VUV Spectroscopy of Atoms, Molecules and Surfaces

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70 Lifetimes of molecular negative ions DETACHMENT YIELD 10 3 10 2 10 1 0 10 20 30 40 TIME (ms) Figure 4.3: Semilogarithmic plot of the detachment yield versus time after injection for a CO − 2 beam produced by charge exchange of CO+ 2 ions. The solid curve represents a fit to two exponentials and a constant background. Error bars represent a statistical uncertainty of one standard deviation. repeatedly, successively disregarding more and more data points from the left (short times). In any case, data points corresponding to the first few revolutions of ions in the ring are disregarded, since they could be influenced by initial state effects like, e.g., slit scattering of the beam. While the absence of a long-lived (1–5 s) component was expected, the presence of one or more millisecond components instead of the expected ∼90 µs componentis surprising. Showing a slight systematic deviation from linearity on a semilogarithmic scale, the data were fitted to two exponentials added to a constant background, as measured with no ions in the ring. By this procedure, lifetimes of 3.3 ± 0.4ms and 7.8 ± 0.3 ms, respectively, were obtained for the data presented in figure 4.3. The two lifetimes may be attributed to different vibrational levels of the same electronic state and are sufficiently close that more than two components may be hidden in the decay. The values stated should thus be considered vibrational averages, sensitive to the conditions prevailing in the ion source. The resolution of the experiment would have permitted observation of the ∼90 µs component, if represented by at least ∼1 %oftheCO − 2 ions. The ap- parent absence of such CO − 2 ions may be explained by the production method applied. Assuming the CO + 2 ions from the ion source to be in the 2 Πg ground state upon entering the charge-exchange cell and considering the ionization energies of 13.76 eV and 4.33 eV for CO2 [51] and potassium, respectively,
4.5 CO − 2 the lowest-lying CO − 2 state is probably not reached. Electron capture by CO + 2 (2Πg) ions will populate highly excited states of CO2, and subsequent electron capture may likely lead to the formation of CO − 2 in an excited state that happens to be long-lived. Starting out from the doublet CO + 2 state, the long-lived CO − 2 state must be bound to a singlet or triplet state of CO2 and therefore either be a doublet or quartet state. Whether these states of CO2 are bent or linear are open questions, since the charge-exchange and CO − 2 process leaves a scope of at least 2×4.33 eV for their energies. Besides, the mapping of electronic excited states of CO2 is far from being complete with only the lowest excited singlet and triplet state energies being calculated by Spielfiedel et al. [52]. They have considered the 1,3Σ− u , 1,3Σ + u , 1,3∆u and 1,3Πg states associated with the 1σ2 g1σ2 u2σ2 g2σ2 u3σ2 g3σ2 u4σ2 g1π4 u1π3 g2π1 u and ...1π4 u1π3 g5σ1 g linear electronic configurations, obtained by excitation of a 1πg electron from the CO2( 1Σ + g ) ground state of electronic configuration . According to the Walsh rules, these linear configurations corre- ...1π4 u1π4 g late with bent 1,3A2 and 1,3B2 states that either raise or lower the energy with respect to the corresponding linear states [51]. If such a bent state turns out to be the energetically most favourable, it is pushed 4–5 eV down with respect to the corresponding linear state [52] and it is thus very difficult to predict the order of states without performing detailed calculations. A complete picture of the variation of equilibrium bend angle with excitation energy can therefore not yet be given. All that can be argued is that between the linear CO2 ground state and the bent CO + 2 ground state there is a low-lying band of bent states and a high-lying band of linear states. The present experimental result has stimulated theoretical investigations of the lowest-lying excited states of CO − 2 and their excited CO2 parent states. The quartet states 4A2 and 4B2 of CO − 2 associated with the lowest triplet states 3B2 and 3A2 of CO2, turned out to be rather short-lived [53] and further investigations therefore concentrated on higher-lying quartet states and on doublet states. The lowest-lying excited 2A2 state of the isoelectronic NO2 molecule is known to be metastable, decaying radiatively to the 2A1 ground state with a lifetime of ∼44 µs [54,55]. NO2is bent in the 2A1 ground state with an equilibrium angle of 133◦ —similar to the lowest-lying CO − 2 state—while the 2A2 state is associated with a 110◦ geometry [56, 57]. By analogy, the observed metastable state of CO − 2 may be a 2A2 state of strongly bent geometry decaying radiatively on a millisecond timescale to the CO − 2 (2A1) state, followed by autodetachment to the CO2( 1Σ + g ) ground state with a lifetime of ∼90 µs. The possibility of an excited metastable state of CO + 2 surviving to the charge-exchange cell and giving rise to a CO − 2 state which is not a doublet or a quartet cannot be ruled out. In fact, an excited state of CO + 2 located 71
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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
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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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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