VUV Spectroscopy of Atoms, Molecules and Surfaces

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66 Lifetimes of molecular negative ions Figure 4.1: Potential-energy curves for the 4Πu and 4Σ − g states of N − 2 and the associated 3 + Σ u and 3Πg parent states of N2 calculated by Sommerfeld and Cederbaum [36]. beam through the charge-exchange-cell, in agreement with a negative result previously reported by Bae et al. [37]. From further investigations at the Aarhus Tandem Accelerator an upper limit of 5×10−10 for the conversion efficiency of N + 2 ions to N−2 ions could be estimated [38]. There, the N−2 ion formation process was studied with a stripping technique which allowed the masses of N − 2 and Si− to be separated. Long-lived N − 2 ions were detected upon sputtering of TiN and BN surfaces, but could not be generated from NaN3. Considering the differing chemical environment of N atoms bound in the three compounds, it seems likely that the long-lived N − 2 ions are formed by interaction between two sputtered N atoms and not from a precursor like, e.g., N − 3 present in NaN3. A ∼25 pA beam of N − 2 ions generated by sputtering of TiN was used for the lifetime measurement. Since the TiN compound was expected to be contaminated with Si from pump-oil deposits, a reference measurement was performed on a pure Si− beam produced by sputtering of a pure Si surface. Present in the Si− decay were a stable component and a short-lived component fitted to 2.0±0.2 ms, with the stated uncertainty reflecting the span of lifetimes obtained when disregarding data-points corresponding to the first few round-trips in the ring. Since Si− is known to possess the three stable states 4S, 2D and 2P with binding energies of 1.39 eV, 0.53 eV and ∼30 meV, respectively [39], the long-lived component must be atttributed to the 4S and 2D states with the 2 ms component left for the 2P state. A typical decay for N − 2 is shown on a semilogarithmic scale on figure 4.2, illustrating a
4.4 N − 2 NEUTRAL YIELD 5 C-120-445 10 10 4 10 3 10 0 2 4 6 8 10 TIME (ms) 2 Figure 4.2: Semilogarithmic plot of the N2 signal as a function of time after injection for a 100 keVN − 2 beam. The solid curve represents a fit to four exponentials. number of not clearly resolvable lifetime components. A stable component is present in the decay and is attributed to the presence of Si− . When fitting the data, two of the components were therefore fixed at the values dictated by the Si− decay curve. The minimum number of exponentials required to give a reasonable fit is four, meaning that at least two different N − 2 lifetime components are present. Due to the very rapid initial decay, only the first three data points could be disregarded in the fit, which is just on the edge of influence from initial state effects. The result thus obtained are N − 2 lifetimes of 120±13 µs and 515±25 µs, with the major contribution to the uncertainty arising from the removal of data points. ions cannot be generated by charge ex- The fact that the long-lived N − 2 change provides evidence against a long-lived quartet state of N − 2 which has as a parent a triplet state of N2. The present result has stimulated Veseth to re-calculate the potential-energy curves associated with the 4Πu and 4Σ− g states, using a many-body perturbation method for general spaces [38]. An adiabatic electron affinity of −0.32 eV was obtained for the 4Πu state, while the 4Σ− g state was found to be bound with respect to the 3Πg state of N2 but unstable with respect to autodetachment to the 3Σ + u state of N2. Attention was subsequently drawn to the sextet states ...1π 3 u 3σ1 g 1π2 g 3σ1 u 6 Πg and ...1π 2 u3σ 2 g1π 2 g3σ 1 u 6 Σ + u of N − 2 associated with the quintet parent states ...1π 3 u3σ 1 g1π 2 g 5 Πu and ...1π 2 u3σ 2 g1π 2 g 5 Σ + g of N2, respectively. Using the CASSCF approach with a AUG-cc-pVTZ basis set, Olsen predicted both 67
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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 26 and 27: 16 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-
Page 110 and 111: 100 Chapter 5. Femtosecond VUV core
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116 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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