VUV Spectroscopy of Atoms, Molecules and Surfaces

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68 Lifetimes of molecular negative ions of the sextet states to be located below both of the quintet parent states and to be stable with respect to autodetachment and photon emission [38]. Equilibrium bond lengths of 2.48 ˚A and 2.65 ˚A were obtained for the 6 Σ + u and 6 Πg states, respectively, and the minima of the potential-energy curves were attained at equal energies within the accuracy of the calculation. These results were later found to be in qualitative agreement with CCSD(T) cal- state to be lowered culations by Dreuw and Cederbaum, predicting the 6Σ + u by ∼0.5 eV with respect to the 6Πg state [40]. They also found the 5Σ + g minimum to be located at a somewhat lower energy, in fact coinciding with state but at a very different bond length. Dreuw and Ceder- that of the 6Σ + u baum have performed similar calculations on the iso-electronic CO− ,ion, predicting a 6Πg potential-energy curve with a shallow minimum at a bond length of 3.5 ˚A, while that of the 6Σ + u state is entirely repulsive [41]. The loosely bound 6Πg state of CO− can be considered the result of a C− ion bound with respect to the dipole that it induces to the O atom [41]. This due to the picture certainly does not carry on to the sextet states of N − 2 instability of N− .The 6Πg state of CO− should probably be associated with long-lived CO− ions observed by Middleton and Klein in a mass spectrometer [10], while the long-lived N − 2 ions of the present experiment can be attributed state. This assignment is supported by optical emission studies to the 6Σ + u of N2 molecules formed in sputtering, providing signatures of N2 formation in the 5Σ + g state by the recombination of two sputtered N( 4S)atoms[42]. This also agrees with the observation at the Aarhus Tandem Accelerator that long-lived N − 2 ions could be generated from TiN and BN but not from NaN3 [38]. 4.5 CO − 2 The existence of metastable, autodetaching CO − 2 ions in the gas phase was first reported in 1970 by Paulson [43], who studied reactions of O− with CO2. Subsequently, Cooper and Compton observed formation of metastable, autodetaching CO − 2 ions in collisions of electrons or Cs atoms with cyclic anhydrides containing as a basic unit CO2 with a bend angle of 120◦ [44, 45]. The lifetimes of the produced species were measured with a time-of-flight spectrometer to be 26 ± 5 µs and60± 5 µs by electron impact, and 71 ± 10 µs and 62 ± 10 µs by Cs collisions for succinic anhydride (C4H4O3) and maleic anhydride (C4H2O3), respectively. The observation of two distinct lifetimes was explained by the population of different vibrational levels of the same electronic state, with the long lifetime attributed to the lowest vibrationel level, exhibiting the smallest Frank-Condon overlap with the neutral parent state. A study of CO − 2 ions formed in collisions of alkali-metal atoms with
4.5 CO − 2 linear CO2 molecules resulted in a mean lifetime of 90 ± 20 µs and a value of −0.6 ± 0.2 eV for the adiabatic electron affinity of the CO2 ground state which was considered the parent state of the observed metastable species [46]. If attributed to the lowest ro-vibrational state, the lifetime 90 ± 20 µs is in agreement with calculations by Rauk et al. [47]. The measured electron affinity of CO2 is also in agreement with the theoretical value of −0.67 eV reported recently by Gutsev et al. who have calculated the potential-energy curves for the lowest-lying electronic states of CO2 and CO − 2 , 1Σ + g and 2A1, respectively [48]. While the ground state of CO2 is linear, the minimum of the potential-energy curve of CO − 2 is obtained at a bend angle of 138◦ and energetically located below the ground-state potential-energy curve of CO2 evaluated at this angle [48]. The metastability of the observed CO − 2 state and the non-existense of stable linear CO − 2 ions [49] are thus accounted for by the characteristics of the ground-state potential-energy curves. Recently, Mid- dleton and Klein have detected CO − 2 69 ions by accelerator mass spectrometry upon sputtering of a Ti cathode with a 0.5 mm diameter hole drilled perpendicularly through the surface for CO2 gas inlet [10]. A lower limit of 170 µs for the lifetime was estimated with no comments as to the possible identity of the state in question. Finally, Schröder et al. studied the formation of CO − 2 from CO + 2 ions by charge exchange in Xe gas and from collisional activation of carboxylate ions, by means of a mass-spectrometric technique [50]. The motivation for using Xe is the similarity of its ionization potential with that of CO2, favouring the formation of CO − 2 ions in the lowest-lying electronic state, 2 A1. A lower limit of 50 µs was estimated for the autodetachment lifetime of the generated CO − 2 ions. The purpose of the present experiment was a test and improvement of the above mentioned results with the condition that the method of CO − 2 ion formation applied might influence the outcome of the experiment. Long- ions were produced either directly in a sputter-ion source [30] or lived CO − 2 by charge exchange of CO + 2 ions. By the sputter process, CO2 gas was let in to a Ti cathode sputtered with Cs + ions, resulting in a 0.15 nA CO − 2 beam. For the charge-exchange process, a 1 µA beamofCO + 2 ions produced in a plasma type ion source was guided through the charge-exchange cell where a minor fraction was converted to CO − 2 ions. Considering first the long-lived CO − 2 ions formed by charge-exchange, the negative-ion beam had decayed completely after ∼50 ms. A typical decay of the neutral-molecule signal as a function of time after injection is shown on a semilogarithmic scale in figure 4.3. The spread in data points reflects betatron oscillations of the ions in the ring, causing the detection efficiency of neutrals to vary due to the finite detector size. In the data processing, the uncertainty thereby introduced is accounted for by fitting the decay curve
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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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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-
Page 110 and 111: 100 Chapter 5. Femtosecond VUV core
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118 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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