VUV Spectroscopy of Atoms, Molecules and Surfaces

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24 Negative ions ing the binding energy of 0.75 eV [8]. H− is also speculated to play a role in the mechanism of formation of molecular hydrogen in the interior of the interstellar medium [9]. Due to the absence of a dipole moment in the ground state, H2 cannot simply be formed by the radiative association of two groundstate H atoms; another type of particle is needed to carry away the excess energy required for the binding [10, 11]. This particle could, for example, be an electron emitted in a H− –H collision and it has recently been proposed to look for signatures of H− in the VUV part of the absorption spectra of interstellar clouds [9]. Also heavier atomic- and molecular negative ions have been considered to play a role in the chemistry of the stars [12] and the interstellar medium [13]. Recently, absorption spectra of small, linear carbon-chain anions obtained by laboratory laser-spectroscopic measurements were found to compare remarkably well with astrophysically observed diffuse interstellar bands [14]. Negative molecular ions are, in addition, abundant 20–120 km above the surface of the Earth in the form of O − 2 ,OH− ,ClO− ,O − 3 ,NO−2 , O − 4 ,NO− 3 ,CO− 3 etc. [15] and must be included in theoretical models of the atmosphere [16, 17]. In the laboratory, negative ions are the key ingredients in accelerator mass spectroscopy which has proven a very sensitive technique in terms of detection efficiency and mass resolution for, e.g., carbon 14dating [18] and investigations of rare- or heavy negative ions [19]. Negative ions are also abundant in plasmas and discharges [20] where they play a role in laser action and semiconductor device processing [21]. 2.2 Negative-ion structure From the above it is hopefully evident that negative ions are worth further investigations, but one could start by asking whether the binding of an electron to a neutral atom or -molecule is at all possible. Indeed, the structure of a negative ion is very different from that of a neutral system, especially for small atoms where the ratio of negative- to positive charge is large and electron-correlation effects therefore more pronounced. The binding of an extra electron to the ground state of a neutral atom is accomplished by it inducing to the atom an electric dipole moment which acts back on it as an attractive 1/r 4 short-range potential [20, 22]. In a classical picture the neutral atom (or molecule) feels an electric field � E ∝ �r/r 3 from the electron approaching a distance r away. This induces to the atom a dipole moment �P ∝ α � E, α being the dipole polarizability, which gives rise to the attractive Φ ∝ � P ·�r/2r 3 ∝ 1/r 4 potential [23]. Unlike the long-range Coulomb potential acting on an electron of a neutral atom, the short-range potential implies a finite number of bound states. In general, only a single bound state exists, but the single-electron continuum may reveal a rich structure of doubly-excited
2.2 Negative-ion structure 25 states, or resonances, that are often associated with excited states of the neutral parent atom. The doubly-excited states usually decay by emission of an electron, leaving the resulting neutral atom in its ground state or a lowerlying excited state. This process is called auto-detachment and is analogous to the autoionization mechanism known for neutral systems. Resonances can be excited by photon absorption from the ground state of the negative ion or by collisions of the corresponding neutral atom or molecule with, e.g., electrons. Laser spectroscopy is superior to electron scattering in terms of energy resolution but is—because of radiative selection rules— restricted to angular momentum states that can be reached by one- or two-photon absorption from the ground state or a metastable state of the negative ion. InthecaseofH − an extra electron may be bound with respect to an excited state of the neutral atom by the long-range 1/r 2 dipole potential resulting from a mixing of the degenerate angular momentum states by the electric field of the approaching electron. This leads to series of doubly-excited states, resonances, converging exponentially to the thresholds defined by excited states of the neutral hydrogen atom [24]. The neutral hydrogen atom is the only element exhibiting such a degeneracy, and in general resonances of negative ions cannot be characterized by such a systematic behaviour. Even for the hydrogen atom the degeneracy is not exact due to the relativistic fine structure and QED splitting of the excited states, implying a deviation from the ideal dipole potential and a termination of the exponential series. Traditionally, doubly-excited states are divided into the idealized classes of Feshbach- and shape resonances [25, 26, 27]. A shape resonance is considered the result of trapping the extra electron behind a barrier formed by the short-range potential and the centrifugal barrier of the neutral atom, thus existing for non-zero orbital angular momenta. The bound state is energetically located above an excited state of the neutral atom and decays by tunneling of the electron through the barrier, resulting in a relatively short lifetime (∼10 −15 s), or a width on the order of an eV. A Feshbach resonance may be considered the result of attaching the extra electron to an electronically excited state of the neutral atom or molecule with the electrons on average sharing the total available energy. By arbitrary energy fluctuations one of the electrons will eventually become energetic enough that the system autodetaches with a relatively long lifetime (10 −11 –10 −13 s), corresponding to a width in the µeV–meV range. A doubly-excited state may, however, live for considerably longer time (microseconds) if its decay is forbidden by autodetachment selection rules. Such a state is considered metastable [28] and can only be observed in a time-of-flight measurement (cf. chapter 4). For molecular negative-ion resonances the additional possibility exists that the electron can be associated with a vibrationally- or rotationally excited state
Page 1: VUV Spectroscopy of Atoms, Molecule
Page 4 and 5: ii CONTENTS 4 .6 Conclusion . . . .
Page 6 and 7: iv Experimental and theoretical stu
Page 8 and 9: vi me to the principle of negative-
Page 10 and 11: viii
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 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
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74 Lifetimes of molecular negative
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76 REFERENCES [19] V. V. Petrunin e
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78 REFERENCES [66] F. Robicheaux, P
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80 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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