6 mA electron beam, corresponding to a density of 5×10 6 cm −3 , is emitted by a 1-cm-diameter tungsten cathode and accelerated through an anode grid to 457 eV. The electron beam is guided in and out of the interaction region by solenoid magnetic fields of 100–200 G connected with two deflection toroids. Since the electrons are continuosly renewed, the electron-cooling section may be considered a heat exchanger where the velocity spread of an ion beam with longitudinal velocity equal to that of the electron beam is reduced as the ions moving too fast (slow) are decelerated (accelerated) by collisional energy transfer [21]. By the cooling process an equilibrium situation with equal ion- and electron-beam temperatures is approached, implying a velocity spread of the ion beam which is reduced with respect to that of the electron beam by a factor � M/m, withM and m being the ion- and electron masses, respectively (assuming Maxwellian velocity distributions). The
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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
Page 8 and 9: vi me to the principle of negative-
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Page 12 and 13: 2 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-
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98 Chapter 5. Femtosecond VUV core-
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100 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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