VUV Spectroscopy of Atoms, Molecules and Surfaces

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74 Lifetimes of molecular negative ions 4.6 Conclusion The utility of an ion-storage ring for time-of-flight measurements of autodetachment lifetimes of metastable negative ions has been demonstrated. This was illustrated by the examples of N − 2 and CO−2 for which the lifetime measurements provided signatures of previously unobserved electronic highspin states. The method could be used for lifetime measurements on other metastable negative ions, such as Ne − 2 [63], as well as on metastable states of stable negative ions associated with excited electronic states of the corresponding neutral molecule. In the latter case, eventual contamination of the ion beam with other stable- or metastable species would be more difficult to identify and should be carefully evaluated. In this respect, a higher degree of selectivity of the present technique would be desirable, either in the negative-ion formation or -detection process. This might be achieved by the application of a more advanced detection system, e.g. capable of massanalyzing the neutrals generated in the autodetachment process or energyanalyzing the emitted electrons. For radiative lifetimes of bound, excited states of negative- or positive ions, selectivity can be obtained by means of laser excitation- and ionization schemes [64] or by detection of the fluorescence emitted during the decay [65]. Such approaches would obviously be very difficult to adapt to studies of autodetaching states of negative ions. The newly installed electron-beam ion source (EBIS) at ASTRID for production of multiply-charged positive ions offers an alternative possibility for selectivity in the negative-ion formation process. Recently, an experiment has been initiated, aiming at the generation of Li− ions in the long-lived 1s2s2p2 5 ++ P state which may be formed from Li by the capture of three electrons in a (sequential) charge-exchange procees. The present technique is very well suited for initial tests of the existence of theoretically predicted negative-ion species, especially for ions which are expected to be on the edge of stability. Along these lines one could, e.g., search for the Mg − 2 ,HeH− ,NeH−and ArH− ions which have been predicted to form stable, weakly-bound states [66, 67, 68]. They are examples of socalled Tango-states, representative of three-body systems consisting of two particles that are able to form a bound state and a third particle which cannot bind to any of them. The existence of such states relies entirely on electron correlation, making them important test cases for theoretical models. Other interesting ions could be H2O− , whose stability is still a matter of debate [69], or HCN− which has been predicted to exist as a dipole-bound species witha6cm−1binding energy [70].
References [1] C. Blondel, Physica Scripta T58, 31 (1995). [2] G.L.Gutsevet al., J.Phys.Chem.A104, 5374(2000). [3] R. D. Mead et al., inGas Phase Ion Chemistry (Academic Press, New York, 1984). [4] C.F.Bungeet al., Nucl. Inst. Meth. 202, 299 (1982). [5] C.A.Nicolaideset al., J. Molecular Structure (THEOCHEM) 199, 283 (1989). [6] N. A. Piangos and C. A. Nicolaides, J. Phys. B 31, L147 (1998). [7] E. Träbert et al., J.Phys.B25, L353 (1992). [8] S. Mannervik et al., J.Phys.B13, L441 (1980). [9] C.F.Bunge,Phys.Rev.A22, 1 (1980). [10] R. Middleton and J. Klein, Phys. Rev. A 60, 3786 (1999). [11] M. Hochlaf et al., J.Chem.Phys.110, 11835 (1999). [12] C. W. Bauschlicher and S. R. Langhoff, J. Chem. Phys. 101, 80 (1994). [13] S. R. Langhoff et al., J.Chem.Phys.89, 7354(1988). [14] C. W. Bauschlicher et al., Chem. Phys. Lett. 216, 341 (1993). [15] R. P. Saxon and B. Liu, J. Chem. Phys. 67, 5432 (1977). [16] T. Kudo and M. S. Gordon, J. Chem. Phys. 102, 6806 (1995). [17] T. Andersen et al., Phys.Rev.A47, 890 (1993). [18] P. Balling et al., Phys. Rev. Lett. 69, 1042 (1992).
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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
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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Page 132 and 133: 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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